A register is an electronic device that holds bit patterns. A general purpose register is a register that is visible to assembly language. The floating point registers, although visible to assembly language, are not regarded as general purpose registers.
| Register Number | Mnemonic Name | Conventional Use | Register Number | Mnemonic Name | Conventional Use | |
|---|---|---|---|---|---|---|
| $0 | $zero | Permanently 0 | $24, $25 | $t8, $t9 | Temporary | |
| $1 | $at | Assembler Temporary | $26, $27 | $k0, $k1 | Kernel | |
| $2, $3 | $v0, $v1 | Value returned by a subroutine | $28 | $gp | Global Pointer | |
| $4-$7 | $a0-$a3 | Subroutine Arguments | $29 | $sp | Stack Pointer | |
| $8-$15 | $t0-$t7 | Temporary | $30 | $fp | Frame Pointer | |
| $16-$23 | $s0-$s7 | Saved registers | $31 | $ra | Return Address |
Although the registers are called "general purpose", it is conventional in software to use specific registers for specific purposes. This is not required by hardware. It is just a way for programmers to avoid confusion by establishing conventions. The extended assembler allows you to use mnemonic names for registers that show how they are conventionally used.
For example, registers $8
through $15 are conventionally used to
hold temporary values.
Mnemonic names for these registers are
$t0 through $t7.
As
far as hardware is concerned,
only registers $0 and $31
are different from the rest
($0 is always full of zeros and $31 is automatically used
by some subroutine linkage instructions to hold the return address).
The remaining registers are electronically identical.
It is a software convention to use different sets
of registers for different purposes.
Are programs written in extended assembly code executed on the same MIPS processor we have been looking at?