MIPS Architecture
For reference, read Volume IA. Quick reference
https://pages.cs.wisc.edu/~markhill/cs354/Fall2008/beyond354/conventions.html
MIPS ABI: https://dmz-portal.mips.com/wiki/MIPS_ABI
Much of this information is taken from the System V Application Binary Interface (MIPS RISC Processor).
ABI Document: https://math-atlas.sourceforge.net/devel/assembly/mipsabi32.pdf
Data representation
A byte is 8 bits. A halfword is 16 bits. A word is 32 bits. A doubleword is 64 bits. We follow the big-endian convention, where the most significant byte is stored at the lowest address.
CPU Registers
- 32 32-bit general purpose registers
- 2 special 32-bit registers to hold the results of multiplication and division.
- 1 32-bit program counter (pc)
The general registers are named $0 to $31.
Special CPU registers
Register usage conventions
| Register Name | Software name | Use |
|---|---|---|
| $0 | $zero | Hard-wired to 0 |
| $1 | $at | Reserved for assembler |
| $2-$3 | $v0-$v1 | Return values from functions |
| $4-$7 | $a0-$a3 | Arguments to functions |
| $8-$15 | $t0-$t7 | Temporary registers |
| $16-$23 | $s0-$s7 | Saved registers (saved across calls) |
| $24-$25 | $t8-$t9 | More temporary registers |
| $26-$27 | $k0-$k1 | Reserved for kernel |
| $28 | $gp | Global pointer |
| $29 | $sp | Stack pointer |
| $30 | $s8/$fp | Frame pointer |
| $31 | $ra | Return address |
Only $16-$23 and $28-30 are saved across function calls. Register $28 is not preserved when calling position independent code.
More on $gp The global pointer points to the middle of a 64KB block of memory in the heap that holds constants and global variables. Objects in this heap can be quickly accessed with a single load or store instruction. The global pointer is set by the operating system when the program starts.