Impractical Example

Here is a program that illustrates memory allocation. Of course, it is not a practical program except for use as an example.

The program: (1) asks SPIM for a block of memory four bytes long. It (2) makes a safe copy of the block's address in $s0. This program does not really need to do this, but it is good practice.

Next, (3), an integer is stored in the block. The address of the block is determined at run time. You can't get to it using a symbolic address in the program. But at run time the address is in $s0. So sw $t0,0($s0) stores 32 bits into the block.

# MallocOne.asm
#
# Allocate one block of memory, put an integer into it,
# print out the integer.

        .text
        .globl  main

main:     
        li      $v0,9             # (1) Allocate a block of memory
        li      $a0,4             # 4 bytes long
        syscall                   # $v0 <-- address
        move    $s0,$v0           # (2) Make a safe copy
        
        li      $t0,77            # (3) Store value 77
        sw      $t0,0($s0)        # into the block
        
        lw      $a0,0($s0)        # (4) Load from the block
        li      $v0,1             # into $a0. 
        syscall                   # (5) Print the integer

        li      $v0,10            # Return to OS
        syscall       

## end of file

To illustrate that the first three steps worked as expected, the program next (4) loads register $a0 from the block of memory and (5) prints out that integer.