Comp 255Q - 1M: Computer Organization Name: Lab #3
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1. Test this Insert instruction 7500 to skip on minus accumulator If it is not negative then N must have been zero or negative to begin with which contradicts the precondition that N is positive so Jump to the Halt instruction at the end of the program leaving P equal to zero otherwise skip over the jump and continue with the multiplication Save a copy of your machine code file gt w h comp255 1lab03e o0b then use NotePad to print out a copy of your machine code Verification 5 Hand In This lab assignment sheet Print outs of programs lab03a obj labO3b obj lab03c obj lab03d obj and lab03e obj2. complete code first Code should start at address 0200 data at address 0300 Allocate four words of memory for Counter N M and P using memory locations 0300 0302 address contents comment description 0300 0000 Counter 0301 0003 N 3 0302 0005 M 5 0303 0000 P 2 Go back to the PDP 8 simulator Reset memory and go to page 1 SAT gt p 0200 3 Entering Your Code 4 Trace single step the execution of the code remember your value are all in octal What is 3 5 base 8 5 Save a copy of your machine code file gt w 1ab03b 0b7 then use NotePad to print out a copy of your machine code Verification 2 Part 3 Subtraction Write a program to subtract b from a putting the result in c i e c a b Since there is no PDP 8 subtract instruction we have to first negate b then add a to it Again using the common convention we will place this code on page beginning at address 0200 for code Data will be placed beginning at 0300 with a being stored at 0300 b at 0301 and c at 0302 Do the following 1 Using a PDP 8 Machine Code Form write out the machine code Note that to compute a b you must first load b i e add it into a zeroed out accumulator negate it then add a thus obtaining b a_ Otherwise your code is similar to Program 1 above clear accumulator and link load b negate b adda store result to c halt jump to 0200 As per our convention locate a b and c at addresses 0300 0302 For dat
3. Comp 255Q 1M Computer Organization Name Lab 3 Machine Language Programs for the PDP 8 January 26 2010 Introduction In this lab you will write test and execute a number of simple PDP 8 machine code programs Because of its complexity and with the advent of assemblers programs which translated assembly language program in to machine code that simplified programming computers machine code programming is not done anymore although there might be some pedagogical merit in having done it at least once Instructions The programs to be written will be very simple and will only require a handful of machine code instructions The nine that we will use four MRI and five op code 7 instructions are given below Four MRI instructions where xxx is indirect bit memory page bit 7 bit offset lxxx Twos Complement Add TAD 12xx or 13xx 2xxx Increment and Skip on Zero ISZ 22xx or 23xx 3xxx Deposit and Clear Accumulator DCA 32xx or 33xx 5xxx unconditional Jump JMP 5200 or 53xx Five op code 7 Instructions 7300 Clear Accumulator and Link CLA CLL 7041 Complement Accumulator and Increment CMA IAC or CIA same as Negate Accumulator 7500 Skip next instruction on Minus Accumulator SMA 7510 Skip next instruction on Positive or zero Accumulator SPA 7402 Halt Operand Addresses Although the PDP 8 has four addressing modes we ll restrict ourselves to using current page direct addressing for our MRI instructions In curr
4. a feel free to choose convenient values for a and b but make sure a is greater than b Set c to 0 2 Use the r Reset command to clear out memory and all registers then use the p PC command to move the memory cursor to address 0200 and display page 1 of memory Enter your code starting at address 0200 and the data at starting at address 0300 3 Trace single step the execution of this code Position the memory cursor on the location of the first instruction address 0200 and use the Space Bar to single step through the code Hit the Space Bar 6 times noting how the PC AC and IR registers change 4 Run Again Using different values for a and b execute your program again Enter the following values into a b and c 0300 0017 0301 0023 0302 0000 Position the memory cursor at location 0200 i e set the PC equal to 0200 and hit the G Go key What happened Is this answer correct Think what would a negative number look like Explain what happened 5 Save a copy of your machine code Use the w Write command see below to save your code to a file Call your file Lab03c obj The code is saved in address content format i e nnnn mmmm When saved open your file using NotePad attach name and date and print out a copy of the file h Comp255 lab03c obj Verification 3 Part 4 Modify program 3 so that you always subtract the smaller value from the larger This will require using a conditional branch After subtractin
5. ent page addressing the effective address is obtained by concatenating the page number of the address of the MRI instruction bits 0 4 with the contents of the offset field of the instruction This means the operand referenced by the MRI instruction is always on the same page as the instruction For example 0205 3320 address contents octal 000 010 000 101 011 011 010 000 address contents binary 00001 0 000 101 011 0 1 1 010 000 page offset MRI format 00001 1010000 000 011 010 000 page MRI offset concatenated 0320 Effective Address for instruction Determining the offset value for an instruction from a known effective address is easier assuming current page addressing simply convert the octal value into binary 12 bits and use the last 7 bits 5 11 for the offset value Placing Code in Memory PDP 8 programs have at least two segments a data segment a contiguous block of words where data is stored and a code segment It s important to separate the two as you don t want to execute your data and you don t want to operate on your code The standard PDP 8 convention seems to be that code starts at location 02000 beginning at the upper half page 1 and data goes somewhere below We ll position data starting at location 03000 beginning at the lower half of page 1 Since our initial programs are small code and data will not overlap Part 1 In this part we will write test and execute a PDP 8 machine code program tha
