C64 Serial IO _toolkit_
Contents
1. Send Serial Byte Deferred EDDD EEE4 EDEE EEF5 Called by JMP from Kernal CIOUT vector at FFA8 JSR at F3FE F4BA in Send OPEN LOAD or SAVE Command to Device JSRs at F61C F6B4 F621 F6B9 and F62B F6C3 in Save to Serial Device JMP at F1D8 F288 in Determine Output Device This routine to send a character to the serial bus maintains a one byte buffer 95 The character to be sent is stored in this buffer This routine first tests a flag 94 which indicates whether the buffer 95 already contains a character If the buffer contains a character first it sends the buffered character in 95 to the serial bus Then it stores the current byte in the buffer 95 If the buffer is empty at entry it 15 simply stores the character to be sent in the buffer 95 Operation 1 See if the high bit of the serial deferred flag 94 is set If so branch to step 3 2 If no character is in the buffer set the high bit of 94 by setting the carry and rotating the carry into bit 7 ROR 94 Then branch to step 6 3 Push the byte to be buffered onto the stack 4 JSR ED40 EE49 to send the byte in 95 the serial deferred character over the serial data output line 5 Pull the byte to be buffered from the stack 6 Store the byte in 95 the serial deferred character 7 CLC and RTS Set Status Word EDAD EEB4 EDB8 EEBF Called by BCS at ED47 EES1 in Send Serial Byte Command or Data alternate entry at EDBO EEB7 by BCC at ED6F EE7D and BNE at EDA2. Use the instructions just mentioned in step 16 to bring the serial clock output line low and the serial data output line high 18 Decrement the serial byte transfer counter A5 19 If all the bits in this byte are not yet sent if A5 is not 0 branch to step 11 to check for a timeout between bits and to send the next bit if no timeout occurred 20 After all eight bits have been sent set CIA 1 timer B VIA 2 timer 2 to 0400 to set a delay of approximately 1 millisecond or 1000 microseconds For the VIC storing the 04 in 9129 the high byte of the latch value for timer 2 will load the counter from the latch values ni A 13 clear the VIA interrupt register and start the timer countdown However for the 64 you must separately order timer B to load its counter from the latches and start counting by storing 19 into DCOF the control register for timer B Also it is necessary to LDA DCOD to clear any pending timer B interrupts 21 Ifa timer B timer 2 interrupt occurs determined by looking at DCOD 912D the interrupt flag register branch to EDBO EEB7 the Set Status Word routine to indicate a frame error condition 22 If the serial data input line stays high loop to step 21 23 If the listener brings the serial data line low before the timer B timer 2 interrupt occurs i e within one millisecond then it is acknowledging that the byte has been received so CLI and RTS Send OPEN LOAD or SAVE Command to Device F3D5
3. 184 times 552 cycles TAX 2 cycles RTS 6 cycles The JSR to call this routine also takes 6 cycles Thus for the 64 the total number of cycles is6 2 2 368 552 2 6 938 cycles which when divided by the 64 s clock frequency of 1 022 370 cycles per second is 917 microseconds or approximately one millisecond For the VIC 1 LDA 04 2 STA 9129 VIA 2 high byte of timer 2 counter When this STA is done the timer 2 interrupt flag for VIA 2 is cleared the low latch of the counter is transferred to the low counter and the counter begins decrementing at the system clock rate ON Aare 3 LDA 912D VIA 2 interrupt flag register 4 AND 20 5 BEQ to step 3 as long as the timer 2 flag has not been set 6 RTS A timer 2 value of 0400 counts down at the system clock rate 0400 1024 decimal and 1024 cycles divided by 1 022 370 cycles per second is 1001 microseconds approximately 1 millisecond Save to Serial Device F5FA F692 F641 F6D9 Called by Falls through from Determine Device for SAVE routine at FSED F685 To save to a serial device a filename must be specified If no filename is given jump to the error routine to display the MISSING FILE NAME error message and exit Send the LISTEN command to the current serial device send the secondary address command of 61 to indicate a SAVE operation Then if the device is present send all the characters in the filename If the device is not present exit
4. Device routine The sequence called the TALK LISTEN turnaround that converts the 64 VIC into a listener and a serial device into the talker is described in Figure 8 4 Bring Serial Bus Attention Line High EDBE EEC5 EDC6 EECD Called by Falls through from EDBB EEC2 in Send Secondary Address After LISTEN JSR at EDD0 EED7 in Send Secondary Address After TALK and Do TALK LISTEN Turnaround JSR at EF03 EF09 in Send UNLISTEN Command JSR at F281 F33A in Open Serial Output Channel This routine brings the serial bus attention line high The serial attention output line from the CIA VIA chip passes through an inverter before reaching the serial port Thus setting the CIA VIA pin low 0 brings the serial attention line high at the serial port No separate routine exists for the converse function of bringing the serial attention output line low However see the code at EDF3 EEF9 in Send UNTALK Command for a sample of how to bring the serial attention output line low Operation 64 AND the contents of the CIA register at DD00 with F7 and store the result back into DDOO thus turning off bit 3 in CIA 2 data port A Bit 3 is the serial attention output line from the CIA chip VIC AND the contents of the VIA register at 91 IF with 7F and store the result back into 91 IF thus turning off bit 7 in VIA 1 data port A Bit 7 is the serial attention output line from the VIA chip Bring Serial Bus Data Line High EE97 E4A0 EE9F E4A8 Called by
5. EEBE in Set Status Word alternate entry at EE06 EFOC by JSRs at EF48 VIC and EE7D EF7C in Receive Byte from Serial Device This routine performs the sequences necessary to send the UNLISTEN command to all serial devices 1 LDA 3F the command for all devices to unlisten 2 JSR ED11 EEIC to do the attention handshake and send the command byte in the accumulator over the serial data output line 3 EE03 EF09 JSR EDBE EECS to bring the serial attention output line high 4 EE06 EFOC TXA to preserve the X register value in the accumulator then LDX with 0A 0B and DEX until it is zero to introduce a delay of approximately 50 60 microseconds After the delay TAX to restore the X register value 5 JSR EE85 EF84 to bring the serial clock output line high 6 JMP EE97 E4A0 to bring the serial data output line high and exit the routine Close Logical File for Serial Device F2EE F3AE F2F0 F3B0 Called by BCS at F2A5 F353 in Determine Device for CLOSE If the current secondary address lt 80 128 decimal send the CLOSE command to the serial device and command all serial devices to unlisten 27 If the current secondary address gt 80 128 decimal do not send the CLOSE or UNLISTEN commands Fall through to the Common Exit for Close Logical File Routines routine at F2F1 F3B1 see chapter 5 The common CLOSE routine decrements the number of open files and removes entries for this file from the secondary address logical file a
6. EOI handshake has been sent the talker brings the clock line low within 60 microseconds after the listener has brought the data line high to complete the EOI handshake When the talker brings the clock line low either for EOI or non EOI the talker is ready to send the actual bits for this byte of data The routine then sets the serial byte data transfer counter A5 to 8 The byte is sent one bit at a time from low to high The data output line is set high for a bit value of 1 and low for a bit value of 0 Each bit has a set up time of around 70 microseconds during which time the clock line is held low by the talker Once the talker is ready to send the bit it releases the clock line to high to signal to the listener that the listener should sample the data line for the value of this bit The listener typically samples the data line within 20 microseconds However if the listener is the 64 the 6567 VIC II chip s cycle stealing for video matrix access or sprite access requires that a minimum of 60 microseconds be allowed for the 64 to sample the data line Between each bit the data line should be high and the clock line held low If the serial data input line is low between bits this routine branches to the read write timeout status routine For each bit that is sent the serial data transfer counter A5 is decremented It continues sending bits until all eight bits have been sent or unless a read write timeout occurs Once the A5 counter reaches
7. and RTS Get Character from Serial Input Channel FIAD F264 F1B7 F26E Called by BCS at F173 F22A in Determine Input Device If any I O status errors occur load the accumulator with 0D ASCII carriage return and exit from the routine If no I O status errors occur JMP EE13 EF19 to receive a byte from the serial device Exit with the byte in the accumulator Operation 1 If any I O status errors are indicated in 90 LDA 0D CLC and RTS 2 If no I O status errors are indicated JMP EE13 EF19 to receive a byte from the current serial device Exit with the carry clear and with the accumulator containing the byte received Open Serial Output Channel F279 F332 F290 F349 Called by BCS at F266 F31F in CHKOUT Execution This routine opens an output channel for a serial device for subsequent CHROUTs to the serial device It sends a LISTEN command to the serial device and if a secondary address lt 128 decimal was specified the secondary address is also sent Entry requirements The accumulator should contain the device number B9 should hold the current secondary address Operation 1 TAX to preserve the accumulator value in X then JSR EDOC EE17 to send the LISTEN command to the current serial device 2 Ifthe secondary address B9 lt 80 decimal 128 branch to step 4 3 If the secondary address gt 80 JSR EDBE EECS to set the serial attention output line high Branch to step 5 4 JSR EDB9 EECO to send the
