Zeus Assembler
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1. It is sometimes convenient to be able to generate code at a different location from the one at which it is ultimately intended to run for example if the latter is occupied at present The monitor may be used to relocate the code to the ORG address For example given the following two statements at the start of a source file ORG 30000 DISP 10000 the source following would be assembled from 40000 but would only normally run at 30000 Set an entry point The X command executes the assembled code from the last ENT directive in the source file 15 EQU Short for EQUATE or EQUALS A label symbol may have a value assigned to it using a statement of the form label EQU value DEFB nn nn Inserts bytes nn at the current assembly address DEFW nnnn nnnn Inserts words addresses nnnn at the current assembly address DEFM string The text enclosed in the delimiters will be inserted at the current assembly address As with all assembly language instructions directives may be prefixed by a label eg DATA1 DEFW 40000 3456 6789 OFFSETS DEFB 3 2 14 20 9 2 USRADDR ENT 5 6 Screen editor ZEUS provides a full screen editor to facilitate modification of the source file First list the text by using the L command The cursor may now be moved to any point in the listing by using the cursor control keys CAPSHIFTED 5 6 7 8 To replace the character at the current cursor position simply enter the new characte2. Q short for QUIT and use SAVE source 1 CODE 32768 144 to save the source to tape Check your recording with VERIFY CODE To return to ZUES enter PRINT USR 57344 Before you can list the source you must enter O to tell ZEUS to retrieve the old source file The O command can also take a 11 parameter that being the start address of the file it is to retrieve You May create a source file at a different address by either Using the N short for NEW command before you enter the program The parameter specifies the address from which the source will be built or Reloading an old source file to a different location from that from which it was saved see Section 6 T Command When you began to enter lines of text ZEUS automatically started the source from the default of the N command e address 32768 or 8000 hex If you have a ZX Printer attached then you may make a hard copy of the source file To turn the printer on enter P1 Now enter L 16 When the printer is on all output sent to the VDU is also sent to the printer To turn the printer off enter PO You may interrupt the printer by using the BASIC BREAK key Return to ZEUS in the normal way You are now ready to test the routine Enter X short for EXECUTE If all is well the screen apart from the border will fill with the current ink colour ie white If this does not happen list the source and check it carefully against the lis
3. 3 Type in the following lines of text pressing ENTER after each to insert the line in the source file and to produce the next line number LD HL 16384 LD DE 16385 LD A 255 LD HL A LD BC 6143 If you enter a character incorrectly use ERASE CAPSHIFTED 0 and re type 4 Exit from auto line number mode CAPSHIFTED 1 followed by ENTER You may now use the full screen editor which as the name suggests allows the text to be edited by altering it as it is displayed on the screen First obtain a listing of the source file Clear the screen and enter L short for LIST The LIST command takes up to three parameters of the form Lxyz where x line number to list from y line number to list up to z number of lines to list before pausing To alter a particular character in the text use the cursor control keys CAPSHIFTED 5 6 7 amp 8 to move the cursor to the desired point in the listing and simply type in the replacement character The cursor will move automatically to the next character position Remember that the modified line will be inserted into the text on pressing ENTER only when the cursor is sitting on that line For example to change line 30 to read 00030 LD A 254 first obtain a listing of the text and using the cursor control keys place the cursor in the same character square as the right hand 5 digit Type 4 and press ENTER CLEARSCREEN and LIST You may now change the number back to 255 either by
