5.61 · exercise

Predict: Subroutine State and Linkage

Act as the CPU. Trace the register states and stack allocations that a subroutine call sets up.

0 of 4 correct

Mental Trace

// the bl below sits at address 0x400040
bl swap
add w0, w0, 1

After the bl executes, what value does x30 hold? Answer in hex, written as 0x followed by the digits.

Mental Trace

// C, where sum takes ten int arguments
sum(10, 20, 30, 40, 50, 60, 70, 80, 90, 100);

How many bytes must the calling code allocate on the stack for the arguments that do not fit in registers? Answer in decimal.

Mental Trace

cube:
    mul w9, w0, w0
    mul w0, w9, w0
    ret

cube calls nothing and writes only w0 and w9. How many bytes does it allocate on the stack? Answer in decimal.

Mental Trace

// C, where struct mystruct is 64 bytes of int members
struct mystruct lvar;
return lvar;

This structure is far too wide to come back in the result registers, so the caller passes a hidden pointer to the space that receives it. Which register carries that pointer? Write the register name, for example x0.