Problems with using registers as local variables for a function

The number of CPU registers is limited
CPU registers are shared:

I will illustrate what can go wrong if you use registers to store value while you call another function next

Example to show what can go wrong when you call a function

We call a square( ) function that computes the square of its input parameter in register r0 and returns the square value in register r0:

// result = square(3); /* ------------------------------------------------- Call square(3) ------------------------------------------------- */ mov r0, #3 // Pass parameter 3 in r0 bl square // Call square(3) /* ------------------------------------------------- Save return value (in r0) in variable result ------------------------------------------------- */ movw r1, #:lower16:result movt r1, #:upper16:result str r0, [r1]

DEMO: /home/cs255001/demo/asm/8-sub/reg-problem.s

Example to show what can go wrong when you call a function

Now let's executes: result = square(3) + square(4)

// result = square(3) + square(4); /* ------------------------------------------------- Call square(3) ------------------------------------------------- */ mov r0, #3 // Pass parameter 3 in r0 bl square // Call square(3) mov r1, r0 // Save return value in r1 /* ------------------------------------------------- Call square(4) ------------------------------------------------- */ mov r0, #4 // Pass parameter 4 in r0 bl square // Call square(4) add r0, r0, r1 // Computes square(3) + square(4) ?? /* ------------------------------------------------- Store in result ------------------------------------------------- */ movw r1, #:lower16:result movt r1, #:upper16:result str r0, [r1]

Fails because the "saved" value was lost !!!

Example to show what can go wrong when you call a function

Rule of a function call:

// result = square(3) + square(4); /* ------------------------------------------------- Call square(3) ------------------------------------------------- */ mov r0, #3 // Pass parameter 3 in r0 bl square // Call square(3) mov r1, r0 // Save value in r1 /* ------------------------------------------------- Call square(4) ------------------------------------------------- */ mov r0, #4 // Pass parameter 4 in r0 bl square // Call square(4) Values in all registers (except the return value) are invalid !!! add r0, r0, r1 // Computes square(3) + square(4) ?? /* ------------------------------------------------- Store in result ------------------------------------------------- */ movw r1, #:lower16:result movt r1, #:upper16:result str r0, [r1]

The safe way (place) to store parameters (and store local variables

The safest way to store parameters (and to store local variables) is:
(Registers can store different variables at different times !)
Because the parameters (and local variables) are created and destroyed in the LIFO order (= function call/return order) :