MIPS Instruction Set

This is a **partial list** of the available MIPS32 instructions, system calls, and assembler directives. For more MIPS instructions, refer to the Assembly Programming section on the class Resources page.

In all examples, $1, $2, $3 represent registers. For class, you should use the register names, not the corresponding register numbers.

Arithmetic Instructions

Logical

Data Transfer

Variations on load and store also exist for smaller data sizes:

• 16-bit halfword: lh and sh
• 8-bit byte: lb and sb

Conditional Branch

All conditional branch instructions compare the values in two registers together. If the comparison test is true, the branch is taken (i.e. the processor jumps to the new location). Otherwise, the processor continues on to the next instruction.

Note 1: It is much easier to use a label for the branch instructions instead of an absolute number.  For example: beq $t0, $t1, equal.  The label "equal" should be defined somewhere else in the code.

Note 2: There are many variations of the above instructions that will simplify writing programs! Consult the Resources for further instructions, particularly H&P Appendix A.

Comparison

Note: There are many variations of the above instructions that will simplify writing programs! Consult the Resources for further instructions, particularly H&P Appendix A.

Unconditional Jump

Note: It is much easier to use a label for the jump instructions instead of an absolute number.  For example: j loop.  That label should be defined somewhere else in the code.

System Calls

The SPIM simulator provides a number of useful system calls.  These are simulated, and do not represent MIPS processor instructions. In a real computer, they would be implemented by the operating system and/or standard library.

System calls are used for input and output, and to exit the program.  They are initiated by the syscall instruction. In order to use this instruction, you must first supply the appropriate arguments in registers $v0, $a0-$a1, or $f12, depending on the specific call desired. (In other words, not all registers are used by all system calls). The syscall will return the result value (if any) in register $v0 (integers) or $f0 (floating-point).

Available syscall services in SPIM:

Notes:

• The print_string service expects the address to start a null-terminated character string. The directive .asciiz creates a null-terminated character string.
• The read_int, read_float and read_double services read an entire line of input up to and including the newline character.
• The read_string service has the same semantics as the C Standard Library routine fgets().
• The programmer must first allocate a buffer to receive the string
• The read_string service reads up to n-1 characters into a buffer and terminates the string with a null character.
• If fewer than n-1 characters are in the current line, the service reads up to and including the newline and terminates the string with a null character.
• There are a few additional system calls not shown above for file I/O:  open, read, write, close (with codes 13-16)

Assembler Directives

An assembler directive allows you to request the assembler to do something when converting your source code to binary code.

Registers

MIPS has 32 general-purpose registers that could, technically, be used in any manner the programmer desires. However, by convention, registers have been divided into groups and used for different purposes. Registers have both a number (used by the hardware) and a name (used by the assembly programmer).

This table omits special-purpose registers that will not be used in ECPE 170.

Based on:

• http://labs.cs.upt.ro/labs/so2/html/resources/nachos-doc/mipsf.html
• http://logos.cs.uic.edu/366/notes/mips%20quick%20tutorial.htm
• http://www.mrc.uidaho.edu/mrc/people/jff/digital/MIPSir.html