What are callee and caller saved registers?

Question

I'm having some trouble understanding the difference between caller and callee saved registers and when to use what.

I am using the MSP430 :

procedure:

mov.w #0,R7 
mov.w #0,R6 
add.w R6,R7 
inc.w R6 
cmp.w R12,R6 
jl l$loop 
mov.w R7,R12
ret

the above code is a callee and was used in a textbook example so it follows the convention. R6 and R7 are callee saved and R12 is caller saved. My understanding is that the callee saved regs aren't "global" in the sense that changing its value in a procedure will not affect it's value outside the procedure. This is why you have to save a new value into the callee reg at the beginning.

R12, the caller saved is "global", for lack of better words. What the procedure does has a lasting effect on R12 after the call.

Is my understanding correct? Am I missing other things?

Answer 1

Caller-saved registers (AKA volatile registers, or call-clobbered) are used to hold temporary quantities that need not be preserved across calls.

For that reason, it is the caller's responsibility to push these registers onto the stack or copy them somewhere else if it wants to restore this value after a procedure call.

It's normal to let a call destroy temporary values in these registers, though.

Callee-saved registers (AKA non-volatile registers, or call-preserved) are used to hold long-lived values that should be preserved across calls.

When the caller makes a procedure call, it can expect that those registers will hold the same value after the callee returns, making it the responsibility of the callee to save them and restore them before returning to the caller. Or to not touch them.