4. from start to finish L 1050 List the text from line 10 to line 50 L 50 List the text from the start to line 50 String parameters are represented by enclosing the string within delimiters Delimiters can be any character except or or any digit eg F LDIR finds all occurrences of the string LDIR F LDIR ditto Commands Ax Assemble the source file and display error messages After x errors have been encountered and displayed assembly will stop To continue assembly press ENTER DEFAULT x 14 17 Dxy F string x y z Ixy Lxyz 18 Delete all lines between line x and line y inclusively If no parameters are given no action will be taken This command searches the source file for all occurrences of the given string The search commences at line x and finishes at line y Any lines containing the string are displayed and a pause initiated after z lines have been displayed DEFAULT x lowest line number y highest line number z 14 string After entry of this command ZEUS will automatically generate line numbers starting with line number x Any text entered will be inserted in the correct place in the source file and another line number generated Parameter y is the spacing between successive line numbers Deleting the line number and pressing ENTER will halt auto line numbering DEFAULT x 10 y 10 List the source file from line x to line y Parameter z gives the number of lines to be listed befo
5. table which contains the alphanumeric symbols and labels defined in the source file along with their associated values It starts at 57343 and increases in size downwards at a rate of sixteen bytes per label used Note if the symbol table produced on assembly becomes close enough to the source to be in danger of overwriting it the message Out of symbol space is issued Similarly if when entering text the source file becomes too close to ZEUS the message Out of memory is produced If either of these occurs the source file must be moved down in memory This can be done using the Monitor COPY command However great care should be exercised to ensure that if necessary RAMTOP is redefined using the BASIC command CLEAR before copying down 27 Appendix five Useful Zeus routines You may wish to call some of the following ZEUS subroutines from your own programs NAME ADDRESS FUNCTION INPUTCHAR F652 Wait for a key to be pressed result inA PRINTCHAR F503 Print char in A May also be a control character see below If printer flag is non zero will also output to printer PRINTDECIMAL E5A3 Write HL in decimal followed by a space PRINTHEXBYTE F2DF Write A in hex followed by a space PRINTHEXWORD E571 Write HL in hex followed by a space PRINTSTRING HE4E3 Write the string following the call string must be terminated by byte zero INPUTLINE F6E2 Enter screen editor put line which cursor is on in buf
6. the same method or by entering 8 30 LD A 255 Before proceeding CLEARSCREEN and LIST The next part of the program involves a loop It is often desirable to indent loops in order to make a program more readable To speed the process of entering sections of indented text ZEUS provides a user definable TAB facility The TAB stops may be set up from within the Monitor Type M and press ENTER The word Monitor appears to remind you of the mode you are in Next type S and move the cursor along until it is lined up with the column of Ls in the listing at the top of the screen Now type a non space character eg 1 followed by say 5 spaces a further 1 and ENTER CAPSHIFTED 2 will now move the cursor to the stops you have just defined Return to ZEUS enter Z Now you are ready to add the loop First enter 60 to invoke auto line numbering from line 60 Type in the word loop which you may wish to put in lower case characters by using the CAPSHIFT key Use CAPSHIFTED 2 to put the cursor at the next TAB stop Now type LD A HL and press ENTER Enter the following lines remembering to indent by using CAPSHIFTED 2 LD DE A INC HL INC DE DEC BC LD A B ORC JR NZ loop Note that each reference to a particular label must be identical character for character to that label Hence if you have used lower case in line 60 then you must do the same in line 130 Enter the last line without the indent ie 00140
7. 00 to 6502 Finally it prints 6503 followed by contents of that address The above sequences can be combined with the enter modify mode command M eg M 6000 FF 42 enters modify mode puts FF into 6000 and 42 into 6001 and leaves modify mode Output the value y to I O port X Set TAB stops Any non space character on the same line as the S will define a TAB stop provided the line is entered CAPSHIFTED 2 advances the cursor between the stops eg S 1 1 sets TAB stops at the columns occupied by the 1 character Tabulate memory from address x The contents of memory are displayed eight bytes at a time in hexadecimal form Each line is prefixed by the address of the first byte on that line After y lines have been displayed tabulation will halt To continue press ENTER any other key will return you to normal command mode Note that is is possible to modify memory by the following method tabulate the required area of memory as described above and after returning to command mode enter modify mode M Using the cursor control keys move the cursor up into the tabulated listing After altering all the required values on a given line press ENTER to modify the actual memory locations Exit from modify mode as before by typing a full stop and pressing ENTER Z Return control to the assembler Note that all constants entered in the Monitor must be hexadecimal with the exception of the A command which
