MIPS Assembly/Register File
Registers[edit | edit source]
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 | $ra | Return address |
Zero Register[edit | edit source]
The zero register ($zero or $0) always contains a value of 0. It is built into the hardware and therefore cannot be modified.
$at Register[edit | edit source]
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[edit | edit source]
The $v Registers are used for returning values from functions. They are not preserved across function calls.
Argument Registers[edit | edit source]
The $a registers are used for passing arguments to functions. They are not preserved across function calls.
Temporaries[edit | edit source]
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[edit | edit source]
Saved Temporary registers are used to store longer lasting values. They are preserved across function calls.
$k Registers[edit | edit source]
The k registers are reserved for use by the OS kernel. They may change randomly at any time as they are used by interrupt handlers.
Pointer Registers[edit | edit source]
- Global Pointer ($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.