6. ero Accumulator 0204 2300 Increment Counter and Skip on Zero 0205 5204 Jump to Address 0204 Increment 0206 7402 Halt 0207 5200 Jump to 0200 0300 0000 Counter 0301 0010 N 10 octal The program will loop N times but to do so the value in N is first negated and stored in a second location called Counter The Increment and Skip on Zero increments the negative value in Count skipping over the Jump instruction when Counter equals 0 Study this code until you are sure you understand it This loop does nothing the body of the loop is empty Modify this loop that it does the multiplication P M N by added M to P N times assuming N is positive That is Clear Accumulator and Link Deposit to P and clear accumulator Load Accumulator with N Negate Deposit to Counter and clear accumulator Add M into Accumulator Increment Counter and Skip on Zero Jump to Instruction Add M Deposit to P and clear accumulator Halt Jump to 0200 Each step above is one machine code instruction Note the two instructions that are inserted into the code Add M and Store P The product is accumulated in the Accumulator within the body of the loop and then stored in P after the loop ends The Jump instruction has also been changed to jump two instructions back and not one Note that efficient use is made of the and clear accumulator feature for the Deposit instruction Do the following 1 Using a PDP 8 Machine Code Form write out the
7. g b from a test to see if the accumulator is positive or zero If positive or zero Skip over the next instruction otherwise execute an instruction to negate the result Do the following 1 Using the pseudo code write out the code on a PDP 8 Machine Code Form Use the same addresses for a b and c clear accumulator and link load b negate b add a skip if positive negate accumulator making it positive store accumulator to c halt jump to beginning 2 Use the r Reset command to clear out memory and all registers then use the p PC command to move the memory cursor to address 0200 and display page 1 of memory Enter your code starting at address 0200 and the data at starting at address 0300 3 Execute your program use either Go and the trace option to make sure it works There are three options here a lt b a b and a gt b Be sure to test all three Save a copy of your machine code Use the w Write command see below to save your code to a file Call your file lab03d obj The code is saved in address content format i e nnnn mmmm When saved open your file using NotePad attach name and date and print out a copy of the file gt w h Comp255 lab03d obj Verification 4 Part 5 with time There is a small problem with Part 2 N is assumed to be positive but what happens if N 0 Think The fix is simple After you Load Accumulator with N and Negate it a strictly positive N should now be negative
8. nge 4 Enter different values for a and b and re execute the program Use the values below 0300 0022 0301 0017 0302 0000 Position the memory cursor at location 0200 i e set the PC equal to 0200 and hit the G Go key What happened Try it a third time with 0300 7776 0301 0002 0302 0000 Explain what happened Hint Check the Link bit 5 Save a copy of your machine code Use the w Write command see below to save your code to a file Call your file Lab03a ob 4 The code is saved in address contents format i e nnnn mmmm When saved open your file using NotePad and print out a copy of the file You can also attach comments to each line of machine code explaining what each line of machine code does Be sure to attach your name and date as a comment gt w h Comp255 lab03a obj Note The gt l load command can read this file back into PDP 8 memory use gt 1 h comp255 lab03a ob Reset clear the memory first use gt r for reset and try it Verification 1 Part 2 Multiplication using a Simple Counting Loop On page 3 11 of The PDP 8 Emulator User s Manual there is a program that executes a simple counting loop The program demonstrates how the ISZ and JMP instructions are used to control a loop although the loop itself addresses 0204 0205 is empty address contents description 0200 7300 Clear Accumulator and Link 0201 1301 Load Accumulator with N 0202 7041 Negate 0203 3300 Deposit to Counter and Z
9. t adds two integers specifically the program will sum 2 plus 3 and will look like the c program below All addressing will be current page direct addressing int main int ie b 3 c declare variables a 23 a b l 1 Start by writing out the machine code for this program using the PDP 8 Machine Code Form The code will be placed on page 1 beginning at address 0200 all values are in octal data will be placed on page 1 beginning at address 0300 2 When you have written out the machine code start the PDP 8 Emulator and go to the Debugger Screen to enter the machine code directly into the PDP 8 memory Use the P command gt P200 enter to place the memory cursor at address 0200 on page 1 You should see addresses 0200 through 0377 in the memory display upper right Enter the code from the PDP 8 Machine Code Form starting at address 0200 As you enter each 4 digit code hit Enter This will automatically advance the memory cursor Use the arrow keys to move the cursor to address 0300 and enter the data values the same way Before continuing check that the program and code data you entered agrees with your code 3 Trace single step the execution of this code by positioning the memory cursor on the location of the first instruction address 0200 this also sets the Program Counter to 0200 and use the Space Bar to single step through the code Hit the Space Bar 5 times noting how the PC AC and IR registers cha
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