8. 140 ORC 00150 JR NZ loop 00160 RET 10 Now you are ready to assemble the source file to produce the object code commonly called the machine code Type A short for assemble and press ENTER If all is well the cursor will simply move below the A If you have made a mistake an error report will be issued followed by the offending line If the error is not apparent consult the appendix on error report codes At this stage the easiest mistake to make is to omit a space from within an instruction This will produce error 0 incomplete statement You may find it worthwhile to purposely introduce an error by for example removing the space in line 30 CLEARSCREEN LIST place the cursor in the space between D and H DELETE CAPSHIFTED 3 ENTER CLEARSCREEN ASSEMBLE You should see Error 0 0030 LDHL 16384 To correct the error move the cursor to the H and use INSERT CAPSHIFTED 4 to add the space ASSEMBLE CLEARSCREEN and LIST It is advisable to save the source file on tape prior to execution Loading and saving is accomplished by using the BASIC commands and treating the source file as a block of data bytes The BASIC command to save a block of bytes is of the form SAVE filename CODE x y where x address of the first data byte y length of data The values of the parameters x and y are given by the T command Enter T and you should see start of source 32768 length 00144 Return to BASIC by entering
9. RET Exit from auto line number mode CAPSHIFTED 1 followed by ENTER Two additional lines are necessary before you can assemble and test the routine First you must tell ZEUS where to place the assembled code by using the directive ORG Enter 5 ORG 30000 to assemble the machine code from memory location 30000 Second you must set an entry point ie the point from which the execute command X will run the code Enter 6 ENT followed by CLEARSCREEN and LIST You will notice that it is necessary to press ENTER twice to list the complete source file This is because the default value for the z parameter number of lines to list before pausing is 14 and the source listing is now 16 lines long ZEUS also features a renumber command of the form Rxyz See ZEUS Assembler commands Section 6 for definition of the parameters To renumber the source file enter R Now list the first 5 lines by typing L535 and pressing ENTER once only Note that the use of a comma to replace a parameter causes the command to use the default value of that parameter You can see that the source has been renumbered in accordance with the default values of the R command Check carefully that the source file is identical to the following 00010 ORG 30000 00020 ENT 00030 LD HL 16384 00040 LD DE 16385 00050 LD A 255 00060 LD HL A 00070 LD BC 6143 00080 loop LD A HL 00090 LD DE A 00100 INC HL 00110 INC DE 00120 DEC BC 00130 LD A B 00
10. SOFTWARE BY USER MANUAL FOR ZEUS ASSEMBLER First published in 1983 by Sinclair Research Limited 25 Willis Road Cambridge CB1 2AQ England ISBN 0 85016 004 X Copyright Crystal Computing Ltd 1983 All rights reserved No part of this program packaging or documentation may be reproduced in any form Unauthorised copying hiring lending or sale and repurchase prohibited Printed in the UK Contents INTFOUCHON A A AENEA TE KENAL iLO Ae Aana EEEE cess cctoets Loading and running ZEUS ooccccccciccccocccnonanoncconanonononononannannnnnnncnnnnns Writing machine code with ZEUS Entering and editing text DT General form AE e e rae re Sissguslyiaeeeds 5 2 Constants 5 3 Operators 5 4 EXPressSiONS E E E E E oie 5 5 Assembler directives ARUN 516 Seren ll umm ag 6 ZEUS assembler COMMANAS c coccccocccocccocccnoncconcnonn conc o ncnnnncnnnnnnn nn AAA A E TOPE Pera Appendix one Command list ooooonnccconocccinoccccononcncnononononononcnnncnnnnnns 24 Appendix two Error report codes c ccoocococccnoccnocnconcccnnccconananos 25 Appendix three Reserved Words c ccciooccccncocccononcnnanananinncncnn 26 Appendix four ZEUS Memory MAP cocccccoccnocccncncnonanancnanannno 26 Appendix five Useful ZEUS routines ind ss Appendix six Further facilities oooonnnnnncinnnnnnnnconcccnncnncccnnccnnnnnnnnnnns 29 1 Introduction Welcome to ZEUS the assembler based machine code ope
