Past exam of the year 1 of the computer science course of the University of Oxford / 2016 / Digital systems, linear algebra, introduction to formal proof / 3 / a / Solution

The calling convention passes the integer argument in $a0, returns the result in $v0, and uses jal with $ra for a call and its return address. The callee returns using jr $ra. Its stack frame grows downwards from $sp and must be removed before returning, restoring the caller's stack pointer.

The $s processor registers are callee-saved registers: a callee that uses $s0 or $s1 must restore their incoming values. The $a, $v, and $t processor registers are caller-saved registers; a caller must preserve any of their values that it needs after a call. Consequently foo keeps its own argument in $s0 and the first call's result in $s1, where nested calls preserve them. Because a nested jal overwrites $ra, foo also saves its incoming return address. Every recursive activation has its own stack frame.

The supplied MIPS instructions use the paper's simplified convention: pseudoinstructions such as subi are accepted, and control transfers have no branch delay slot. The completed code below follows that convention. A hardware implementation with delayed control transfers requires suitable scheduling or nop instructions throughout the supplied code as well.