CSC263 Comp Org/Arch

Introduction to MIPS

a word in MIPS is 32 bits, or 4 bytes
MIPS instructions are 32 bits (4 bytes) long
MIPS memory addresses are 32 bits (4 bytes) long
MIPS has 32 integer registers, each of which is 32 bits (4 bytes) long
arithmetic and logical operations are always done with registers
load operations copy a value from memory to a register
store operations copy a value from a register to memory
MIPS is a load-store architecture: all arithmetic and logical operations are done in registers; only load and store operations access memory
There are three types of instructions: basic instructions, pseudo instructions, and directives

basic instructions are executed directly by the MIPS hardware
pseudo instructions are translated by the assembler into one (or a sequence of) basic instruction(s)
directives are instructions for the assembler itself

there are three instruction formats: R-type, J-type, and I-type (more on these later)
a character is 8 bits (1 byte) long
an integer is 32 bits (4 bytes) long

The components of a MIPS program are as follows:

comments: Comments start with a # sign. Everything is a comment from the # to the end of the line.
labels: Labels are user defined names, assigned to statements and variables. A label starts with a letter, followed by letters and/or digits, and ends with a :
variables: Variables are defined at the beginning of the program. The directive .data starts the variable section.
code: The code comes after the variables. There are two directives for the code. The .globl directive specifies the external name of the function. For now, that name will be main. Later we will discuss how to create additional functions. This is followed by the .text directive, which starts the code section. The label main comes right after the .text directive to indicate where execution should begin.

The layout of a MIPS program is as follows:

# comments describing the program
#
      .data

# variable declarations go here

      .globl   main
      .text
main:

# program code goes here

character: character constants are enclosed in single quotes, for example 'a', 'Q', '4', '&'
numeric: numeric constants are written in base 10, with an optional leading sign, for example 5, -17
string: string constants are enclosed in double quotes, for example "this is a string"