11. TED 3 true video Delete the character under the cursor CAPSHIFTED 4 inv Video Insert a space character at the cursor position CAPSHIFTED 5 6 7 amp 8 Cursor directional control keys CAPSHIFTED 9 graphics Clear the screen and home the cursor Appendix two Error report codes lllegal character or incomplete statement Label too long symbol expected Truncation error or jump out of range expected Context error Redefined label symbol expected lllegal mnemonic Undefined label 00 O 0 25 Appendix three Reserved words The following is a list of reserved words e those which must not be used as labels although any label may contain them A ADC ADD AF AF AND B BC BIT C CALL CCF CP CPD CPDR CPI CPIR CPL D DAA DE DEC DEFB DEFM DEFS DEFW DI DISP DJNZ E El ENT EQU EX EXX H HALT HL IM IN INC IND INDR INI INIR IX IY JP JR L LD LDD LDDR LDI LDIR M NC NEG NOP NV NZ OR ORG OTDR OTIR OUT OUTD OUTI P PE PO POP PUSH RES RET RETI RETN RL RLA RLC RLCA RLD RR RRA RRC RRCA RRD RST SBC SCF SET SLA SP SRA SRL SUB V XOR Z Appendix four ZEUS memory map The following areas of memory are used by ZEUS A ZEUS machine code and workspace 57344 65279 E000 FF00 B Source file Initially starts at 32768 8000 but may be altered with the N command C Assembled Defined by the user through the code Object directives ORG amp DISP code 26 D Symbol table This is the
12. UIT will then return control to the Monitor and Disassembler 29
13. alues either into symbols or into memory 3 Loading and running Zeus If you wish to use your ZX Printer with ZEUS remember to connect it before plugging in your Spectrum 1 Rewind the cassette to the beginning of side A 2 Set the volume on your recorder to about Y maximum and the tone to full treble 3 Connect the EAR socket on your recorder to the EAR socket at the rear of the computer 4 Type LOAD zeus or simply LOAD Press ENTER and start the tape 5 Stop the tape when you see the copyright message and flashing cursor 4 Writing machine code with ZEUS This section is a description of the entry of a short machine code subroutine which along with the program itself has been written to illustrate some of the features of ZEUS The program loads byte 255 into each address memory location on the screen The screen is 6144 bytes in length and starts at address 16384 The outline of the program along with its BASIC equivalent is as follows LD HD 16384 5000 LET HL 16384 LD DE 16385 5010 LET DE 16385 LD A 255 5020 LET A 255 LD HL A 5030 POKE HL A LD BC 6143 5040 LET BC 6143 loop LD A HL 5050 LET A PEEK HL LD DE A 5060 POKE DE A INC HL 5070 LET HL HL 1 INC DE 5080 LET DE DE 1 DEC BC 5090 LET BC BC 1 LD A B 5100 IF BC lt gt 0 THEN GOTO 5050 ORC 5110 RETURN JR NZ loop RET First clear the screen by pressing CAPSHIFTED 9 equivalent to entering graphics mode in Sinclair BASIC Entering li
14. ent address use LOAD filename start address 19 Having loaded an old source file re enter ZEUS PRINT USR 57344 and enter the command O start address to retrieve the source file at start address Execute the machine code produced by the last assembly Execution begins at the ENT statement nearest to the end of the source file If no ENT statement existed an error message is issued and no execution occurs 7 The monitor The co resident Monitor allows you to directly inspect and manipulate memory and the I O ports and also set up several options within ZEUS MONITOR COMMANDS Ax Cxyz 20 This command displays both the decimal and the hexadecimal representation of the value x which may be of either form eg A59 prints HEX 3B DECIMAL 59 A 2A prints HEX 2A DECIMAL 42 Copy a block of memory This command takes three parameters as follows x Start address of block to be copied from y start address of block to be copied to z the number of bytes to copy Execute machine code from address x Print the value of I O port x This command allows the border paper and ink colours to be altered x is the byte which is used to fill the attributes table it is made up of four fields Bits 0 2 control the ink colour Bits 3 5 control the paper colour Bit 6 is set for bright Bit 7 is set for flash y is the required border colour eg KF 1 white ink on blue background with blue border K 38 7 black i