8. five that should be connected to the serial bus at any one time Serial I O can be performed either through BASIC commands or by calling Kernal routines The Kernal serial I O routines are described later in this chapter No examples of serial I O programs are included here See the chapter on Using Disk Storage in Raeto Collin West s Programming the Commodore 64 and Programming the VIC for some examples of using machine language programs for serial disk I O Serial I O is not interrupt driven like I O to tape or to an RS 232 device Tape I O is performed by the specific tape IRQ interrupt handling routines and RS 232 I O occurs from within the NMI interrupt handler Instead of using interrupts to determine when to send data out or to sample data in the Kernal serial I O routines use interrupts from timer B on CIA 1 timer 1 on VIA 2 to detect timeouts during the attention response handshake when a device is not present to know when to perform the EOI handshake during serial receive to detect read timeouts and to detect timeouts during the frame handshake at the end of each byte sent or received So serial I O operates on the theory that everything is going to proceed in a timely fashion that the serial device is going to respond within a prescribed time period and the interrupts are used to enforce these time limits During these detections of timeouts and timing of EOI IRQ interrupts are disabled on the 64 VIC Thus the timer B timer 2 inte
9. now are actual data values loaded into memory or verified against memory e If the current operation is a VERIFY compare the byte received against the byte pointed to by AE If no match set the verify mismatch bit in the status word e If the current operation is a LOAD store the byte received in memory location pointed to by AE e Increment the pointer to the load verify area AE Once the EOI status is received from the serial device the load verify is considered complete and AE points to the end of the load area 1 Any bytes sent by the device after it has indicated EOI are discarded by the 64 VIC For EOI the 64 VIC sends the UNTALK command to all serial devices which thus forces the serial device to send its last buffered character After UNTALK the LISTEN command is sent to the serial device followed by the CLOSE secondary address of EO and the UNLISTEN command to all serial devices The final location of the pointer AE indicating the end of the load area 1 is loaded into X from AE and into Y from AF and the routine exits Entry requirements B7 should hold the number of characters in filename C3 should point to the starting address for LOAD 93 the LOAD VERIFY flag should indicate the operation 0 for LOAD or 1 for VERIFY 90 I O status word should have a value of 0 Operation 1 If there are no characters in the filename JMP F710 F793 to display the MISSING FILE NAME message set the carry set t
10. of data 11 JMP F654 JSR EFO4 At F654 on the 64 do a JSR EDFE These instructions on the 64 and the VIC command all devices on the serial bus to unlisten 12 CLC and RTS Send Secondary Address After LISTEN EDB9 EEC0 EDBD EEC4 Called by JMP from Kernal SECOND vector at FF93 JSR at F286 F33F Open Serial Output Channel JSR at F3EA F4A6 in Send OPEN LOAD or SAVE Command to Device JSR at F612 F6AA in Save to Serial Device JSR at F651 F6E9 in Send Secondary Address for CLOSE This routine stores the secondary address passed in the accumulator in 95 the serial buffered character and does a JSR to ED36 EE40 to send the secondary address to the serial bus as a command Finally it falls through to EDBE EECS to bring the serial attention output line high the setting for transmitting normal data bytes Entry requirements The accumulator should hold the secondary address to be sent Operation 1 STA 95 saving the secondary address in the serial buffered character location 2 JSR ED36 EE40 to bring the serial clock output line low to indicate the talker is ready to send another byte delay one millisecond and send the character in 95 to the serial data output line The serial attention output line should be set low when calling the subroutine so that the character in 95 the secondary address will be considered to be a serial command 3 Fall through to the routine at EDBE EECS to bring the serial attention output line high
11. the routine at F633 F6CB to send the CLOSE command to the serial device load the accumulator with 0 set carry and exit 16 If the STOP key has not been pressed branch to F63A F6D2 to skip the CLOSE command then increment the pointer to the save area AC by JSR FCDB FD1B 17 If this pointer s high byte AD is 0 fall through to step 16 otherwise branch to step 9 18 JSR EDFE EF04 to send the UNLISTEN command to all serial devices 19 Fall through to the routine at F642 F6DA to send the LISTEN command to this device send the secondary address command of E1 to indicate a CLOSE for a SAVE Then command all serial devices to unlisten V 0 N mon Stop Load or Save F633 F6CB F639 F6D1 Called by JMP at F4FE F595 in Load or Verify from Serial Device falls through from F632 F6CA in Save to Serial Device This routine is executed if the keyboard STOP key is detected during a load or save for a serial device JSR F642 F6DA to do the following send the LISTEN command to this device send secondary address command for CLOSE E0 for LOAD E1 for SAVE send the UNLISTEN command to all devices clear the carry and RTS After the JSR returns load the accumulator with 0 set the carry and exit 18 Operation 1 JSR F642 F6DA to send the CLOSE command for SAVE or LOAD E1 or E0 respectively send UNLISTEN CLC and RTS 2 LDA 00 3 SEC 4 RTS Send Secondary Address for CLOSE F642 F6DA F658 F6F0 Called
12. to complete the EOI handshake Then the final data byte is transmitted in normal fashion as shown in Figure 8 3 11 Figure 8 3 Serial Byte Transmission and Handshake High Attention Low 3 High 7 Clock l Low kasd eS bey E repeat for bits 2 6 ae 2 4 6 7 1000 uused Data max Low Bit 0 Bit 1 Bit 7 _ The talker allows the clock line to go high when ready to send 2 The listener allows the data line to go high when ready to receive If the talker does not respond within 200 usec perform EOI handshake see Figure 8 2 The talker brings the clock line low to signal the start of transmission The talker sets the data line low for a bit value of 0 or high for bit value of 1 The talker then brings the clock line high to alert the listener that a valid bit is present on the data line The listener must read the data line before the talker pulls the clock line low again to set the next bit value A Ww ON The talker brings the clock line low and the data line high between bits 7 After all eight data bits have been sent the listener must bring the data line low within 1000 usec to assure the talker that the byte was successfully received The talker brings the clock line low after the listener has signaled it is ready to receive data by bringing the data line high The talker brings the clock line low within 200 microseconds if the EOI handshake is not to be done If the
13. 4 EEAA in Send Serial Byte Command or Data alternate entry at EDB2 EEB9 by JMP at EE44 EF42 Receive Byte from Serial Device Set 90 the I O status word to indicate an error condition The condition indicated depends on the entry point EDAO EEB4 device not present EDBO EEB7 read or write timeout EDB2 EEB9 read timeout Clear the carry enable IRQ interrupts bring the serial attention output line high the serial clock line output line high and the serial data output line low Operation 1 EDAD EEB4 LDA 80 and fall through to step 3 by using a dummy BIT instruction 2 EDBO EEB7 LDA 03 3 EDB2 EEB9 JSR FE1C FE6A to set the I O status word with ORA 90 STA 90 4 Enable IRQ interrupts and clear the carry 5 Branch to EEOE EFO09 to bring the serial attention output line high enter a short delay loop bring the serial clock output line high and bring the serial data output line low Delay One Millisecond EEB3 EF96 EEBA EFA2 Called by JSR at ED3D EE46 in Do Attention Handshake with Serial Device For the 64 a series of instructions is executed that takes approximately one millisecond For the VIC timer 2 of VIA 2 the timer count is initialized to 0400 From this initial value timer 2 takes approximately one millisecond to countdown to zero and generate an interrupt Operation For the 64 1 TXA 2 cycles 2 LDX B8 2 cycles 16 DEX 2 cycles X 184 times 368 cycles BNE to step 3 3 cycles X
