5.41 · exercise
The vanishing value
warning
This program is broken. Find the bug and fix it so the checks pass.
This program reads two values into registers, swaps them, then prints them in their new order. Run it: one of the values has vanished, and the other now appears twice.
Trace the two mov instructions of the swap by hand and watch where the first value goes. Fix the swap so both values survive; the starter already names a spare register tmp_r if you want one.
What is checked
- both values appear, in swapped order
- the program exits cleanly
- the swap really happens in the registers:
x19andx20end up holding each other's values, so reordering the printf arguments is not enough - the same holds for other pairs you do not see, equal and negative ones among them
specification
stdin123 87
stdoutprints the right output
exitexits with the right code
registerleaves the right value in x19
registerleaves the right value in x20
hiddenright output and exit code on 4 more inputs you do not see
We run your program on the input above and on the hidden ones, and compare what it does. Nothing is matched against a stored solution.