Must the nop follow the jr?

Answer:

Yes. (Assuming a real MIPS chip with branch delays.) At run-time, there is no predicting what will be in memory after this jr.

This subroutine will be linked with other subroutines so we better be careful. It is likely that the linker will put another subroutine immediately after this one in memory, and that subroutine's prolog starts with:

sub     $sp,$sp,4      # push the return address
sw      $ra,($sp)

It would be disastrous to unintentionally execute that first instruction when our subroutine returns to its caller.

Maximum of Three Expressions

For the next example, the main program asks the user for integers x and y and writes out the maximum of x*x, x*y, and 5*y. Look ahead to page 22 to see main.

main uses this sub-task:

Write a subroutine that returns the maximum of three expressions: x*x, x*y, and 5*y for integer arguments x and y. The integers use two's complement representation. Use stack-based linking.

With our stack-based linkage convention this is:

Subroutine maxExp

  Input:   $a0 -- x, a two's complement integer
           $a1 -- y, a two's complement integer
           
  Returns: $v0 -- the maximum of x*x, x*y, or 5*y

Answer:

Maximum of Three Expressions

QUESTION 15: