Syscalls & Procedures

Syscalls

Syscalls are a powerful tool, which enables interaction with I/O, files, and dynamic allocation of memory. The MARS editor supports 59 different syscalls. Here's a few of useful ones.

To use syscalls, there are some special registers:

$v0 is used for the code of the syscall
$a0 to $a3 are used for parameters
The output is usually saved in $v0

By setting these registers to the desired values, and using the syscall instruction, the OS will run the operation.

Files

service	v0	arguments
open file	13	$a 0 = a dd resso f n u ll - t er mina t e d s t r in g co n t ainin g f i l e nam e < b r / >$ a1 = flags $a 2 = m o d e ∣$ v0 contains file descriptor (negative if error)
read from file	14	$a 0 = f i l e d escr i pt or < b r / >$ a1 = address of input buffer $a 2 = ma x im u mn u mb ero f c ha r a c t ers t ore a d ∣$ v0 contains number of characters read (0 if end-of-file, negative if error)
write to file	15	$a 0 = f i l e d escr i pt or < b r / >$ a1 = address of output buffer $a 2 = n u mb ero f c ha r a c t ers t o w r i t e ∣$ v0 contains number of characters written (negative if error)
close file	16	a0 = file descriptor

Hello World!

.globl main

.data
    string: .asciiz "Hello World!"

.text
    main: 
        li v0, 4
        la a0, string
        syscall

Procedures

Procedures are pieces of code that take parameters, and return a result. They're useful to make the code cleaner and more modular.

.globl main

.text
    main:
        li a0, 5 #; first parameter
        li a1, 6 #; second parameter
        jal function #; call function

        return:
            li v0, 17
            li a0, 0
            syscall #; we have to exit, or the execution will continue

    function:
        subi sp, sp, 12 #; we need 4 bytes * 3 registers 
        sw ra, 8(sp) #; return address
        sw a0, 4(<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">s</span><span class="mord mathnormal">p</span><span class="mclose">)</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:0.02691em;">w</span></span></span></span>a1, 0(sp)

        #; function body...
        #; I can use jal, because we have saved in memory ra

        lw <span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">a</span><span class="mord">1</span><span class="mpunct">,</span><span class="mspace" style="margin-right:0.1667em;"></span><span class="mopen">(</span></span></span></span>sp)
        lw <span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">a</span><span class="mord">0</span><span class="mpunct">,</span><span class="mspace" style="margin-right:0.1667em;"></span><span class="mord">4</span><span class="mopen">(</span></span></span></span>sp)
        lw <span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">a</span><span class="mpunct">,</span><span class="mspace" style="margin-right:0.1667em;"></span><span class="mord">8</span><span class="mopen">(</span></span></span></span>sp)
        addi <span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.1944em;"></span><span class="mord mathnormal">s</span><span class="mord mathnormal">p</span><span class="mpunct">,</span></span></span></span>sp, 12 #; reset stack pointer
        
        jr <span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8889em;vertical-align:-0.1944em;"></span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">a</span><span class="mord">‘‘‘</span><span class="mord mathnormal" style="margin-right:0.13889em;">T</span><span class="mord mathnormal">h</span><span class="mord mathnormal">e</span><span class="mord mathnormal">b</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">oc</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="mord mathnormal">t</span><span class="mord mathnormal">h</span><span class="mord mathnormal">e</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">u</span><span class="mord mathnormal">n</span><span class="mord mathnormal">c</span><span class="mord mathnormal">t</span><span class="mord mathnormal">i</span><span class="mord mathnormal">o</span><span class="mord mathnormal">n</span><span class="mord mathnormal">t</span><span class="mord mathnormal" style="margin-right:0.03148em;">ak</span><span class="mord mathnormal">es</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">m</span><span class="mord mathnormal">t</span><span class="mord mathnormal">h</span><span class="mord mathnormal">e</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord">∗</span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal">c</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord">∗</span><span class="mord mathnormal">i</span><span class="mord mathnormal">sc</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.01968em;">ll</span><span class="mord mathnormal">e</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.8889em;vertical-align:-0.1944em;"></span><span class="mord">∗</span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal">c</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">am</span><span class="mord mathnormal">e</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.4653em;"></span><span class="mord">∗</span><span class="mord mathnormal" style="margin-right:0.02778em;">or</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord">∗</span><span class="mord mathnormal">a</span><span class="mord mathnormal">c</span><span class="mord mathnormal">t</span><span class="mord mathnormal">i</span><span class="mord mathnormal" style="margin-right:0.03588em;">v</span><span class="mord mathnormal">a</span><span class="mord mathnormal">t</span><span class="mord mathnormal">i</span><span class="mord mathnormal">o</span><span class="mord mathnormal">n</span><span class="mord mathnormal" style="margin-right:0.02778em;">recor</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord">∗</span><span class="mord">.</span><span class="mord mathnormal" style="margin-right:0.13889em;">W</span><span class="mord mathnormal">ec</span><span class="mord mathnormal">an</span><span class="mord mathnormal">u</span><span class="mord mathnormal">se</span><span class="mord">‘</span></span></span></span>fp` to point to the start of the **activation record**, it's rendundant and rarely used.

## Recursions

Functions calling themselves!

> TODO: complete factorial

```armasm
.globl main

.text
    main:
        li $a0, 5
        jal factorial

        print:
            move $a0, $v0 #; integer = result of factorial function
            li $v0, 1 #; print integer
            syscall

        return:
            li $v0, 17
            li $a0, 0
            syscall

    factorial:
        #; jump by 1
        #; recursive step
        #; base case
        returnFactorial:
            lw $ra, ($sp)
            addi $sp, $sp
            jr $ra

Computer Science @ Sapienza

Syscalls & Procedures

Syscalls

Files

Hello World!

Procedures