MIPS Assembly/Register File

Registers

MIPS has 32 general-purpose registers and another 32 floating-point registers. Registers all begin with a dollar-symbol ($). The floating point registers are named $f0, $f1, ..., $f31. The general-purpose registers have both names and numbers, and are listed below. When programming in MIPS assembly, it is usually best to use the register names.

Number	Name	Comments
$0	$zero, $r0	Always zero
$1	$at	Reserved for assembler
$2, $3	$v0, $v1	First and second return values, respectively
$4, ..., $7	$a0, ..., $a3	First four arguments to functions
$8, ..., $15	$t0, ..., $t7	Temporary registers
$16, ..., $23	$s0, ..., $s7	Saved registers
$24, $25	$t8, $t9	More temporary registers
$26, $27	$k0, $k1	Reserved for kernel (operating system)
$28	$gp	Global pointer
$29	$sp	Stack pointer
$30	$fp	Frame pointer
$31	$ra	Return address

Zero Register

The zero register ($zero or $0) always contains a value of 0. It is built into the hardware and therefore cannot be modified.

$at Register

The $at (Assembler Temporary) register is used for temporary values within pseudo commands. It is not preserved across function calls. For example, with the (slt $at, $a0, $s2) command, $at is set to one if $a0 is less than $s2, otherwise it is set to zero.

$v Registers

The $v Registers are used for returning values from functions. They are not preserved across function calls.

Argument Registers

The $a registers are used for passing arguments to functions. They are not preserved across function calls.

Temporaries

The temporary registers are used by the assembler or assembly language programmer to store intermediate values. They are not preserved across function calls.

Saved Temporaries

Saved Temporary registers are used to store longer lasting values. They are preserved across function calls.

$k Registers

The k registers are reserved for use by the OS kernel. They may change randomly at any time as they are used by interrupt handlers.

Pointer Registers

• Global Pointer ($gp) - Usually stores a pointer to the global data area (such that it can be accessed with memory offset addressing).
• Stack Pointer ($sp) - Used to store the value of the stack pointer.
• Frame Pointer ($fp) - Used to store the value of the frame pointer.
• Return Address ($ra) - Stores the return address (the location in the program that a function needs to return to).

All Pointer Registers are preserved across function calls.