Review Lat lecture, we learnt addi, and, andi, or, ori, xor, xori, nor, beq, j, bne An array is stored sequentially in the memory The instructions are also stored sequentially in the memory. Executing the code is to load then execute the instructions one by one, unless we encounter a branch condition.
Review Label: A label is associated with exactly one instruction. We can understand it as the address of the instruction in the memory. Go to a label means that fetch that instruction from the memory and execute it. A label in MIPS looks like L1: add $t0, $t1, $t2
Review We talked about the if-else in MIPS. When we want to do if ($t0 == $t1) $t0 = $t1 + $t2; else $t0 = $t1 - $t2; One of the correct ways is beq $t0, $t1, L1 sub $t0, $t1, $t2 j exit L1: add $t0, $t1, $t2 exit:
Review Suppose $t0, $t1, $t2 are storing 1,2,3, respectively. What will the values in $t0 be after we come to these instructions? beq $t0, $t1, L1 sub $t0, $t1, $t2 L1: add $t0, $t1, $t2 exit:
Exercise 2 How to implement this with only the instructions we learnt? if ($t1 > $t2) $t0 = $t1; else $t0 = $t2;
Exercise 2 # if ($t1 > $t2) $t0 = $t1; # else $t0 = $t2; sub $t3, $t1, $t2 srl $t3, $t3, 31 bne $t3, $zero, L1 ori $t0, $t1, 0 j L2 L1: ori $t0, $t2, 0 L2:
slt, slti slt $t3, $t1, $t2 – set $t3 to be 1 if $t1 < $t2 ; else clear $t3 to be 0. “Set Less Than.” slti $t3, $t1, 100 – set $t3 to be 1 if $t1 < 100 ; else clear $t3 to be 0. Using slt, the code is simpler.
Using slt slt $t3, $t1, $t2 bne $t3, $zero, L21 ori $t0, $t1, 0 j L22 L21: ori $t0, $t2, 0 L22:
Some Comments Being able to write if-else, we can have all other fancy things like for loop, while loop…. That is why we do not have an instruction for the for loop or while loop, but we build it from the if-else.
Compiling a while loop in C 4/13/2015 week04-3.ppt 11 How to translate the following to MIPS assembly? We first translate into a C program using if and goto
Compiling a while loop in C 4/13/2015 week04-3.ppt 12 Assume that i and k correspond to registers $s3 and $s5 and starting address of array save is in $s6