Assembler Directives

1. Assembler Directives • Example program: .data # DATA segmentcity: .asciiz “Seattle” .align 2num: .word 2210, 2341, 26, 0x022ec, 0x11110087 .byte 65, 79, 253 .text # CODE segment .globl main # declaring “main” as globalmain: li $v0, 4 # syscall for print_string la $a0, city # argument in $a0 syscall la $t0, num # get address of first number lw $s0, 0($t0) # s0 = 2210 lw $s1, 4($t0) # s1 = 2341 add $s0, $s0, $s1 # s0 = 2210+2341 li $v0, 1 # syscall for print_int move $a0, $s0 # argument in $a0 syscall .end main

2. General format of MIPS programs • Xspim on our machines automatically loads some assembly code that takes care of calling “main” • Need to write code that looks roughly like the following: .data # DATA segmentlab1: .byte ……… lab2: .asciiz …… # some static data .float ………lab3: .double ……... .text # CODE segment .globl main # declaring “main” as globalmain: <your part of the code> … ... .end main

3. If-then-else in MIPS • What does the following C code looks like in MIPS assembly language: if (a < b) { a = a + 1; b = b - 1; } else { a = a * 2; b = b / 2; }Assume $s0 contains a, and $s1 contains b.

4. If-then-else (cont.) • # if (a < b) slt $t0, $s0, $s1 beq $t0, $zero, else_label# a = a + 1, b = b - 1 addi $s0, $s0, 1 subi $s1, $s1, 1 j end_label# else a = a * 2, b = b / 2 else_label: sll $s0, $s0, 1 sra $s1, $s1, 1# end of if-then-else end:<statements following if-then-else>

5. From C Program to Machine Language • Stages in the transformation: C program Compiler Assembly Language program Assembler Object : Machine Language Module Library routines Machine Lang Linker Executable (Machine Language) Loader Memory