14. DDOO CIA 2 data port A VIC LDA 912C VIA 2 peripheral handshaking control register 2 ORA 20 binary 0010 0000 to turn on bit 5 3 64 STA DDO to set bit 5 of the port serial data out to 1 VIC STA 912C to hold the CB2 handshaking line high Bring Serial Bus Clock Line High EE85 EF84 EE8D EF8C Called by JSR at ED2B EE35 in Do Attention Handshake with Serial Device JSRs at ED49 EE53 and ED7D EE8B in Send Serial Byte Command or Data JSR at EDD3 EEDA in Send Secondary Address After TALK and Do TALK LISTEN Turnaround JSR at EEOD EF13 in Send UNLISTEN Command JSR at EE18 EF1E in Receive Byte from Serial Device JSR at FE2E in System Reset VIC This routine brings the serial bus clock line high The serial clock output line from the CIA VIA chip passes through an inverter before reaching the serial port Thus setting the CIA VIA pin low 0 brings the serial clock line high at the serial port Operation 1 64 LDA DDOO CIA 2 data port A VIC LDA 912C VIA 2 peripheral handshaking control register 2 64 AND EF binary 1110 1111 to turn off bit 4 VIC AND FD binary 1111 1101 to turn off bit 1 3 64 STA DDOO to set bit 4 of the port serial clock out to 0 VIC STA 912C to hold the CA2 handshaking line low Bring Serial Bus Clock Line Low EE8E EF8D EE96 EF95 Called by JSR at ED37 EE40 in Do Attention Handshake with Serial Device JSR at ED5F EE6C in Send Serial Byte Command or Data JSR at EDFO EE
15. DDOO bit 6 0 91 IF bit0 0 Data In High False 1 DDO bit 7 1 91 IF bit 1 1 Low True 0 DDO0 bit 7 0 91 IF bit 1 0 Clock Out High False 1 DDO0 bit 4 0 Low True 0 DDOO0 bit 4 1 CA2 control 111 Data Out High False 1 DDO0 bit 5 0 CB control 110 Low True 0 DDOO0 bit 5 1 CB2 control 111 In examining the serial I O routines just looking at the code without referring to other sources is not very enlightening Jim Butterfield s article How the VIC 64 Serial Bus Works COMPUTE July 1983 pages 178 84 helps make the serial I O logic comprehensible Also Raeto Collin West s Programming the PET CBM like subsequent volumes on the VIC and 64 clarifies the active low principle in which true is low 0 while false is high 1 The serial I O routines perform various handshaking sequences between the controller the 64 VIC the talker either the 64 VIC or a serial device and the listeners either the 64 VIC or serial devices Whenever the controller needs to send a command to a serial device it brings the serial attention line low The actual transmission of a byte of data and the handshaking sequences that occur for the data byte transfer are discussed in this chapter see the Send Serial Byte Command or Data routine and Figure 8 3 The EOI handshake sequence is also covered there and illustrated in Figure 8 2 The EOI sequence when the 64 VIC is the listener is also covered in the Receive Byte from Serial
16. E8E EF8D to bring the serial clock output line low 11 JSR EE97 E4A0 to bring the 64 VIC serial data output line high allowing the serial bus data line to go high 12 JSR EEB3 EF96 to delay one millisecond 1000 microseconds On the VIC VIA 2 timer A is used to generate an interrupt after counting one millisecond On the 64 a delay loop of instructions that takes one millisecond to execute is used 13 Fall through to the Send Serial Byte Command or Data routine at ED40 EE49 Send Serial Byte Command or Data ED40 EE49 EDA C EEB3 Called by JSR at ED19 EE24 and fall through from ED3D EE46 in Do Attention Handshake with Serial Device JSR at EDE7 EEEE in Send Serial Byte Deferred This routine is called to send either a command or a data byte on the serial data output line The data output line is from bit 5 of DDOO CIA 2 data port A on the 64 and from the CB2 handshaking line of VIA 2 port B on the VIC 10 The 64 VIC first makes sure that it is not holding the data line low The routine then tests the data line and if it is not low the addressed serial device is not responding and is considered not present If the data line is low the device has responded and a byte can be sent to it If the EOI flag A3 has its high bit on indicating this will be the last data byte to be sent the EOI handshake shown in Figure 8 2 is performed before sending the buffered character Figure 8 2 EOI Handshake Sequence High Attent
17. F495 F408 F4C6 Called by JSR at F37F F43F in OPEN Execution JSR at F4C8 F56A in Load or Verify from Serial Device JSR at F605 F69D Save to Serial Device For OPEN LOAD or SAVE operations to a serial device the current device is commanded to listen then the secondary address and the filename are sent on the serial bus Finally all devices on the serial bus are commanded to unlisten The secondary address must be less than 128 decimal and the filename must contain at least one character Before sending the secondary address this routine ORs the secondary address with FO Thus only secondary addresses of 0 15 are valid for OPEN LOAD or SAVE A secondary address of 0 indicates a LOAD operation and a secondary address of 1 specifies a SAVE Secondary addresses 2 15 are available for OPEN Entry requirements B9 should hold the current secondary address B7 should hold the number of characters in filename BA should hold the current device number BB should point to the current filename These values can be established using the Kernal SETLFS and SETNAM routines Operation 1 If B9 the current secondary address gt 80 128 decimal branch to step 12 Only secondary addresses lt 80 128 are acceptable for OPEN LOAD or SAVE 2 If there are no characters in the filename branch to step 12 B7 contains the number of characters in the filename 3 64 Store 0 in 90 to clear the I O status indicator 4 LDA BA the cu
18. F6 in Send UNTALK Command JSR at FF36 in System Reset VIC This routine brings the serial bus clock line low The serial clock output line from the CIA VIA chip passes through an inverter before reaching the serial port Thus setting the CIA VIA pin high 1 brings the serial clock line low at the serial port Operation 1 64 LDA DDOO CIA 2 data port A VIC LDA 912C VIA 2 peripheral handshaking control register 2 64 ORA 10 binary 0001 0000 to turn on bit 4 VIC ORA 02 binary 0000 0010 to turn on bit 1 3 64 STA DDOO to set bit 4 of the port serial clock out to 1 VIC STA 912C to hold the CA2 handshaking line high Read Serial Data In and Serial Clock In EEA9 E4B2 EEB2 E4BB Called by JSRs at ED44 EE4D ED50 EESA ED55 EE60 ED5A EE66 and EDA6 EEAC in Send Serial Byte Command or Data JSR at EDD6 EEDD in Send Secondary Address After TALK and Do TALK LISTEN Turnaround JSRs at EE B EF21 and EE37 EF35 in Receive Byte from Serial Device On the 64 this routine first forces CIA 2 data port A DDO0 to stabilize then loads the accumulator from DDOO and does an ASL This ASL leaves the bit from the serial data input line in the carry flag of the status register and the bit from the serial clock input line in the high bit of the accumulator On the VIC this routine first forces VIA 1 data port A 91 IF to stabilize then loads the accumulator from 91 IF and does a LSR This LSR leaves the bit from the serial c
19. I O routines but doesn t get into specific details about disk drives and printers The Commodore 1541 disk drive is extensively covered in Inside Commodore DOS by Immers and Neufeld and in The Anatomy of the 1541 Disk Drive from Abacus Software which contains an interesting program that allows spooling of files directly from the disk to the printer while your 64 VIC is busy doing other things The program essentially commands the printer to listen and the disk to talk The hardware aspects of the 1541 are covered in the 1542 Single Drive Floppy Disk Maintenance Manual by Michael G Peltier Also see the chapters about disk I O in Raeto Collin West s Programming the VIC and Programming the Commodore 64 both from COMPUTE Books The Commodore VIC 1541 User s Manual and VIC 1525 User s Manual are also occasionally useful Also the Commodore 64 Programmer s Reference Guide contains diagrams and timings of serial I O functions on pages 364 65 The following table illustrates how the various values for the serial attention clock and data lines may be set or read These values indicate the actual setting of the line to high or low at the serial data port Reading and Setting the Serial Clock Data and Attention Lines Logic Value at Where Set or Line Serial Port Read 64 VIC Attention Out High False 1 DDO0 bit 3 0 91 IF bit 7 0 Low True 0 DDO bit 3 1 91 IF bit 7 1 Clock In High False 1 DDO bit 6 1 91 IF bit0 1 Low True 0
20. JSRs at ED24 EE2E and ED3A EE43 in Do Attention Handshake with Serial Device JSRs at ED41 EE4A and ED7A EE88 in Send Serial Byte Command or Data JSR at EE2A EF26 in Receive Byte from Serial Device JMP at EE10 EF16 in Send UNLISTEN Command This routine brings the serial bus data line high The serial data output line from the CIA VIA chip passes through an inverter before reaching the serial port Thus setting the CIA VIA pin low 0 brings the serial data line high at the serial port Operation 1 64 LDA DDOO CIA 2 data port A VIC LDA 912C VIA 2 peripheral handshaking control register 2 AND DF binary 1101 1111 to turn off bit 5 3 64 STA DDO0 to set bit 5 of the port serial data out to 0 VIC STA 912C to hold the CB2 handshaking line low Data port bit 5 of CIA 1 of the 64 or the CB2 line of VIA 2 of the VIC then passes through an inverter to reach the serial port data output line Bring Serial Bus Data Line Low EEA0 E4A9 EEA8 E4B1 Called by JSR at ED75 EE83 in Send Serial Byte Command or Data JSR at EDCD EED4 in Send Secondary Address After TALK and Do TALK LISTEN Turnaround JSRs at EE47 EF45 and EE76 EF75 in Receive Byte from Serial Device This routine brings the serial bus data line low The serial data output line from the CIA VIA chip passes through an inverter before reaching the serial port Thus setting the CIA VIA pin high 1 brings the serial data line low at the serial port Operation 1 64 LDA
