MIPS Architecture - Compiler Register Usage

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.

Registers for O32 Calling Convention
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
Registers for N32 and N64 Calling Conventions
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).

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