CS274: Computer Architecture - MIPS Conditionals

Activity Goals

The goals of this activity are:

1. To implement if and else statements using MIPS assembly.

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.

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Model 1: An if/else statement in MIPS

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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           # our secret number is 7!     li $s0, 7           # if v0 == our secret number s0     beq $s0, $v0, correct           # incorrect code     li $v0, 4     la $a0, wrong     syscall                 # but skip the correct code!     j exit   correct:     li $v0, 4     la $a0, right     syscall       # What would you add here to print "The secret number was: " followed by the integer 7, followed by a newline?       exit:     # exit     li $v0, 10     syscall        .data     input: .asciiz "Can you guess my secret number between 1 and 10?  Enter it below\n"     wrong: .asciiz "Not quite!\n"     right: .asciiz "You guessed it!\n"

Questions

1. Translate the above code into a C, Java, or Python program. What does it do?
2. What would happen if the j exit line was omitted?
3. How would you modify this program to print the secret number using a syscall?
4. Translate the beq instruction to machine language.
5. Translate the j instruction to machine language.
6. What is the farthest you can move the program counter with a branch instruction? How about a jump instruction?
7. Suppose you wanted to jump to a location that caused the upper four bits of the program counter to be modified. Using a combination of branch and jump instructions, perform this jump.

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Model 2: Implementing MIPS Conditionals

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int main(void) {     int grade = 93;           if(grade > 90) {         printf("A\n");     } else if(grade > 80) {         printf("B\n");     } else {         printf("C or lower\n");     } }

Questions

1. Translate the above C code into a MIPS assembly program

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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.