Another usage of the stack

Recall we had trouble performing the following computation:

result = square(3) + square(4);

This code will not work all the time:

/* ------------------------------------------------- Call square(3) ------------------------------------------------- */ mov r0, #3 bl square mov r10, r0 // Save 3^2 in r10 *** FAIL !!! /* ------------------------------------------------- Call square(4) ------------------------------------------------- */ mov r0, #4 bl square <--- r10 overwritten by this call add r0, r0, r10 // FAIL to compute 3² + 4²

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

Recall the important facts to notice in the last example

Before main( ) make a function call, the program stack was empty
After main( ) has clean up the parameters in the program stack, the program stack is empty again !!!

In general: the stack right before and right fater a function call are equal in content

.... instr1 <------+ push parameters | The content of the bl f | runtime stack are pop parameters | equal at these instructions instr2 <------+

Using the stack to perserve values in registers

There is a very simple technique to perserve values in registers when you need to make a function call:

.... // How to perserve the value in register r0 // we need to make a function call push {r0} // Save r0 on stack <------+ push parameters | The content of the bl f | runtime stack are pop parameters | equal at these instructions <------+ push {r0} // Restore the old r0 value

Using the stack to perserve values in registers

Example: result = square(3) + square(4);

mov r0, #3 bl square push {r0} // Save 3² in stack *** !!! mov r0, #4 bl square pop {r1} // Retrieve 3² from stack *** !!! add r0, r0, r1 // Computes 3²+ 4² /* ------------------------------------------------- Store in result ------------------------------------------------- */ movw r1, #:lower16:result movt r1, #:upper16:result str r0, [r1]

DEMO: /home/cs255001/demo/asm/reg-problem.s
Note: this technique will also work when parameters + local vars are in the stack