一 概要

mips擁有32個通用暫存器，一個pc暫存器，一個HI和LO暫存器，另外協處理器也有自己的暫存器，如CP0有32個單獨的暫存器(不同的廠商可以實現自己獨特的暫存器，用MTC0 rt, rd, sel， sel進行區分)，浮點協處理單元也有自己獨立的暫存器。

二 32個通用暫存器

s0–s8: By convention, subroutines must guarantee that thevalues of

these registers on exit are the same as they were on entry, either by not

using them or by saving them on the stack and restoring them before

exit. This makes them eminently suitable for use as register variables or

for storing any value that must be preserved over a subroutine call.

k0, k1: Reserved for use by an OS trap/interrupt handlers,which will use

them and not restore their original value; they are of little use toanyone

else.

gp: This is used for two distinct purposes. In the kind ofpositionindependent

code (PIC) used by Linux applications, all out-of-module

code and data references go through a table of pointers known as the

GOT (for Global Offset Table). The gp register is maintained to point to

the GOT. See Chapter 16 for details.

In regular non-PIC code (as is used by simpler embedded systems) gp

is sometimes used to point to a link-time-determined location in the midstof your static data. This means that loads and stores to data lying

within 32 KB of either side of the gp value can be performed in a single

instruction using gp as the base register.

Without the global pointer, loading data from a static memory area takes

two instructions: one to load the most significant bits of the 32-bitconstant

address computed by the compiler and loader and one to do the

data load.

To create gp-relative references, a compiler must know at compile time

that a datum will end up linked within a 64-KB range of memory locations.

In practice it can’t know; it can only guess. The usual practice is

to put small global data items (eight bytes and less in size) in thegparea

and to get the linker to complain if it still gets too big.

sp: It takes explicit instructions to raise and lower thestack pointer, so

MIPS code usually adjusts the stack only on subroutine entry and exit;

it is the responsibility of the subroutine being called to do this.spis

normally adjusted, on entry, to the lowest point that the stack will need

to reach at any point in the subroutine.Nowthe compiler can access stack

variables by a constant offset from sp. Once again, see section 11.2.1 for

conventions about stack usage.

fp: Also known as s8, a frame pointer will be used by asubroutine

to keep track of the stack if for any reason the compiler or programmer

was unable or unwilling to compute offsets from the stack pointer.

That might happen, in particular, if the programdoes things that involve

extending the stack by an amount that is determined at run time.

Some languages may do this explicitly; assembly programmers are

always welcome to experiment; and C programs that use the alloca()

library routine will find themselves doing so.

If the stack bottom can’t be computed at compile time, you can’t access

stack variables from sp, so fp is initialized by the function prologue to

a constant position relative to the function’s stack frame. Cunning use

of register conventions means that this behavior is local to the function

and doesn’t affect either the calling code or any nested function calls.

ra: On entry to any subroutine, return address holds theaddress to

which control should be returned—so a subroutine typically ends with

the instruction jr ra. In theory, any register could be used, but some

sophisticated CPUs use optimization (branch prediction) tricks, which

work better if you use a jr ra.

Subroutines that themselves call subroutines must first save ra, usually

on the stack.

詳細可以參考：see mips run.pdf

三 CP0暫存器

1. Registers used in memory management.
2. Registers used in exception processing.
3. Registers used in debug.

詳細可以參考：4KcProgMan.pdf （MD00016）和 MIPS® Architecture For Programmers Vol. III: MIPS32®/microMIPS32™ Privileged Resource Architecture （MD00090）