CS274: Computer Architecture - MIPS Procedures

Activity Goals

The goals of this activity are:
  1. To read and write procedure calls
  2. To save and restore the stack when calling procedures
  3. To use registers and save registers according to standard MIPS calling conventions

The Activity

Directions

Consider the activity models and answer the questions provided. First reflect on these questions on your own briefly, before discussing and comparing your thoughts with your group. Appoint one member of your group to discuss your findings with the class, and the rest of the group should help that member prepare their response. Answer each question individually from the activity, and compare with your group to prepare for our whole-class discussion. After class, think about the questions in the reflective prompt and respond to those individually in your notebook. Report out on areas of disagreement or items for which you and your group identified alternative approaches. Write down and report out questions you encountered along the way for group discussion.

Model 1: MIPS Procedures

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.text
.globl main
  
main:
    # print user prompt string
    li $v0, 4
    la $a0, input
    syscall   
     
    # read integer - will be copied to v0
    li $v0, 5
    syscall
 
    # call procedure
    addi $a0, $v0, 0 # set a0 to the v0 value that the user just typed in
    jal add5
     
    # print the answer: be sure to set a0 before v0 so we don't lose it!
    addi $a0, $v0, 0 # print the value that add5 just returned as v0
    li $v0, 1
    syscall
     
    # exit
    li $v0, 10
    syscall 
     
add5:
    # Save one register on the stack
    addi $sp, $sp, -4
    sw $s0, 0($sp)
     
    # add 5 to the input argument, and set it to the return value
    li $s0, 5
    add $v0, $a0, $s0
     
    # Restore the register from the stack
    lw $s0, 0($sp)
    addi $sp, $sp, 4
     
    # return
    jr $ra
     
.data
input: .asciiz "Enter an integer\n"       

Questions

  1. Why was it important to set a0 prior to executing syscall 1 above?
  2. Given that register values are 32 bits in size, why did we subtract 4 from the stack pointer prior to saving register s0, and add 4 back to it when restoring the register from memory?
  3. Why was it necessary to save register s0 to the stack in the add5 function?
  4. If we had used t0 instead of s0 in add5, would we have had to save it to the stack? What is the significance of this?
  5. Why is the syscall 10 call important in this code? What would happen if it was not present?

Model 2: Writing Procedures

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.text
.globl main
  
main:
    la $t0, val1
    la $t1, val2
     
    lw $t2, 0($t0)
    lw $t3, 0($t1)
     
    sw $t2, 0($t1)
    sw $t3, 0($t0)
     
    # exit
    li $v0, 10
    syscall 
     
.data
val1: .word 5
val2: .word 6

Questions

  1. What does this program do?
  2. Modify this program to execute the algorithm in a procedure, using the argument registers to correspond to the two memory addresses to be swapped. Call the procedure from main.

Submission

I encourage you to submit your answers to the questions (and ask your own questions!) using the Class Activity Questions discussion board. You may also respond to questions or comments made by others, or ask follow-up questions there. Answer any reflective prompt questions in the Reflective Journal section of your OneNote Classroom personal section. You can find the link to the class notebook on the syllabus.