21. Serial I O Routines Serial I O refers to the I O input output operations that occur over the serial bus The serial port connects the 64 and VIC to the serial bus Several I O devices such as disk drives and printers can be connected to the serial bus Each I O device is an intelligent device that contains its own microprocessor and ROM control program that allows the device to know how to respond to the commands it receives over the serial bus from the 64 VIC During serial I O data is transmitted one bit at a time over the serial bus with the series of bits normally representing a byte of data or a command RS 232 communications also occur in a bit by bit serial manner but it uses separate lines for transmitting and receiving while serial I O only uses one Two other lines on the serial bus the serial clock line and the serial attention line are also used during serial I O Within the 64 and VIC the data and clock lines are both divided into separate lines for incoming and outgoing signals serial data input serial data output serial clock input and serial clock output Only a serial attention output line is connected from 64 VIC hardware to the serial bus attention line The unused serial attention input line is connected to pin 9 of the user port The serial bus has one additional signal line serial service request input However the Kernal serial I O routines do not make use of this line Another difference between RS 232 and serial I O is
22. TEN SECOND CIOUT sequences it should be possible to use secondary addresses from 0 31 and device numbers 0 30 The following tables compare some of the serial input and output routine sequences available on the 64 VIC Serial Kernal Jump Table Input Routine Similarities TALK TKSA ACPTR Sequence TALK JSR ED09 EE14 to send 4x TALK to the serial device TKSA JSR EDC7 EECE to send a secondary address command and do TALK LISTEN turnaround ACPTR JMP EE13 EF19 to get the byte from the serial data input line OPEN CHKIN CHRIN Sequence OPEN JSR EDOC EE17 to command the current serial device to LISTEN JSR EDB9 EECO to send Fx command the secondary address for OPEN and send the filename JMP EDFE EF04 to send an UNLISTEN command to the serial device CHKIN The X register contains the logical file number at entry JSR ED09 EE14 to send the TALK command to the serial device JSR EDC7 EECE to the send secondary address and do the TALK LISTEN turnaround Store BA in 99 the current input device CHRIN If location 99 contains a value greater than 3 this is a serial device If the device is not present it returns 0D JMP EE13 EF19 to get byte from the serial data input line Serial Kernal Jump Table Output Routine Similarities LISTEN SECOND CIOUT Sequence LISTEN JMP EDOC EE17 to send 2x LISTEN to the serial device SECOND JMP EDB9 EECO to send a secondary address command to the serial device CIOUT JMP EDDD EEF4 to send the buffer
23. Then use a dummy BIT instruction to skip to step 20 For a LOAD operation store the byte received from the serial device at the current location pointed to by AE Increment AE the pointer to the load verify area See if the I O status word indicates EOI If not branch to step 13 If EOI has been detected JSR EDEF EEF6 to send the UNTALK command to all serial devices JSR F642 F6DA to send the LISTEN command to this serial device send the CLOSE secondary address command of E0 and send the UNLISTEN command to all serial devices Clear the carry LDX from AE LDY from AF and RTS Send TALK Command to Device ED09 EE14 ED10 EE1B Called by JMP from Kernal TALK vector at FFB4 JSR at F238 F2F1 in Open Serial Input Channel JSR at F4CD F56F in Load or Verify from Serial Device The accumulator which contains the device number is ORed with 40 turning on bit 6 to indicate a TALK command RS 232 interrupts are disabled The routine then falls through to ED11 EEIC the Do Attention Handshake with Serial Device routine to send the serial TALK command 21 Operation 1 ORA 40 Use a BIT instruction to fall through to step 2 bypassing the Send LISTEN Command to Device routine s entry point at EDOC EE17 2 JSR FOA4 F160 to disable RS 232 interrupts 3 Fall through to the routine to send a serial command at ED11 EEIC Send Secondary Address After TALK and Do TALK LISTEN Turnaround EDC7 EECE EDDC EEE3 Called by JMP from Ker
24. ads and saves 2 14 are available and 15 is the error channel As the above table indicates you cannot send UNTALK or UNLISTEN commands just one serial device these commands are sent to all serial devices The secondary address can range from 0 31 decimal Notice also that there are no specific OPEN or CLOSE commands Instead OPEN and CLOSE are combinations of commands OPEN is a LISTEN possible devices 4 30 followed by the Fx OPEN secondary address followed by the filename followed by an UNLISTEN to all devices CLOSE is a LISTEN possible devices 4 30 followed by the Ex CLOSE secondary address Notice that OPEN for any device in its final step automatically causes all serial devices to unlisten It seems that only devices 0 30 are valid for LISTEN and TALK and 4 30 for OPEN because device 31 is preempted for indicating the UNLISTEN and UNTALK commands and devices 0 3 are screened by the 64 VIC Kernal routines for OPEN to indicate the keyboard tape screen and RS 232 devices The TALK and LISTEN routines do not screen for device numbers 0 3 Notice that you can use a secondary address of 0 31 for TKSA and SECOND while you are limited to 0 15 for OPEN and CLOSE The Kernal serial routines also apparently prevent secondary addresses gt 128 decimal 80 from being sent for serial OPEN LOAD or SAVE Where to Get More Information Serial I O is a topic that could easily fill an entire book This chapter discusses the Kernal serial
25. and STA B9 to set the secondary address to 61 indicating a SAVE 2 If the filename does not contain any characters if B7 contains a 0 JMP F710 F793 to display the MISSING FILE NAME error message set the accumulator to 8 set the carry bit to 1 and exit 3 JSR F3D5 F495 to send the LISTEN command send the secondary address of 61 send all the characters in the filename and then send the UNLISTEN command 4 JSR F68F F728 to display SAVING and the filename 5 JSR EDOC EE17 to send the LISTEN command to the current device BA Then JSR EDB9 EECO to send the secondary address of 61 to the device 6 JSR FB8E FBD72 to set the pointer to the start of the save area AC from Cl Cl was set from two bytes in page zero during the Jump to SAVE Vector routine see chapter 5 Send the low byte of the starting address of the save area AC to the serial bus by JSR EDDD EEE4 Send the high byte of the starting address of the save area AD to the serial bus by JSR EDDD EEE4 JSR FCD1 FD11 to compare AC to AE to see if AC the pointer to the save area is greater than or equal to the pointer to AE the end of the save area 1 12 If AC gt AB branch to step 16 13 LDA AC Y to get the next byte from the save area The Y register is zero 14 Send this byte onto the serial bus with JSR EDDD EEEF4 15 JSR FFE1 the Kernal STOP vector to see if the STOP key on the keyboard has been pressed If it has fall through to
26. at the alternate point ED36 EE40 from the routines to send a secondary address after TALK or LISTEN only steps 3 4 and 5 in Figure 8 1 are performed since the serial attention line should still be low Once step 5 in Figure 8 1 is finished this routine falls through to the Send Serial Byte routine at ED40 EE49 where the Send Serial Byte routine expects to find that a serial device has responded to the attention request by bringing the serial data line low If it doesn t find this condition it assumes the device is not present Figure 8 1 Attention Request from 64 VIC High Attention Low Clock Low l l l 5 l 1 1 4 High l Data i Low i ___ 1000 usec el Allow serial data line to go high Bring serial attention line low Bring serial clock line low Allow serial data line to go high Delay one millisecond Listening devices must bring the data line low within this period to be recognized as present on the bus NBWN re One interesting question about the routine on the VIC is why the delay of one millisecond is done with IRQ interrupts enabled the SEI is not until EE49 Although it probably is unlikely an interrupt would occur between the one millisecond delay and the following instruction it is possible and could force a false device not present condition Indeed on the 64 this oversight is corrected by inserting the SEI before the one millisecond delay and also before bringing the clo
