5.49 · exercise

The forgetful helper

warning

This program is broken. Find the bug and fix it so the checks pass.

main reads a running total into x19 and a bonus, asks a helper to double the bonus, and adds the result. With 100 and 20 the total should be 140. Run it: it prints 80.

The helper doubles correctly, but it uses x19 as scratch. ARM's rules for function calls make x19 through x28 callee-saved: any function that changes them must put them back before it returns, because its caller is still counting on them. Scratch work belongs in x9 through x15, which a caller must assume are gone after any bl.

Fix the helper so it does its scratch work in a scratch register and leaves x19 alone.

What is checked

  • the correct total is printed
  • the program exits cleanly
  • the helper is still called (bl present), and x19 no longer appears in double_it
  • the same holds for other totals and bonuses you do not see, zero and negative ones among them

specification

stdin100 20
stdoutprints the right output
exitexits with the right code
sourceuses bl
sourcedoes not use x19 or w19 in double_it
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.

loading editor...

regfile

N clearZ clearC clearV clear

x0–x30 are the integer registers.

X0arg00x0000000000000000
X1arg10x0000000000000000
X2arg20x0000000000000000
X3arg30x0000000000000000
X4arg40x0000000000000000
X5arg50x0000000000000000
X6arg60x0000000000000000
X7arg70x0000000000000000
X8ind0x0000000000000000
X90x0000000000000000
X100x0000000000000000
X110x0000000000000000
X120x0000000000000000
X130x0000000000000000
X140x0000000000000000
X150x0000000000000000
X16ip00x0000000000000000
X17ip10x0000000000000000
X18pr0x0000000000000000
X190x0000000000000000
X200x0000000000000000
X210x0000000000000000
X220x0000000000000000
X230x0000000000000000
X240x0000000000000000
X250x0000000000000000
X260x0000000000000000
X270x0000000000000000
X280x0000000000000000
X29fp0x0000000000000000
X30lr0x0000000000000000
SP0x0000000080000000
PC0x0000000000400000
console

Output prints here as your program runs.

Press step or run under the editor, or feed stdin from the box below.

not assembled

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