COMS 361 Computer Organization

1. COMS 361Computer Organization Title: Instructions Date: 9/30/2004 Lecture Number: 11

2. Announcements • Homework 4 • Due 10/05/04

3. Review • SPIM • MIPS simulator • SPIM program (exit.s) • Comments • Assembler directives • .text • .data • Labels • Instructions • Data • MIPS DataPath • Pseudo Instructions

4. Outline • SPIM • MIPS simulator • Assembler directives

5. Allocate space in the data segment of the SPIM memory model Allocates 4096 bytes in the data segment Assembler Directives • Used to create data structures that are available at run-time • To allocate a one-dimensional array in C++ int ARRAY[1024]; • Corresponds to the MIPS assemble directive .data ARRAY: .space 4096

6. Assembler Directives • Allocate and initialize a one-dimensional array int ARRAY[] = { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 }; • Corresponds to the MIPS assemble directive .data ARRAY: .word 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 la $a0 ARRAY # $a0 = &ARRAY[0] lw $s0 8($a0) # $s0 = MEM[2] $a0 contains a pointer to the array

7. Assembler Directives • String literal definition .data helloStr: .ascii “Hello, World!\n” • helloStr is the memory location of the array of characters • The string is not null terminated • Can cause problems (hell01.s) .data helloStr: .asciiz “Hello, World!\n”

8. Pseudo Instructions • Give the illusion of a richer instruction set • Translated into one or more “real” MIPS instructions la $a0 label • The problem in implementing this instruction is that label should be a 32-bit address • It will not fit into a single instruction lui $a0 upper 16-bits of label ori $a0 lower 16-bits of label

9. Control Flow • MIPS conditional branch instructions bne $t0, $t1, Label beq $t0, $t1, Label • Example • C/C++ code: if(i==j) { h=i+j; } • MIPS code: bne $s0, $s1, Label add $s3, $s0, $s1 Label: .... • Implemented using reverse logic • Turned the equality condition around

10. Control Flow if(i != j) { h = i + j; }else { h = i - j; } beq $s4, $s5, Lab1 add $s3, $s4, $s5 j Lab2 Lab1: sub $s3,$s4,$s5 Lab2: … Turned the test around Branched over the then part

11. Control Flow • beq, bne are useful • What about branch-if-less-than? • New instruction if($s1 < $s2) $t0 = 1; else $t0 = 0; • Vanilla MIPS does not provide a ble instruction • It can be synthesized from other MIPS instructions

12. Control Flow • The ble pseudo-instruction is implemented as two “real” MIPS instructions • slt $at, $s1, $s2 (set $t0 to 1 if $s1 < $s2) • beq $at, $0, Target if ($t0 == $0) goto Target0 • The assembler needs an extra register to convert this pseudo instruction into a sequence of actual MIPS instructions • $at can NOT contain a currently needed value

13. Example • Implement the following while loop in MIPS assembly language • Assume an array save has 10 elements and they are filled with integer values: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 int j = 0, sum = 0; int save[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; while (j < 10) { sum += save[j++]); } cout << “save array sum: “ << sum << endl;