the forgetful helper

main keeps a running total of 100 in x19callee-saved register (x19-x28): a routine must restore it before it returns., asks a helper to double a bonus of 20, and adds the result. The total should be 140. Run it: the program says something else entirely.

The helper works. The arithmetic works. The bug is a broken promise: AAPCS64 says x19callee-saved register (x19-x28): a routine must restore it before it returns. through x28callee-saved register (x19-x28): a routine must restore it before it returns. belong to the caller, so a subroutine that touches them must save and restore them. Scratch work belongs in x9caller-saved temporary (x9-x15): a called routine may overwrite it. through x15caller-saved temporary (x9-x15): a called routine may overwrite it., which the caller must assume are gone after any blBranch with link; writes return address to X30..

Fix the helper so it stops trampling its caller's registers.

what is checked

• the correct total is printed
• the program exits cleanly
• the helper is still called (blBranch with link; writes return address to X30. present)