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.

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.