27. built 22 Again read and stabilize DD00 911F 23 64 ASL to move the serial clock input line value to the high bit of the accumulator VIC LSR to shift the serial clock input line value into the carry 24 Loop to step 22 until the serial clock input line goes low When it does the data is no longer considered valid as the talker is doing the setup for the next bit 25 Decrement A5 the serial byte transfer counter 26 If A5 is not 0 after the decrement branch to step 17 to read the next bit of data 27 If A5 is now 0 all eight bits of serial input have been received JSR EEAO E4A9 to bring the serial data output line low to indicate to the talker that the 64 VIC has accepted this byte 28 64 Check 90 the I O status word to see if the EOI status bit is set VIC Check 90 to see if any I O status bits are set If the status is not flagged branch to step 30 29 If EOI 64 or any I O status word condition VIC is indicated JSR EE06 EFO06 to first execute a delay loop of approximately 50 60 microseconds JSR EE85 EF84 to bring the serial clock output line high and then JMP EE97 E4A0 to bring the serial data output line high 30 LDA A4 to exit with the accumulator containing the serial byte that was built from the eight bits of serial data received 31 Enable IRQ interrupts CLC and RTS Open Serial Input Channel F237 F2F0 F24F F308 Called by BCS at F221 F2DA in CHKIN Execution The current device with a device number g
28. by Falls through from F641 F6D9 in Save to Serial Device JSR at F2EE F3AE in Close Logical File for Serial Device JSR at F52B F5C2 in Load or Verify from Serial Device JSR at F633 F6CB in Stop Load or Save alternate entry at F6F4 by JMP at F406 in Send OPEN LOAD or SAVE Command to Device 64 If the current secondary address lt 80 128 decimal send the LISTEN command to the serial device send the secondary address for a CLOSE command of Ex x varies depending on what called this routine send the UNLISTEN command to all serial devices clear carry and RTS If the current secondary address gt 80 128 decimal just CLC and RTS Operation 1 Ifthe current secondary address gt 80 branch to step 6 2 JSR EDOC EE17 to send the LISTEN command to the current serial device 3 LDA B9 the current secondary address Then AND EF 1110 1111 binary to clear bit 4 and ORA E0 to set bits 5 7 to 1 Thus the command for CLOSE is produced Ex where x is the secondary address option The x will be 0 for a CLOSE following LOAD 1 for a CLOSE following SAVE and 2 15 for a CLOSE following OPEN 4 JSR EDB9 EECO to send the secondary address command to the current device 5 JSR EDFE EF04 to command all serial devices to unlisten 6 CLC and RTS Load or Verify from Serial Device F4B8 F55C F532 F5C9 Called by Falls through from F4B7 F55B in Determine Device for LOAD When loading from a serial device you must specify a f
29. ck line low and allowing the data out to go high If this routine is entered from falling through from the serial send TALK command or the serial send LISTEN command the TALK or LISTEN command is placed in the serial buffered character location to be sent on the serial data output line Bringing the serial attention line low causes all devices on the serial bus to listen for a command Valid device addresses for the command are 0 30 The listening devices on the serial bus can check the first five bits of the command to see if they are the device to which this command is being sent If the device number is 31 all serial devices are being addressed for an UNLISTEN or UNTALK command The serial device that recognizes its address then reads the command from the three high bits of the command For the alternate entry points for sending a secondary address after LISTEN or TALK the serial device should still be listening under attention and should read a secondary address of 0 31 from the lower five bits and read the secondary address command identifier in the three high bits Operation 1 If the serial output deferred flag 94 has its high bit off branch to step 5 2 If the serial output deferred flag 94 has its high bit on the Send Serial Byte Deferred routine at EDDD EEEF4 has been executed to set this flag The flag indicates a character is waiting to be sent If the serial device is currently listening the flag will be set Set the high bi
30. data line to go high The data line will remain low however because the 64 VIC is holding its data output line low 5 The serial device is now the talker and the 64 VIC is now the listener Transmission of bytes will proceed as shown in Figure 8 3 22 does when it is the talker and is ready to send data The talker the serial device then sends the data to the listener the 64 VIC and any other listening serial devices as shown in Figure 8 3 The routine enables IRQ interrupts and then exits If the secondary address gt 80 128 decimal when opening a serial channel the secondary address is not sent Entry requirements The accumulator should contain the secondary address Operation 1 STA the secondary address in 95 the serial buffered character JSR ED36 EE40 to send the secondary address as a command on the serial data output line EDCC EED3 Disable IRQ interrupts JSR EEA0 E4A9 to hold the serial data output line low JSR EDBE EECS to bring the serial attention output line high JSR EE85 EF84 to bring the serial clock output line high JSR EEA9 E4B2 to read the serial clock input line Loop until the serial clock input line is brought low by the serial device 8 Enable IRQ interrupts and RTS Sy he oh Receive Byte from Serial Device EE13 EF19 EE84 EF83 Called by JMP from Kernal ACPTR vector at FFA5 JSRs at F4D5 F577 F4E0 F582 and F501 F598 in Load or Verify from Serial Device JSR at F1B5 F26C in Ge
31. data line to high The VIC performs the data low data high operations in steps 12 and 13 while the 64 performs the data low data high operations in steps 13 and 6 Set the EOI status bit in 90 the I O status word with LDA 40 and JSR FEIC FE6A Increment A5 which will now be used to indicate that if another timer B timer 2 interrupt occurs it is a timeout and not an EOI Loop to step 5 step 6 When the serial has indicated that it is ready to send the byte set A5 the serial byte data transfer counter to 8 Read and stabilize the data port for the serial data input line DDOO CIA 2 port A 91FF VIA 1 port A 64 ASL to move the value of the serial clock input line to the high bit of the accumulator 24 and the value of the serial data input line into the carry VIC LSR to shift the value of the serial clock input line into the carry and the value of the serial data input line into the low bit of the accumulator 19 Loop to step 17 until the serial clock input line goes high It is brought high by the serial device the talker in this case when the device has completed the setup of the data line and valid data exists The valid data that exists when the clock line is brought high has already been read by the 64 VIC in step 17 20 VIC LSR to shift the serial data input line value into the carry for the following ROR 21 ROR A4 to move the serial bit just received now in the carry bit into the high bit of A4 the serial byte being
32. dress specified when the load was called is 0 This secondary address was saved in the X register at entry to this routine If it is 0 the starting address for the load is taken from the setting of the X and Y registers at the time load was called If the secondary address is 0 the X and Y values found now in C3 and C4 are stored in AE and AF Thus a secondary address of 0 for a load allows a relocatable load After this JSR F5D2 to display the LOADING or VERIFYING message VIC A patch is used for this test of the secondary address JSR E4C1 to test the secondary address in the same manner as the 64 did and then JMP F66A to display the LOADING or VERIFYING message Clear the I O status word read error bit JSR FFE1 the Kernal STOP vector to test for the STOP key If the STOP key is detected JSR F633 F6CB to send a LISTEN command to this serial device send the CLOSE secondary address command of E0 send the UNLISTEN command to all serial devices LDA 00 SEC and exit JSR EE13 EF19 to receive a byte from the serial device If the status word 90 indicates read timeout branch to step 13 Thus a read timeout for the individual bytes of data does not abort the entire load or verify If this is a LOAD operation branch to step 19 If this is a VERIFY operation see if the last byte received from the serial device is the same as the byte pointed to by AE If not set the verify mismatch bit in the I O status word with LDA 10 JSR FE1C FE6A
33. ed character on the serial data output line OPEN CHKOUT CHROUT Sequence OPEN JSR EDOC EE17 to command the current serial device to LISTEN JSR EDB9 EECO to send Fx command the secondary address for OPEN and send the filename JMP EDFE EF04 to send an UNLISTEN command to the serial device CHKOUT The X register contains the logical file number at entry JSR EDOC EE17 to send the LISTEN command to the serial device JSR EDB9 EECO to send the secondary address Store BA in 9A the current output device CHRIN If 9A contains a value greater than 3 this is a serial device JMP EDDD EEEF4 to send the buffered character on the serial data output line While the serial attention line is held low bytes sent are considered to be commands The 64 and VIC use the command values shown in the table below Serial Commands 2x OOld dddd LISTEN 3F 0011 1111 UNLISTEN all devices 4x 010d dddd TALK 5F 0101 1111 UNTALK all devices 6x Olls ssss Secondary address Ex 1110 aaaa Secondary address for CLOSE Fx 1111 aaaa Secondary address for OPEN F1 1111 0001 SAVE memory to serial device FO 1111 0000 LOAD memory from serial device aaaa Secondary address 0 15 for OPEN CLOSE s ssss Secondary address 0 31 d dddd Device address 0 30 for LISTEN and TALK Device 31 is pre empted by use for UNLISTEN and UNTALK As the 1541 disk drive manual states channel numbers secondary addresses 0 and 1 are reserved for operating system lo
