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Lab #4

1. a PAL source code file Be aware that if your use a text editor like Notepad to create your source code files avoid using the tab key since the PDP 8 Editor does not handle tab characters well they display as an odd looking o character The assembler has no problems with them but the editor does Part 1 As a class exercise create assemble and execute the multiplication program below This is the same multiplication program from Lab03 Name File lab04a pal Date February 2 2010 Desc A program to multiply two integers P N M iE Code Segment if 0200 start at address 0200 Main cla cll clear AC and Link dca P initialize product to 0 tad N load N cia negate it dca CNT store in loop counter Li tad M add multiplicand isz CNT increment loop counter jmp L1 A and loop unless 0 dca P store product hlt halt jmp Main return to beginning Data Segment 0300 CNT 0 loop counter N 3 multiplier M 7 multiplicand P 0 product SMain Start the PDP 8 simulator program and do the following 1 From the Edit Window read in the file template pal Enter the code given above 2 Assemble the program If successful save the list file as h comp255 lab04a Ist otherwise go back and fix any syntax errors Normally it is not necessary to save the list file but in this case do so Since the PDP 8 simulator program does not have a print capability you will n
2. extracted digits 0300 DO 0 D1 0 D2 0 D3 0 Number 3527 number to be unpacked Mask 0007 mask for AND operation C4 4 constant 4 CNT 0 loop counter There is still an addressing problem Inside the loop how do you change the address of DO to be D1 then D1 to be D2 etc That is inside the body of the loop how does the deposit instruction dca DO become dca Dl One answer is to use self modifying code Attach a label to the dca DO instruction and insert the instruction isz A after the A dca DO instruction A dca DO isz A This will solve the problem but why Think about it Now write the program and test that this works There is one problem with self modifying code is that you can t re run it since it changes itself This can be fixed by first saving the instruction that s modified then restoring it afterward So allocate a temporary storage and before you enter the main loop cla cll tad A get the instruction at A dca temp and store it at temp Afterward restore the instruction cla cll tad temp restore instruction dca A at Loop Still there must be a better way
3. Clear accumulator and link Deposit to Q and clear accumulator Deposit to R and clear accumulator Load B Negate it Deposit result to minusB and clear accumulator Load A L1 Add minusB the L1 loop is where division occurs Skip next instruction if positive or zero you exit the loop when you go negative Jump to L2 Increment Q counting the number of subtractions Jump to L1 L2 Add B to restore the remainder since you overshot you need to Deposit to R and clear accumulator restore the dividend which is now the reminder Halt Jump to Main There are 5 variables A and B which should be initialized to non zero values for the dividend and divisor respectively and Q R and MB minus B the latter being used to store B This is needed for subtraction The variables A B Q R and MB should be stored in order at beginning at address 0300 See below Data Segment 0300 A 13d dividend B 5 divisor Q 0 quotient R 0 reminder MB 0 minus B SMain Begin with the template pal file Follow the steps from Part 1 but save the source code file as lab04b pal you do not need to create a list file When your program runs correctly have the instructor verify your work Print out your source code file to be handed in with the lab assignment Verification 2 Part 3 Extracting Octal Digits We begin by introducing four more instructions Assembler Machine Description Mnemonic Code AND Onnn Log
4. Comp 255Q 1M Computer Organization Name Lab 4 PDP 8 Assembler Language Programming February 2 2010 Grade 10 Introduction We ll begin this lab with a brief overview of Assembler Language and Assembler Language programming We ll begin with the AddO1 pal program found on page 4 1 of the PDP 8 Emulators User s Manual 1 Using the Built in editor to create PAL source code file 2 Elements of PDP 8 Assembler Instructions Expressions Allocating Storage Directives Notes 3 Assembling and running programs thru the Debug screen Notes 4 A Small Set of PDP 8 Instructions PDP 8 Lite see page 4 9 5 LST Files Non Relocatable Code Using Notepad to Print PAL files Notes 6 Creating a PDP 8 template file Name lt your name here gt File Date Desc Code Section 0200 Code starts at Address 0200 Main cla cll Clear AC and Link Data Section SMain Save the file on your h drive as template pal Note For all pledges assignments the pledge statement must be included in the header comment block and signed see below Since labs are not pledged the pledge statement was removed I affirm that my work upholds the highest standards of honesty and academic integrity at Wittenberg and that I have neither given nor received unauthorized assistance AS SG ER Any ASCII text editor Notepad MS Edit or the PDP 8 Editor can be used to create edit
