1. Syscall in MIPS
Xinhui Hu
Yuan Wang

2. MARS What is MARS? How to get it? How to use it? MIPS simulator
Java based
How to use it?
Use the tutorial in the website
Assemble->execute->step-by-step execute

3. 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!

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

5. 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 0x

6. 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

7. 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)

8. 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

9. 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

10. 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!”

11. 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

12. 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