34. eration 1 The accumulator which contains the current device number in the low five bits is ORed with 20 binary 0010 0000 to set bits 5 7 to 001 to indicate a LISTEN command 2 JSR FOA4 F160 to disable RS 232 interrupts 3 Fall through to ED11 EEIC which is the routine to send a serial command Do Attention Handshake with Serial Device ED11 EE1C ED3F EE48 Called by Fall through from ED10 EE1B after Send TALK Command to Device or Send LISTEN Command to Device JSR at EE00 EF06 in Send UNTALK Command or Send UNLISTEN Command alternate entry at ED36 EE40 by JSR at EDBB EEC2 in Send Secondary Address After LISTEN JSR at EDC9 EEDO in Send Secondary Address After TALK and Do TALK LISTEN Turnaround If the serial deferred flag indicates that a character is buffered to be sent on the serial bus the end or identify EOI handshake and the buffered character are sent on the bus before the command control character is sent Also the serial deferred flag and the EOI flag are then turned off The command in the accumulator at entry which had been temporarily saved on the stack is stored in 95 the serial buffered character This buffered character is used later in the Send Serial Byte routine when it is sent as a command with the serial attention line held low Next the attention request sequence in Figure 8 1 is performed by the controller the 64 VIC to inform the serial devices that a command is coming If the routine is entered
35. he accumulator to 8 and exit 2 64 LDX with the current secondary address B9 VIC JSR to a patch area at E4BC to LDX B9 3 JSR FSAF JMP F647 from the patch area at E4BE on the VIC to display SEARCHING FOR and the filename 4 Set the current secondary address B9 to 60 5 JSR F3D5 F495 to send the LISTEN command to the current device the secondary address of 60 the characters of the filename and to send the UNLISTEN command to all serial devices 6 JSR EDO9 EE14 to command the current device BA to talk 7 JSR EDC7 EECE to send the current secondary address in B9 of 60 and to do the TALK LISTEN turnaround Now the serial device is the talker and the 64 VIC and possibly other serial devices is the listener 8 JSR EE13 EF19 to receive a byte from the serial bus with the byte received returned in the accumulator 9 STA AE since the first byte received should be the low byte of the pointer to the end of the load area 1 10 If the I O status word 90 indicates a read timeout branch and JMP F704 F787 to display 20 11 12 13 14 15 16 17 18 19 20 21 22 23 24 the FILE NOT FOUND error message set accumulator to 4 set carry and exit JSR EE13 EF19 to receive a byte from the serial bus with the byte received returned in the accumulator STA AF since the second byte received should be the high byte of the pointer to the end of the load area 1 64 See if the secondary ad
36. ilename The routine displays SEARCHING FOR and the filename It sends the LISTEN command to the current serial device a secondary address of 60 indicating a LOAD and the filename It then sends the UNLISTEN command to all serial devices Next it sends the TALK command to the current serial device and the current secondary address of 60 and then receives a byte from the serial bus Sending the TALK secondary address command of 60 performs the TALK LISTEN turnaround sequence where the serial device becomes the talker and the 64 VIC becomes the listener If the I O status word indicates the byte was not returned fast enough a read timeout has occurred and FILE NOT FOUND is displayed The first two bytes received from the serial device are used as the pointer to the starting address of the load verify area in AE However if you set the secondary address to 0 and call 19 the load routine the X and Y registers at entry to load are used to set the pointer to the start of the load verify area in AE After initializing AE the routine displays the message for LOADING or VERIFYING Next the following loop is executed until the status word 90 indicates EOI end of file e Turn off the status word read error bit e See if the STOP key is down If so send the LISTEN command to the serial device the CLOSE command secondary address of E0 and the UNLISTEN command then exit e Receive a byte from the serial device bytes being received
37. in the number of bits that make up a discrete unit of transmission With RS 232 a unit can be less than eight bits while in serial I O a unit is always eight bits Each byte of serial data is transmitted and received from the low bit to high bit direction For example ASCII A with a bit value of 0100 0001 is transmitted in this sequence 10000010 The tables below show which CIA data port bits are used in serial I O for the 64 and which VIA data port bits are used in serial I O for the VIC Commodore 64 Serial Port CIA Map DDO0 CIA 2 Data Port A Bit 3 Serial attention output Bit 4 Serial clock output Bit 5 Serial data output Bit 6 Serial clock input Bit 7 Serial data input DCOD CIA 1 Interrupt Control Register Bit 4 FLAG IRQ serial service request input not used by Kernal serial I O routines VIC 20 Serial Port VIA Map 911F VIA 1 Data Port A without handshaking Bit 0 Serial clock input Bit 1 Serial data input Bit 7 Serial attention output 9120 VIA 2 Data Port B CB1 Serial service request input not used by Kernal serial I O routines CB2 Serial data output 9121 VIA 2 Data Port A with handshaking CA2 Serial clock output The following table shows the serial I O line functions as viewed from the serial port Serial Port I O Lines Pin Function Serial service request input Ground Serial attention input output Serial clock input output Serial data input output Reset NNBWNR The serial I O lines are connec
38. ion Low l 4 l Clock l Low l I i j 60 usec max High 2 4 l Data 200 usec min i I Low The talker allows the clock line to go high when ready to send The listener allows the data line to go high when ready to receive If the talker has not brought the clock line low within 200 usec the listener assumes this is an EOI handshake To confirm this the listener brings the data line low again for 60 usec then brings it high again WN eR 4 To confirm the EOI handshake the talker must not bring the clock line low within 60 usec Rather than the talker bringing the clock line low as it does when it is ready to transmit normal bytes of data the talker just loops waiting for the data line to go high The data line goes high when the listener is ready to receive data The listener can hold off indefinitely allowing the data line to go high until it is finally ready to receive data The listener must monitor the clock line and if the talker does not bring the clock line low within 200 microseconds it is notifying the listener that the EOI sequence is to be performed and that the next byte is the last byte of the file The listener confirms the EOI sequence by bringing the data line low for at least 80 microseconds for an external listener or 60 microseconds if the 64 VIC is the listener then allowing the data line to go high again This routine for the 64 VIC watches for this sequence of data line low data line high
39. lock input line in the carry flag of the status register and the bit from the serial data input line in the low bit of the accumulator The serial data and clock input lines from the serial port to the CIA VIA chips do not pass through any inverters Thus reading a value of 1 indicates the corresponding serial line is high and reading a value of 0 indicates the corresponding serial line is low Operation 1 Wait for DD00 911F to stabilize by loading its value into the accumulator and comparing this value to the current value in DD00 911F until the two are the same 2 64 ASL to shift the bit for the serial data input line into the carry bit and the bit for the serial clock input line into the high bit of the accumulator 3 VIC LSR to shift the bit for the serial data input line into the low bit of the accumulator and the bit for the serial clock input line into the carry bit Send LISTEN Command to Device EDOC EE17 ED10 EE1B Called by JMP from Kernal LISTEN vector at FFB1 JSR at F648 F6E0 in Send Secondary Address for CLOSE JSR at F27A F333 in Open Serial Output Channel JSR at F3E3 F49F in Send OPEN LOAD or SAVE Command to Device JSR at F60D F6A5 in Save to Serial Device This routine prepares the accumulator which contains the device number to send a LISTEN command to the device The device number should be 0 30 RS 232 interrupts are disabled then this routine falls through to the Send Serial Control Character routine Op
40. lock output line high If A3 the EOI flag has its high bit off branch to step 8 If the high bit is on perform the EOI handshake see Figure 8 2 as this byte to be sent will be the last data byte for a file Loop until the serial data input line is brought high by the listener Loop until the serial data input line is brought low by the listener Loop until the serial data input line is brought high by the listener JSR EE8E EF8D to set the serial clock output line low 10 Set the serial byte data transfer counter A5 to 08 11 Force DDOO CIA 2 data port A 911F VIA 1 data port A to stabilize 12 If the serial data input line is low 0 between data bits branch to EDBO EEB7 to set read write timeout status 13 ROR 95 the serial buffered character rotating the low bit into the carry 14 If the carry is clear after the ROR JSR EEA0 E4A9 to send a 0 by holding the serial data output line low 15 If the carry is set after the ROR JSR EE97 E4A0 to send a 1 by holding the serial data output line high 16 JSR EE85 EF84 to set the serial clock output line high Then delay for four NOP instructions about eight microseconds When you combine the time required for these NOP instructions with the time for the following LDA DD00 912C AND DF ORA 10 02 STA DD00 912C you see that the serial clock output line is held high for about 20 cycles These 20 cycles 20 microseconds give the listener time to sample the data line 17
