CS 2504 Intro Computer Organization Homework 2: Basic MIPS
Last modified: 9/6/2006 10:05 AM 1
Prepare your answers to the following questions in a plain text file. Submit your file to the Curator system by the posted
deadline for this assignment. No late submissions will be accepted.
You will submit your answers to the Curator System (www.cs.vt.edu/curator) under the heading HW2.
1. [25 points] Consider adding support for the following pseudo-instruction to the MIPS assembler:
asum $rd, $rl, $rr # place sum of |$rl| and |$rr| in $rd;
# the contents of $rl and $rr are not changed
This will require that the assembler replace an occurrence of the pseudo-instruction above with a sequence of one or
more native MIPS instructions. What instruction(s) might the assembler use for the replacement? (Note: the native
MIPS absolute value instructions abs.f and abs.d are not suitable for integer values.) You may use any of the
temp registers ($tx) as needed.
2. [25 points] In section 2.6 of P&H, a MIPS assembly translation is given for the following segment of C code:
while ( save[i] == k )
i += 1;
However, the given translation uses both a conditional branch and an unconditional branch operation each time
through the loop. Only poor compilers, and poor MIPS assembly programmers, would write something that
inefficient. Rewrite the assembly code so that it uses at most one branch or jump operation each time through the loop.
3. [25 points] Consider the following short MIPS assembly program:
.data
x: .word 12
y: .word 17
z: .word 0
.text
main:
lw $s0, x
lw $s1, y
blt $s0, $s1, less
add $s0, $s0, $s1
sw $s0, z
j finish
less: sub $s1, $s1, $s0
sw $s1, z
finish:
li $v0, 10 # exits program
syscall
Write an equivalent C/C++ program.