15. f logical operators addition subtraction amp logical AND logical OR No operator priority is observed expressions are evaluated strictly from left to right 5 4 Expressions Wherever a constant is required in an instruction an expression may be used in its place Expressions are built from labels and or constants separated by operators eg LD A ADDRESS offset LD HL START 3 IN A PORT 8 FF LABEL FF00 Note that expressions will be computed by the assembler to determine the actual memory location which will be inserted into the object code Expressions are computed at assembly time not at 14 program execution time 5 5 Assembler directives The following pseudo operations either provide parameters for the operation of ZEUS or instruct the assembler to store values either as symbols or directly into memory ORG nnnn DISP nnn ENT Short for ORIGIN This directive instructs ZEUS to assemble the block of machine code as translated from the source listing after the ORG statement from address nnnn provided that the current DISP value is zero see below Multiple ORGs within the same source file are allowed Each ORG statement will redirect the address from which subsequent code is assembled Short for DISPLACEMENT A DISP instruction alters the place from which subsequent code is generated even though the code so produced is assembled to run at the address specified by the current ORG parameter
16. fer BUFFER FE00 32 character buffer terminated by a byte zero PRINTERFLAG F4CB Non zero to enable printer PRINTDECNZ E59E Print decimal number in HL without leading zeros 28 Notes All prints send characters to the printer as well as VDU if PRINTERFLAG is non zero PRINTCHAR also allows the use of screen editing commands For example printing byte HOF will clear the screen The list of editor control characters is as follows 04 Delete character under cursor 05 Insert space at cursor position 06 Move cursor position to next tab position 07 Clear line containing cursor 08 Move cursor left 09 Move cursor right 0A Move cursor down 0B Move cursor up 0C Move cursor left and blank character under cursor 0D Move cursor to beginning of next line Scroll if on bottom line 0F Clear screen and cursor home Appendix six Further machine code facilities from Crystal Computing ZX Spectrum Machine Code Monitor and Disassembler Package This program provides additional facilities to aid the inspection analysis and debugging of your routines with this and ZEUS co resident in memory your Spectrum becomes a truly comprehensive Z80 programming unit NB When you intend to use both programs at once load ZEUS in FIRST then QUIT and load in the 16K Monitor Disassembler You may enter both utilities separately but it is convenient to enter ZEUS by using the Monitor command Goto E000 The use of the ZEUS command Q
17. like the assembler defaults to decimal prefix hex values with a symbol 23 Appendix one Command List 1 Assembler Ax Dxy F string x y z Ixy Lxyz M Nx Ox 2 Monitor Ax Cxyz Ex Ix Kxy Mx Oxy S Txy Z 24 ASSEMBLE Print x errors at a time DELETE from line x to line y inclusive FIND string between line x and line y print z occurrences at a time INSERT auto line number starting at line x with increments of y LIST from line x to line y in steps of z lines Enter MONITOR Create a NEW source file at address x Recover OLD source file at address x PRINTER control x 0 for off x 1 for on QUIT Return to BASIC RENUMBER the source file from line z to commence with line x and increments of y Print the SYMBOL TABLE x lines at a time Display the start address and length of the source file EXECUTE object code at last ENT statement Print x in decimal and hex COPY a block memory from address x to address y for a length z bytes EXECUTE a machine code routine from address x Print the value of I O port x Set up the paper ink and border colours Enter MODIFY mode at address x OUTPUT byte y to I O port x SET up the TAB stops TABULATE memory from address x y lines at a time Return to the assembler 3 Screen editor CAPSHIFTED 1 edit Clear the line containing the cursor CAPSHIFTED 2 caps lock Move the cursor to the next TAB stop CAPSHIF