41. nal TKSA vector at FF96JSR at F245 F2FE in Open Serial Input Channel JSR at F4D2 F574 Load or Verify from Serial Device alternate entry at EDCC EED3 by JSR at F23F F2F8 in Open Serial Input Channel For the normal entry point store the secondary address in the serial buffered character location at 95 and send the buffered character as a command with the serial attention line held low on the serial bus From this point on the normal and alternate entry points perform the same functions Disable IRQ interrupts Then perform the TALK LISTEN turnaround sequence shown in Figure 8 4 Once this turnaround is complete the device is now the talker and the 64 VIC is the listener The talker is holding the clock low and allowing the data to go high The listener 64 VIC is holding the data line low After these operations the talker brings the clock line high to indicate it is ready to send data just as the 64 VIC Figure 8 4 TALK LISTEN Turnaround Sequence 2 High Attention Low 3 4 5 High Clock Low High Data Low l 1 After the desired device has been commanded to talk the 64 VIC brings the data output line low This causes no immediate change because the device will already be holding the data line low 2 The 64 VIC brings the attention line high 3 The 64 VIC brings the clock line high and waits for the clock line to go low again 4 To acknowledge the turnaround the serial device must bring the clock line low and release the
42. nd device number tables Operation 1 JSR F642 F6DA If the secondary address lt 80 128 decimal send the LISTEN command to the current device convert the secondary address to Ex to indicate the CLOSE command and send this secondary address Then send the UNLISTEN command to all serial devices 2 Fall through to the common close logical file routine at F2F1 F3B1 Set Serial Timeout Value FE21 FE6F FE24 FE72 Called by JMP from Kernal SETTMO vector at FFA2 The IEEE timeout flag 0285 is set to the value passed in the accumulator However although the purpose of this Kernal routine has been described as setting a flag for timeout conditions on the IEEE bus nowhere in BASIC or the Kernal is this flag read Operation 1 STA 0285 the timeout flag 2 RTS 28
43. or the timer in the interrupt flag register DCOD 912D If an interrupt has occurred branch to step 10 If no timer B timer 2 interrupt occurs JSR EEA9 E4B2 to read the serial clock input line If the serial clock input line is still high branch to step 7 If the serial clock input line is low continue with step 9 When the serial clock input line goes low before a timer B timer 2 interrupt the serial device is indicating that the EOI handshake should not be performed for the byte to follow so branch to step 16 When a timer B timer 2 interrupt occurs before the serial clock input line is brought low the serial device is assumed to be calling for an EOI handshake If A5 the transfer counter is 0 branch to step 12 If A5 is nonzero this is the second time an interrupt has occurred while the clock line is high Thus the serial device has not responded to the EOI handshake from the 64 VIC Set the read timeout status with LDA 02 JMP EDB2 EEB9 which also exits from this routine JSR EEAO0 E4A9 to bring the serial data output line low for the EOI handshake 64 JSR EE85 to allow the serial clock output line to remain high VIC JSR EFOC At EFOC first enter a delay loop for about 52 microseconds then JSR EF 84 to allow the serial clock output line to remain high and JMP E4A0 to bring the serial data output line high Both the 64 and VIC acknowledge the EOI by bringing the data output line low for at least 60 microseconds and then releasing the
44. rrent device number 5 JSR EDOC EE17 to send a LISTEN command to the current device The serial attention output line is brought low to send the LISTEN command and it will remain low upon return 6 LDA B9 the secondary address then ORA FO to prepare to send the secondary address for OPEN SAVE or LOAD 7 JSR EDB9 EECO to send the secondary address to the device The attention line is still low from step 5 when the subroutine is called However the serial attention output line will be set high upon return Thus the following filename is sent as regular bytes of data not as a command This is contrary to the information about how filenames are sent as commands in 14 the article How the VIC 64 Serial Bus Works by Jim Butterfield COMPUTE 1983 8 Test bit 7 of 90 the I O status word If bit 7 is set to 1 the preceding subroutine detected that the specified device is not present In this case pull the current return address from the stack so that the JMP to F707 F78A to display the DEVICE NOT PRESENT error message will RTS to the routine that called for the OPEN SAVE or LOAD 9 If the device is present see if there are any characters in the filename If not branch to step 11 10 If the filename contains characters go through a loop that gets the next character in the filename and outputs this character on the serial bus until all characters in the filename have been transmitted JSR EDDD EEF4 to send each character as a byte
45. rrupt that occurs just sets the interrupt flag register at DCOD 912D to indicate a timer B timer 2 timeout has occurred The serial routines that check for timeouts check the interrupt flag in DCOD 912D rather than letting the actual IRQ interrupt from the CIA VIA occur Indeed the IRQ Interrupt Handler routine has no check for these timer B timer 2 interrupts Thus the use of interrupts is quite different with serial I O than it is with tape or RS 232 I O Kernal Jump Table routines that specifically send or receive data to or from the serial bus ACPTR CHRIN CHROUT CIOUT on a byte by byte basis or LOAD SAVE routines that handle multiple byte transfers are used to initiate and control serial I O Although both the 64 and VIC Programmer s Reference Guides state that bringing the serial service request SRQ line low results in the 64 VIC servicing the device that brought SRQ low no Kernal routines check for this condition Of course you could write your own routine to enable SRQ interrupts and then handle any that occurred Another interesting item in serial I O is that CHRIN from a serial device seems to function almost like ACPTR CHROUT and CIOUT are also very similar One difference between the routines is that a filename is only sent during the OPEN sequence Another difference between these routines is that by using the OPEN sequence you are limited to secondary addresses 0 15 and device numbers 4 30 By using the TALK TKSA ACPTR or LIS
46. secondary address in B9 to the current serial device Return with the attention line set high 5 TXA to retrieve the device number saved in step 1 then test the I O status register 90 If the high bit of 90 is 1 JMP F707 F78A to display the DEVICE NOT PRESENT error message 26 set the accumulator to 5 set the carry and exit 6 If the device is present BPL to F275 F32E to set 9A the current output device number from the device number value in the accumulator 7 CLC and RTS Send UNTALK Command EDEF EEF6 EE12 EF18 Called by JMP from Kernal UNTALK vector at FFAB JSR at F340 F400 in Clear Serial Channels and Reset Default Devices JSR at F528 F5BF in Load or Verify from Serial Device This routine performs the sequence necessary to send the UNTALK command to all serial devices Operation 1 JSR EE8E EF8D to set the serial clock output line low 2 Bring the serial attention output line low by storing a 1 in bit 3 of DDOO bit 7 of 91 IF 3 LDA 5F the command for all devices to untalk Use a dummy BIT instruction to fall through to EE00 EF06 step 2 of the following routine Send UNLISTEN Command Send UNLISTEN Command EDFE EF04 EE12 EF18 Called by JMP from Kernal UNLSN vector at FFAE JSR at F339 F3F9 in Clear Serial Channels and Reset Default Devices JSR at F4C2 in Send OPEN LOAD or SAVE Command to Device VIC JSRs at F63F F6D7 and F654 F6EC in Save to Serial Device alternate entry at EE03 EF09 by BCC at EDB7
47. t 4 is commanded to talk If the current secondary address lt 80 128 decimal the secondary address is sent on the serial bus However a secondary address that gt 80 128 decimal is not sent If the serial device is not present the DEVICE NOT PRESENT message is displayed If the serial device is present the current device number BA is stored as the current input device number 99 Entry requirements The accumulator should hold the device number B9 should hold the current secondary address Operation 1 TAX to preserve the accumulator value in X then JSR ED09 EE14 to send a TALK command to the serial device whose device number was passed in the accumulator 2 If the secondary address gt 80 no secondary address is sent Instead just JSR EDCC EED3 to do the TALK LISTEN turnaround then JMP to step 4 25 3 If the current secondary address lt 80 JSR EDC7 EECE to send the secondary address and do the TALK LISTEN turnaround Return with the attention line set high 4 TXA to retrieve the device number value saved in step 1 then check bit 7 of 90 the I O status word which indicates the device not present condition If the specified serial device is not present JMP F707 F78A to exit with the DEVICE NOT PRESENT error message set the carry and set accumulator to 5 5 If the serial device is present BPL to F233 F2EC to set the input device number 99 from the current device number in the accumulator 6 CLC
