Syscall in MIPS Xinhui Hu Yuan Wang

MARS What is MARS? How to get it? How to use it? MIPS simulator http://courses.missouristate.edu/KenVollmar/Mars/download.htm Java based How to use it? Use the tutorial in the website Assemble->execute->step-by-step execute

Format of a MIPS program Every line contains at most one instruction # marks beginning of comment assembly language program require more comments! at least one per line!

Parts of a MIPS program Identify data segment and text (code) segment .globl main main: # start of code On simulator, data segment starts at 0x10010000

Data segment Identify data segment and text (code) segment .data .word 7 #one word with initial value 7 .word 3 #one word with initial value 3 .text .globl main main: # start of code On simulator, data segment starts at 0x10010000

Data and labels Locations in data section can be marked with label Label represents the address of where the data is in memory .data first: .word 7 #one word with initial value 7 last: .word 3 #one word with initial value 3

System calls syscall instruction is used for calls to the operating systems input output Basic operation load $v0 with command to execute put output value in $a0 (or $f12) get input result from $v0 (or $f0)

Useful syscall Commands Command Event Arguments Result (in $v0) 1 print int $a0 = integer $a0 is printed out 2 print float $f12 = float $f12 is printed out 4 print string $a0 = pointer to string string is printed out 5 read int $v0 holds integer read 6 read float $f0 holds float read 8 read string $a0 = buffer,a1 = length string is read from console 10 exit program 11 print byte $a0 = byte byte is printed out

Printing an integer Command is 1 Command must be in register Value to print must be in register a0 Example: print the value 10 addi $v0, $v0, 1 # command to print integer addi $a0, $a0, 10 # value to print syscall

Strings for output Define in data section of program Use labels to identify different strings Labels represent addresses in memory Strings are C strings (end with 0) .data prompt: .asciiz "Enter in an integer: " str1: .asciiz "The integer is: " newline: .asciiz "\n " hello: .asciiz”Hello, students of CSE230!”

Printing a string Command is 4 $v0 must hold command $a0 must hold address of string to print Example: print hello li $v0, 4 #command to print string la $a0, hello # load address of string syscall

Reading input Command is 5 $v0 must hold command $v0 get result Example: read number li $v0, 5 # command to read integer syscall move $t0, $v0 # result saved in $t0