5. eed to print out this file using another text editor like Notepad You can do this later If the assemble is successful save the source code file as h comp255 lab04a pal You can print out this file later using Notepad Normally you would save the source code file 3 Since your program does not do I O you will have to run the program from the Debug Screen to see any results Note that when a program is successfully assembled entering the Debug Screen will take you to the page where the code begins Test that your program runs correctly 4 If your program runs correctly use Notepad to print out the list lst and source code pal files otherwise go back to the Editor and fix your errors Hand in the list and source code print outs with this lab assignment Verification 1 Question 1 What happens if the multiplier N is 0 How could you detect this condition and fix the program Question 2 Which is the more efficient calculation 3 7 where the multiplier is 3 or 7 3 where the multiplier is 7 How could you make the program more efficient Part 2 Write a PDP 8 program to divide using repeated subtraction Given two integers A and B your program should compute the quotient Q A B and the remainder R A B Here is the pseudo code you will need all you need to do is translate it into PDP 8 assembler and get it to execute correctly Use the fact that the ISZ instruction can also be used to increment a variable Main
6. ical AND C AC lt C AC AND C Eaddr TAC 7001 Increment Accumulator C AC lt C AC 1 RAL 7004 Rotate Accumulator and link Left SZL 7430 Skip on Zero Link If C L 0 Then C PE lt G PC I The RAL instruction an op code 7 instruction does a Rotate Accumulator Left through the Link bit That is accumulator bit 11 is shifted left to bit 10 bit 10 is shifted left to bit 9 etc all the way to bit 1 which is shifted to bit O Bit 0 is shifted to the Link bit and the Link bit is shifted to bit 11 of the accumulator Rotate operations often used for just rotating the bits within a word Thus the PDP 8 s RAL instruction which includes the Link bit in the rotation is a bit problematic However there is a workaround the code below performs a sort of Rotate Left of the Accumulator only That is bit 0 of the Accumulator is moved to bit 11 of the Accumulator instead of to the Link bit cll ral clear link and rotate left szl skip on zero link iac otherwise add 1 to AC The code above clears the link bit and does a rotate left which moves a 0 into bit 11 since c11 and ral are both Group One Microinstructions they can be combined The Skip on Zero Link SZL instruction is used to test the Link bit which contains the former contents of bit 0 If the Link bit is 0 it skips the next instruction otherwise it adds 1 to the Accumulator which has the effect of setting bit 11 to 1 If we execute this code three times we r
7. me sequence of code three more times rotate left three save amp restore then AND with the value 0007 will allow us to extract all four octal digits so 3527 is unpacked to obtain 3 5 2 and 7 This is why you must save amp restore the rotated results before you AND with 0007 otherwise you will lose the other 3 digits Extracting each octal digit from Number is quite useful since if you add 60 octal to each of the four octal digits extracted you would have converted each to the ASCII code for the corresponding digit Then if you printed out each digit you would have printed out the value of Number Note This sets the stage for doing I O array type data structures and program modularization subroutines Part 4 Self Modifying Code Since much of the code for extracting the digits is repeated can you modify the code to use a loop to extract the four digits Going thru the code four times is done using a counting loop that executes four times something like the following Gla cll Clear AC and link tad C4 Load C4 C4 contains 4 cia Negate it dca CNT and deposit it to the loop counter El Body of loop begin here load Number left rotate 3 times save amp restore mask out upper 9 bits deposit digit to memory isz CNT Increment loop counter jmp L1 ff and loop if not zero Your data segment is augmented to include additional variables for control your counting loop as well as four variable to hold the
8. otate the left most three bits of the accumulator into the right most three bits of the accumulator If we then AND opcode 0 the accumulator with a memory address containing the octal value 0007 called a mask we have effectively extracted the left most three bits of memory by zeroing out the upper nine Important Save the rotated number before you mask out the upper three digits by using the instruction pair dca Number Save AC tad Number Restore AC This save amp restore accumulator must be done if we should want to extract the other octal digits otherwise the masking operation will erase the upper three digits nine bits before they can be extracted Write a program to take any 4 octal digit integer and extract the least significant octal digit to a second memory location called DO You will have to load the integer into the accumulator rotate it 3 times left mask out the upper 9 bits be sure to save the rotated integer and store what s left at the second location in memory Use the following Data Section Data Section 0300 DO 0 Number 3527 number to be unpacked Mask 0007 mask for AND operation Begin with the template pal file Save the source code file as lab04c pal you do not need to create a list file When your program runs correctly have the instructor verify your work Print out your source code file to be handed in with the lab assignment Verification 3 And Beyond Repeating this sa

Lab #4

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