MIPS has two primary types of registers, integer registers and floating point registers. In addition, MIPS has a small number of special purpose control registers.

There are register usage conventions that specify how main programs and subprograms should coordinate their use of registers. Adhering to these conventions is not mandatory, but it is helpful for avoiding problems that are difficult to debug.

Simple programs should only use the following registers:

• $zero for the constant 0
• $s0 - $s8 for main program variables
• $t0 - $t8 for subprogram variables
• $a0 - $a3 for subprogram and syscall parameters
• $v0, $v1 for subprogram return values and syscall codes and return values

For more complex programs, there are register usage conventions that specify how main programs and subprograms should coordinate their use of registers.

Alternate Name	Register Name	Use
$zero	$0	constant value 0
$s0 - $s8	$16 - $23, $30	saved values - preserved across calls
$sp	$29	stack pointer - preserved across calls
$ra	$31	return address - not preserved across calls
$a0 - $a3	$4 - $7	the first four parameters - not preserved across calls
$t0 - $t9	$8 - $15, $24 - $25	temporaries - not preserved across calls
$v0 - $v1	$2 - $3	expression evaluation and subprogram return value - not preserved across calls
$at	$1	reserved by the assembler - dangerous to use
$gp	$28	global pointer - dangerous to use
$k0 - $k1	$26 - $27	reserved by the operating system - dangerous to use

The usage convention designations have significant implications for both subprograms and their callers.

• When a register is designated as "preserved across calls", it means that the caller can count on the register having the same contents before and after a subprogram call. If the subprogram uses one of these registers, it should take measures to save the register value before changing it and restore the value before returning.
• If a register is designated as "not preserved across calls", it means that the caller cannot count on the register having the same contents before and after a subprogram call. Thus the subprogram can use the register freely. If the caller puts a value into one of these registers before a subprogram call and needs the value after the call, then the caller has the responsibility of saving and restoring the value.
• If a register is designated as "dangerous to use", it means that the register is used either by the operating system or the assembler. Most programs should avoid the use of these registers.

Register $0 is special in that it always contains the value 0. If it is specified as a destination operand of an instruction then the result of the instruction is not stored.

When writing MAL code, it is easier to stick to the conventions if you refer to registers by their alternate name.

Alternate Name	Register Name	Use
$zero	$0	constant value 0
$s0 - $s8	$16 - $23, $30	saved values - preserved across calls
$sp	$29	stack pointer - preserved across calls
$ra	$31	return address - not preserved across calls
$a0 - $a3	$4 - $7	the first four parameters - not preserved across calls
$t0 - $t9	$8 - $15, $24 - $25	temporaries - not preserved across calls
$v0 - $v1	$2 - $3	expression evaluation and subprogram return value - not preserved across calls
$at	$1	reserved by the assembler - dangerous to use
$gp	$28	global pointer - dangerous to use
$k0 - $k1	$26 - $27	reserved by the operating system - dangerous to use

Unfortunately, the MIPS floating-point registers do not have alternate names.

Register Name	Use
$f0, $f2	floating point subprogram return value - not preserved across calls
$f4 - $f10	temporaries - not preserved across calls
$f12, $f14	the first 2 floating point parameters - not preserved across calls
$f16, $f18	temporaries - not preserved across calls
$f20 - $f30	saved values - preserved across calls