How to use MIPS $k0 and $k1 registers

Question

I was wondering, what are $k0 and $k1 registers in MIPS architecture. As there is on WikiBooks MIPS Assembly

The k registers are reserved for use by the OS kernel.

But I couldn't find anything about what are they usefull for? and also how to use them?

Thank you.

Answer 1

An interrupt handler must save any general - purpose registers that it is going to use (to be restored at return). But to do so requires you to modify at least one register first (something like sw $t0, saved_t0 expands to two machine instructions using $at).

This situation is resolved by forbidding user programs from using two general - purpose registers, $k0 and $k1 (The k stands for kernel, which an exception handler is part of). The interrupt handler is allowed to use $k0 and $k1 without having to save or restore their values. This allows just enough leeway to start saving registers, as well as making returning from the interrupt handler possible.