SPIM and MIPS programming

Review: Memory Organization Large single-dimension array A memory address is an index into the array Memory stores 8 bits in each address Registers store 32 bits of information For example: Li $t0, 0 lw $t0, 0($t1) Then $t0 will contain the data of the first four memory locations 6 5 4 3 2 1 8 bits of data

Review: Registers MIPS has 32 registers, each containing 32 bits Some registers are listed below $zero: constant 0 $at: reserved for the assembler $v0 - $v1: function results $a0 - $a3: function arguments $t0 - $t9: temporary $s0 - $s7: temporary

Arithmetic Instructions (1/2) Some instructions include: Add Subtract Multiply Divide Must break expression into simple operations use temporary registers ($t0 to $t9) to store intermediate results use load to insert a value into a variable use store to assign a value to a variable

Instructions Instruction Meaning add $s1, $s2, $s3 $s1 = $s2 + $s3 sub $s1, $s2, $s3 $s1 = $s2 – $s3 lw $s1, 100($s2) $s1 = Memory[$s2+100] sw $s1, 100($s2) Memory[$s2+100] = $s1

Global variables C program Assembly Program int variable = -42; .data variable: .word -42 int r1; r1: .space 4 .space allocates empty space in bytes .word allocates 4-byte value followed by the value to store

Assember Directives All assembler directives have names that begin with a period (`.'). Examples: .word w1 allocate a 4-byte word .half h1 allocate 2-bytes .byte b1 allocate a single byte .ascii “hello" allocate a sequence of bytes and store ASCII values .asciiz “hello" allocate a sequence of bytes and store ASCII values. terminate string with 0 byte (end of string) .float .double

Input/Output On real machines (including real MIPS computers), I/O is very complicated usually handled by operating system In SPIM, I/O is managed by system calls syscall instruction SPIM suspends simulation to perform I/O, then resumes simulation

System calls Systems calls are used to interact with the user To make a system call determine which service you want put service’s call code in register $v0 put arguments in registers $a0, $a1, etc. execute the syscall instruction results are in registers $v0, $v1

System calls

Input/Output: Numbers .data value: .space 4 .text main: li $v0, 5 #Syscall 5: read int syscall sw $v0, value #Number is in $v0 li $v0, 1 #Syscall 1: print int lw $t0, value mul $a0, $t0, $t0 #$a0 is what to print li $v0, 10 #Exit

Input/Output: strings .data msg: .asciiz "Type a string: " buff: .space 20 .text main: li $v0, 4 # print string la $a0, msg # $a0: address of string syscall li $v0, 8 # read string la $a0, buff # $a0: max length of buffer li $a1, 20 # $a1: max length of string li $v0, 4 la $a0, buff li $v0, 10 # exit