18. nes of text is very similar to writing a BASIC program Each line must be prefixed by a number which may take any value from 0 to 65534 inclusive It is important to remember that providing the line the cursor is on begins with a valid line number pressing ENTER will insert that line at the correct position in the source file so replacing any previously existing line which had the same number The command to tell ZEUS to provide line numbers automatically is of the form Ixy where x line number from which to start y increment between successive line numbers x and y are called parameters because they specify the particular values which the command will use when it executes For example in along BASIC program you may use LIST 1000 Here the parameter 1000 causes the command to start listing from line 1000 To illustrate the use of parameters type 1100 100 and press ENTER several times To exit from auto line number mode simply delete the current line number eg with CLEARLINE CAPSHIFTED 1 and press ENTER Clear the screen CAPSHIFTED 9 type l and press ENTER several times As you have not specified particular values for the parameters x and y ZEUS has used the default values which are x 10 and y 10 To take an example from BASIC the default of the LIST command parameter is the first line number To enter the first few lines of text 1 Clear the screen CAPSHIFTED 9 2 Type l followed by ENTER once only
19. nk on white background with white border Enter modify mode starting at address x The modify command provides an extremely flexible and hence powerful memory manipulation tool eg M 6000 will print 6000 XX note the position of the cursor XX is the present contents of address 6000 You may now proceed in four ways 1 Alter the contents of the address Type the new value over the old one and press ENTER Modify prints the next address 6001 along with its present contents 2 Alter the contents of the address and subsequent addresses simultaneously Type the new value for the present address followed by the values to be written in the subsequent addresses On pressing ENTER modify will print the address and value of the byte after the last one modified 3 Alter the modification address Type followed by the new address On pressing ENTER modify will move to the new address 4 Exit from modify mode Type a full stop and press ENTER 21 Notes Oxy Txy 22 Characters can be entered by the use of quotes eg after entering M 6000 overwrite the present value with C The ASCII value of the character C is now at address 6000 More complex formats can be entered eg 6000 43 41 40 A C D enters the six values into 6000 to 6005 and leaves modify mode 6000 43 41 40 6500 83 81 80 enters bytes 43 41 40 into 6000 to 6002 It then moves the modify address to 6500 and enters 83 81 and 80 into addresses 65
20. r The cursor will move automatically to the next character position Space may be inserted into a line at the current cursor position by using the inverse video key CAPSHIFTED 4 The character at the edge of the screen is lost The character at the cursor position may be deleted by using the true video key CAPSHIFTED 3 Having altered the line pressing ENTER will insert the new line in the correct place in the text Other commands are CLEARSCREEN The entire screen may be cleared and the cursor 16 placed at the top left hand corner by using the graphics key CAPSHIFTED 9 CLEARLINE The current line may be cleared and the cursor placed at the start of the line using the edit key CAPSHIFTED 1 N B This command does not remove the current line from the text but simply from the screen TAB The Caps lock key CAPSHIFTED 2 moves the cursor to the next tab stop on the current line The tab stops may be set up from within the Monitor See Section 7 6 ZEUS Assembler Commands A command consists of a command letter followed by any number of numerical or string parameters Numerical parameters consist of either a decimal or a hexadecimal constant Entry of numerical parameters overwrites the default parameters for that command If it is desired to alter a later default parameter without upsetting earlier ones this can be achieved by entering a comma for every parameter to be skipped eg L List the text
21. rating system for the ZX Spectrum ZEUS provides the optimum method for writing Z80 machine code on your computer and yet as you will shortly discover it is even easier to use than BASIC The heart of ZEUS is a high speed two pass assembler designed to allow the convenient symbolic representation of your machine code program lines of assembly language mnemonics machine code instructions are entered as simply as you would enter a BASIC program But here the similarity with other assemblers ends for the true power of ZEUS stems from the unique versatility of its operating system Just as the Sinclair ROM enables you to write BASIC so ZEUS provides all the facilities you need to produce machine code programs quickly and efficiently 2 Definitions The following terms are used throughout this manual we give their definitions here for those unfamiliar with assembler terminology SOURCE FILE The text including all assembly language mnemonics labels and comments OBJECT CODE The machine code produced by ZEUS from the source file SYMBOL A string of alphanumeric characters used to represent a numerical value either data or an address LABEL A special type of symbol the value of which corresponds to the address of the instruction which immediately follows it DIRECTIVE An assembler directive is not an actual Z80 instruction but represents an order given by the programmer to the assembler which results in the storing of v
