ch1.adoc

Chapter 1: Data

In MIPS, you can declare global variables in the .data section.

At a minimum, this is where you would declare/define any literal strings your program will be printing, since virtually every program has at least 1 or 2 of those.

When declaring something in the .data section, the format is

variable_name: .directive value(s)

where whitespace between the 3 is arbitrary. The possible directives are listed in the following table:

Table 1. MIPS data types

Directive	Size	C equivalent
.byte	1	char
.half	2	short
.word	4	int, all pointer types
.float	4	float
.double	8	double
.ascii	NA	char str[5] = "hello"; (no '\0')
.asciiz	NA	char str[] = "hello"; (includes the '\0')
.space	NA	typeless, unitinialized space, can be used for any type/array

As you can see it’s pretty straightforward, but there are a few more details about actually using them so let’s move onto some examples.

Say you wanted to convert the following simple program to MIPS:

#include <stdio.h>

int main()
{
	char name[30];
	int age;
	printf("What's your name and age?\n");
	scanf("%s %d", name, &age);
	printf("Hello %s, nice to meet you!\n", name);
	return 0;
}

The first thing you have to remember when converting from a higher level language to assembly (any assembly), is that what matters is whether it is functionally the same, not whether everything is done in exactly the same way.^[1] In this instance, that means realizing that your literal strings and your local variables name and age become globals in MIPS.

.data
age:         .word 0  # can be initialized to anything

ask_name:    .asciiz "What's your name and age?\n"
hello_space: .asciiz "Hello "
nice_meet:   .asciiz ", nice to meet you!\n"

name:        .space 30

.text

# main goes here

As you can see in the example, we extract all the string literals and the character array name and int age and declare them as MIPS globals. One thing to note is the second printf. Because it prints a variable, name, using the conversion specifier, we break the literal into pieces around that. Since there is no built-in printf function in MIPS, you have to handle printing variables yourself with the appropriate system calls.

Arrays

Obviously strings are special cases that can be handled with .ascii or .asciiz for literals, but for other types or user inputed strings how do we do it?

The first way, which was demonstrated in the snippet above is to use .space to declare an array of the necessary byte size. Keep in mind that the size is specified in bytes not elements, so it only matches for character arrays. For arrays of ints/words, floats, doubles etc. you’d have to multiply by the sizeof(type).

"But, .space only lets you declare uninitialized arrays, how do I do initialized ones?"

Actually, it appears .space initializes everything to 0 similar to global/static data in C and C++, though I can’t find that documented anywhere.

Aside from that, there are two ways depending on whether you want to initialize every element to the same value or not.

For different values, the syntax is an extension of declaring a single variable of that type. You specify all the values, comma separated. This actually gives you another way to declare a string or a character array, though I can’t really think of a reason you’d want to. You could declare a .byte array and list all the characters individually.

However, if you want an array with all elements initialized to the same value there is a more convenient option. After the type you put the value you want, a colon, and then the number of elements. So a: .word 123 : 10 would declare a 10 integer array with all elements set to 123. This works for all types in MARS, but SPIM does not support this syntax for floats and doubles. Since you’ll probably not have to deal with floating point types at all, let alone arrays of them initialized to a particular value, this isn’t a huge loss.

Given what we just covered, this:

int a[20];
double b[20];
int c[10] = { 9,8,7,6,5,4,3,2,1,0 };
int d[5] = { 42, 42, 42, 42, 42 };
char e[3] = { 'a', 'b', 'c' };

becomes

.data
a:        .space 80
b:        .space 160
c:        .word 9,8,7,6,5,4,3,2,1,0
d:        .word 42 : 5
e:        .byte 'a', 'b', 'c'

For more examples of array declarations, see array_decls.s. You don’t have to understand the rest of the code, just that it prints out each of the arrays.

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1. Obviously compilers have to follow stricter rules, but for the purposes of learning and actually using assembly directly, there’s no reason to make your life harder than necessary.