Compiler Register Usage
The hardware architecture specifies that:
- General purpose register $0 always returns a value of 0.
- General purpose register $31 is used as the link register for jump and link instructions.
- HI and LO are used to access the multiplier/divider results, accessed by the mfhi (move from high) and mflo commands.
These are the only hardware restrictions on the usage of the general purpose registers.
The various MIPS tool-chains implement specific calling conventions that further restrict how the registers are used. These calling conventions are totally maintained by the tool-chain software and are not required by the hardware.
| Name | Number | Use | Callee must preserve? |
|---|---|---|---|
| $zero | $0 | constant 0 | |
| $at | $1 | assembler temporary | No |
| $v0–$v1 | $2–$3 | values for function returns and expression evaluation | No |
| $a0–$a3 | $4–$7 | function arguments | No |
| $t0–$t7 | $8–$15 | temporaries | No |
| $s0–$s7 | $16–$23 | saved temporaries | Yes |
| $t8–$t9 | $24–$25 | temporaries | No |
| $k0–$k1 | $26–$27 | reserved for OS kernel | |
| $gp | $28 | global pointer | Yes |
| $sp | $29 | stack pointer | Yes |
| $fp | $30 | frame pointer | Yes |
| $ra | $31 | return address |
| Name | Number | Use | Callee must preserve? |
|---|---|---|---|
| $zero | $0 | constant 0 | |
| $at | $1 | assembler temporary | No |
| $v0–$v1 | $2–$3 | values for function returns and expression evaluation | No |
| $a0–$a7 | $4–$11 | function arguments | No |
| $t4–$t7 | $12–$15 | temporaries | No |
| $s0–$s7 | $16–$23 | saved temporaries | Yes |
| $t8–$t9 | $24–$25 | temporaries | No |
| $k0–$k1 | $26–$27 | reserved for OS kernel | |
| $gp | $28 | global pointer | Yes |
| $sp | $29 | stack pointer | Yes |
| $s8 | $30 | frame pointer | Yes |
| $ra | $31 | return address |
Registers that are preserved across a call are registers that (by convention) will not be changed by a system call or procedure (function) call. For example, $s-registers must be saved to the stack by a procedure that needs to use them, and $sp and $fp are always incremented by constants, and decremented back after the procedure is done with them (and the memory they point to). By contrast, $ra is changed automatically by any normal function call (ones that use jal), and $t-registers must be saved by the program before any procedure call (if the program needs the values inside them after the call).
Read more about this topic: MIPS Architecture
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