PROGRAM STRUCTURE

The following is a template on how to organize your program.

.data                           
            (Constant and variable declarations go here.)

            .text                 # Main (must be global).
            .globl main

main:       (Your program starts here.)

            li $v0, 10            # Syscall to exit.
            syscall

REMARKS

1- The order of the data and main sections is interchangeable

2- The main label always must be declare as global.

3- Always finish your program with a syscall to exit

REGISTERS

Registers are what we call variables in a high level language. A MISP processor has 32 register named from 0 to 1 and distributed in the following way.

zero       : It always contains the value 0

at, k0, k1 : Used by the assembler to comunicate with the operating
             system.

v0, v1     : v0 is used to set the value for a system call, and also it
             can be use in addition to v1 to store values returned from a
             system call

a0 - a3    : Used by the assembler to store the values of the arguments
             passed to a system call.

t0 - t9    : temporary registers whose values are not preseved after a
             function call.

s1 - s7    : Temporary registers whose values are preserved during a
             function call

sp         : Stack pointer. It points to the first free location on the
             stack.

fp         : Frame pointer. It is used to save the value of a frame in a
             function call.

gp         : Global pointer. It points into the middle of a 64k block of
             memory in the heap that holds constant and global variables.

ra         : Return address. It is use to save and restore the address
             before and after a function call.





ASSEMBLER DIRECTIVES

Assembler directives are used to declare a section of a program,
variables, strings and/or to align data in memory. The most important
are:

.align n
     Align the next datum on a  byte boundary. For example, .align 2
     aligns the next value on a word boundary. .align 0 turns off
     automatic alignment of .half,.word, .float, and.double directives
     until the next .data or .kdata directive. 

.asciiz str
     Store the string in memory and null-terminate it. 

.data 
     The following data items should be stored in the data segment. If
     the optional argument addr is present, the items are stored
     beginning at address addr. 

.float f1, ..., fn
     Store the  floating point single precision numbers in successive
     memory locations. 

.globl sym
     Declare that symbol sym is global and can be referenced from other
     files. 

.space n
     Allocate  bytes of space in the current segment (which must be the
     data segment in SPIM). 

.text 
     The next items are put in the user text segment. In SPIM, these
     items may only be instructions or words (see the .word directive
     below). If the optional argument addr is present, the items are
     stored beginning at address addr. 

.word w1, ..., wn
     Store the  32-bit quantities in successive memory words. 





SYSTEM CALLS


System calls are a set I/O operations that let you read and print
data. To request a system call do the following:


1- Select the system call that you are going to use an store it in the
   $v0 register.

            li $v0,4 

2- Set the value of the arguments if the system call requires arguments.

            la $a0,message 

3- Use the keyword SYSCALL to execute the system call

            SYSCALL

4- Some system calls return values. this values are stored in the $v0 or
   $f0 (floating point) registers


A complete table containing all the system calls can be found on page 8
of the developers reference.