c - MARS MIPS and struct nodes

Question

typedef struct node {    
    int data;    
    struct node *next;    
} nodeL; 

Assuming I want to translate the above declaration in MIPS assembly language, how am I supposed to do it? Apart from allocating memory (using syscall 9), which is done in the .text segment, what about the .data segment? Also, what about alignment?

Answer 1

Before even mentioning code, you need to clarify whether the struct you want to create will be static data (data segment), local data (stack), or dynamically allocated data (heap). There are different ways to allocate each.

But before discussing that, the very first thing you need to do is determine the layout of the struct instance. At the very least it could be:

------------------
| data - 32-bits |
------------------
| next - 32-bits |
------------------

To create an instance statically, it's simply:

    .data
    .align 2
anInstance:     .word   0,0

And on the heap:

    .text
Allocator.newNode:
    li $a0, 8           #allocate 8 bytes
    li $v0, 9
    syscall             #returns word-aligned ptr
    jr $ra

If placing on the stack, simply allocate 8 bytes for it.

A cleaner way is to use a prototype-based method.

Your object layout becomes:

------------------
| size - 32-bits |
------------------
| atr 1 - 32-bits|
------------------
| atr 2 - 32-bits|
------------------
.
.
.
------------------
| atr n - 32-bits|
------------------

For each struct, you create a prototype which the allocation routine will use to create the instance.

    .data
ListProto:      .word   8,0     #size, head ptr
NodeProto:      .word   12,0,0  #size, data, next ptr    

    .text
main:
    la $a0, ListProto
    jal Allocator.newObject     #create new list instance

    la $a0, NodeProto
    jal Allocator.newObject     #create new node instance

Allocator.newObject:
    lw $a0, 0($a0)      #a0 = object size
    li $v0, 9
    syscall
    jr $ra

Whether you want the instance to actually keep a size field is up to you. With that approach you can simply add prototypes and that's it.