Part 3: MIPS Registers
What is a CPU register?
You can think of a CPU register abstractly as a variable or by analogy a box, it's something you can store values in.
A register is a special area of storage in a CPU that is used for math (addition, subtraction), moving values, and storage of pointers (references to memory)
In hardware, they are a bunch of flip flops, which you can see here
What are the MIPS register conventions?
MIPS features 32 general-purpose registers which can be addressed using register numbers $0 through $31 or by their alternative names such as $sp, $gp, etc.
This table is adapted from this site with modifications.
| Register Number | Alternative Name | Conventional Usage |
|---|---|---|
| $0 | $zero | The value zero, used to move values / NOP |
| $1 | $at | (assembler temporary) reserved by the assembler |
| $2 | $v0 | Used to store syscall number / to store return values of function calls like eax in x86 |
| $3 | $v1 | (value) from expression evaluation and function results |
| $4 - $7 | $a0 - $a3 | (arguments) first four parameters passed to a function, not expected to be saved across function calls |
| $8 - $15 | $t0 - $t7 | (temporaries) expected for functions to use to store temporary values, values not expected to be saved across function calls. |
| $16 - $23 | $s0 - $s7 | (saved values) callee saved, preserved across function calls. functions that use one of these registers are expected to save and restore previous values. |
| $24 | $t8 | (temporaries) the same as $t0 - $t7 |
| $25 | $t9 | (temporaries) the same as $t0 - $t8; however, $t9 is also used to store the address of a shared library during library function calls (example printf). |
| $26 - $27 | $k0 - $k1 | (kernel) reserved for use by the interrupt handler |
| $28 | $gp | (global pointer) points to static data area of program memory |
| $29 | $sp | (stack pointer) points to the top of the stack |
| $30 | $s8 / $fp | (saved value or frame pointer) pretty much the base pointer (ebp) in x86 |
| $31 | $ra | (return address) stores the return address during a function call |
Finally there is the program counter ($pc), which contains the current memory address of the instruction that the CPU is executing.
The table may be a lot to look at once so keep it handy. Here's a link of the table by itself for you to print out.
As a summary for the most important registers to know, $a0, $a1, $a2, and $a3 are used to pass arguments to functions, any register starting with t is temporary and thus when used, the value isn't expected to be saved across function calls, whereas any register beginning with s is saved and expected to persist across function calls.