22. re pausing DEFAULT x start y finish z 14 Enter the Monitor See Section 7 Create a new empty source file at address x NB care must be taken when using this command as placing a source file at certain locations may cause a crash DEFAULT x 32768 8000 Rxyz This command causes the old source file at address x to become the current source it is usually used to retrieve a source file which has been loaded to address x DEFAULT x 32768 8000 Printer operation x 1 turns the printer on All output to VDU is also sent to the ZX Printer Pressing BREAK during printer operation causes a return to BASIC Re enter ZEUS as normal the printer is automatically switched off on re entry x 0 turns the printer off DEFAULT x 0 QUIT Exit to BASIC To return to ZEUS enter PRINT USR 57344 Renumber the source file The parameters are as follows x first new line number y spacing between successive new line numbers z line to start re numbering from DEFAULT x 10 y 10 z lowest current line number Print the symbol table x gives the number of symbols labels and their associated hexadecimal values listed before pausing DEFAULT x 15 Display the start address and length of the current text file in bytes The file may be saved by returning to BASIC Q and using SAVE filename CODE start length To reload an old source file to the same address as it was saved from use LOAD filename CODE a differ
23. ry reference to a particular label must be identical character for character to that label 5 1 2 Instruction An instruction may be any of the standard Zilog Z80 instructions or it may be an assembler directive see below An extra facility is provided by ZEUS for referring to the parity overflow flag When using ZEUS you may refer to jump overflow instead of jump parity even Thus JP PE may be written JP V and JP PO may be written JP NV The two versions are completely interchangeable but their use can help to make a routine more readily comprehensible 5 1 3 Optional comment A comment may be appended to the end of any instruction lt must be separated from the instruction by a semicolon Lines containing comments only are also allowed 13 5 2 Constants Constants may be expressed either in decimal or hexadecimal form as follows Decimal 1 99 234 4096 Hexadecimal HA FE 6843 5C00 Literals can be expressed as follows A 7 9 e _ For example to load the literal C into the accumulator use LD A C Note that in 280 convention parentheses denote an address eg LD A 200 will load the accumulator with the contents of memory location 200 decimal Finally there is also a system constant which is set to the current assembly address eg DJNZ is equivalent to loop DJNZ loop 5 3 Operators In order to further facilitate the writing of symbolic programs ZEUS allows the use o
24. ting above If the computer crashes simply reload ZEUS QUIT and load the source file using LOAD CODE Remember to use the O command to retrieve the old source file In this section you will have gained an appreciation of the way ZEUS can be used to produce a simple subroutine There are many additional features at your disposal to aid the programming of more complex routines The next section provides a comprehensive description of these and also consolidates the features already mentioned 5 Entering and editing text 5 1 General format ZEUS uses the ASCII character set To enter assembly language instructions you must first type in a line number and follow it with the required line of text The line number must lie in the range 0 65534 inclusive 12 The text must consist of one or more statements separated by colons Each statement comprises 1 An optional label 2 An instruction 3 An optional comment 5 1 1 Optional label The following rules govern the use of labels A label may contain upper and lower case letters and digits A label must start with a letter A label can be up to 14 characters in length A label must not be identical to a reserved word eg using LD as a label is not permitted However a label may contain reserved words eq HELD would be valid For a list of reserved words see Appendix three A label must be separated from an instruction by a space Note that eve
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