48. t Character from Serial Input Channel In this routine the serial device is the talker while the 64 VIC is the listener IRQ interrupts are disabled and the serial byte data transfer counter is initialized to 0 The serial clock output line is brought high Next loop until the serial clock input line goes high and then set the serial data output line low Set a timer interrupt for 250 microseconds If the serial clock input line does not go low within the 250 microseconds perform the EOI handshake which consists of bringing the serial data output line low the serial clock output line high then the serial data output line high Set the EOI status bit in the I O status word 90 Check again to see if the serial clock input line goes low within 250 microseconds If it doesn t set the read timeout status If it does the listener is now ready to receive data If the serial clock input line goes low before the first 250 microsecond delay has completed the EOI sequence is not executed the 64 VIC is ready to receive data For receiving the byte loop until the serial clock input line goes low and receive eight bits from the serial data input line As the data comes in a series of shifts builds the serial byte received into location A4 Between bits the serial clock output line goes low After all eight bits have been received bring the serial data output line low If the EOI status is true send the UNLISTEN command to all serial devices Exi
49. t of the end or identify EOD flag A3 to 1 3 JSR ED40 EE49 to send to the serial bus the EOI handshake and the buffered character 4 Clear the high bits of 94 and A3 to indicate no character is awaiting transmission and the EOI handshake is not to be done 5 Store the value from the accumulator on entry to this routine in 95 the serial buffered character 6 64 Disable IRQ interrupts 7 JSR EE97 E4A0 to bring the 64 VIC serial data output line high This will allow the serial bus data line to go high 8 The BNE at ED29 EE33 is an unconditional branch because the previous JSR EE97 E4A0 ANDs the accumulator with DF binary 1101 1111 Thus a value of 3F 0011 1111 could never remain in the accumulator upon return from the JSR and hence the CMP 3F is never equal to the accumulator Both the 64 and the VIC have this apparent bug It is unclear what condition Commodore was trying to check for with this comparison for 3F but if a match would have been found a JSR EE85 EF84 to allow the serial clock line to go high would have been executed 9 Set the 64 VIC serial attention output line low to bring the serial bus attention line low On the 64 store a 1 in bit 3 of DDOO the serial attention output line On the VIC store 1 in bit 7 of 91 IF the serial attention output line Remember that the lines from the CIA VIA chip go through an inverter before reaching the serial port 10 ED36 EE40 Disable IRQ interrupts on the 64 only JSR E
50. t with the accumulator containing the byte received the carry clear and IRQ interrupts enabled Although IRQ interrupts are disabled during this routine the timer interrupt can occur and still be detected by looking at the interrupt flag register in the CIA VIA 23 Operation 1 2 3 4 10 11 12 13 14 15 16 17 18 Disable IRQ interrupts Set the serial byte data transfer counter A5 to 0 JSR EE85 EF84 to set the serial clock output line high This allows the serial bus clock line to go high JSR EEA9 E4B2 to read the serial clock input line Loop until the serial clock input line is brought high by the serial device indicating it is ready to send 64 Set DCO7 the high byte of CIA 1 timer B to 01 preparing it for a timer count of 0100 This will generate an interrupt in about 250 microseconds Store 19 in DCOF to load the timer B counter from the latched value and to start timing VIC JSR E4A0 to bring the serial data output line high indicating to the serial device that the VIC is ready to receive data 64 JSR EE97 to bring the serial data output line high indicating to the serial device that the 64 is ready to receive data Clear any pending interrupts in CIA 1 with LDA DCOD VIC Set 9129 the high byte of VIA 2 timer 2 to 01 to specify a timer count of 0100 This will generate an interrupt in about 250 microseconds See if a timer B timer 2 interrupt occurs by examining the flag bit f
51. ted to all serial devices and each serial device should have a unique address The serial I O lines are active low meaning that the lines remain at 5 volts while inactive and bus activity is indicated by pulling a line to zero voltage Any serial device including the 64 VIC can bring the clock or data lines low The serial attention line is used to tell serial devices that a command is coming No incoming signals on the attention line of the serial port are recognized by the CIA VIA chip there s no serial attention input line connected to the CIA VIA so any attempt by another serial bus device to bring the serial attention line low will not be acknowledged by the 64 VIC The 64 VIC is the only device that controls the serial attention line Thus the 64 VIC is the only controller of the serial bus The TALK LISTEN UNTALK and UNLISTEN commands are sent from the 64 VIC to one particular serial device at a time When the serial attention line is brought low by the 64 VIC the serial devices should then prepare for a command to arrive over the serial bus The first five bits the low five bits of the command contain the address of the serial device to which this command is directed and the last three bits specify the actual command to the serial device With five bits 32 possible serial devices can be addressed However see the caution by Michael G Peltier in the 2542 Single Drive Floppy Disk Maintenance Manual about the maximum number of devices
52. with the DEVICE NOT PRESENT error message Set the pointer to the current byte to save AC from the starting address to be saved This starting address is then sent over the serial bus first AC and then AD The address in AC is incremented after each byte is sent to the serial device When the address in these bytes equals the address in AE the pointer to the end of the memory being saved 1 the save is complete When the save is complete command all serial devices to unlisten and fall through to the routine at F642 F6DA to send the secondary address for a CLOSE command to the serial device The actual operation for saving each byte is to LDA with the next byte from the save area using AC as a pointer and then send this byte over the serial bus The routine also checks to see if the keyboard STOP key has been pressed and if it has the save is halted and the routine falls through to the routine to send the CLOSE command to the device If the STOP key has not been pressed AC the pointer to the save area is incremented If the high byte of the pointer is 00 save is halted and the routine falls through to the routine to send the CLOSE command Thus you can t wrap your save from FFFF to 0000 17 Entry requirements B7 should hold the number of characters in filename BA should hold the current device number Cl should point to start of the save area AE should point to the end of the save area 1 Operation 1 LDA 61
53. zero indicating all eight bits have been sent the computer sets CIA 1 timer B VIA 2 timer 2 to 0400 Then it loops waiting for the serial data input line 12 to go low The serial data input line is brought low by the listener to indicate it has accepted the byte If the serial data input line does not go low within this countdown of 0400 1000 microseconds the timer B timer 2 interrupt that occurs indicates a read write timeout called a frame error for this byte If the serial data input line goes low within 1000 microseconds the listener has accepted the data In this acceptance case the computer will enable IRQ interrupts and RTS During this routine IRQ interrupts are disabled When IRQ interrupts are disabled any IRQ interrupt that occurs will not be serviced by the IRQ interrupt handler However by checking the flag of the interrupt status register you can still detect if a timer B timer 2 interrupt occurred By this method even though IRQ interrupts are disabled interrupts can still occur and be serviced Operation 1 Disable IRQ interrupts 2 JSR EE97 E4A0 to bring the serial data output line high This will allow the serial bus data line to go high 3 JSR EEA9 E4B2 to sample the status of the serial data input line If the serial data input line has not been brought low by the listener BCS to EDAD EEB4 to set the status word 90 to indicate a device not present error condition 4 JSR EE85 EF84 to bring the serial c
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