A proper way to do it in this context would be to push the link register onto the stack at the start of the function, immediately execute your subroutine, and then at the end of the function pop the link register off the stack and put it in the program counter. You can use
push {lr} and
pop {pc} for that. This way the return address is not lost, and restored properly without meddling with other registers. Here's a made-up example:
Code:
Original: Change to:
mov r0, #0xA --- push {lr}
mov r1, #0xB \__ bl 0xC0DE /* pun intended */
add r0, r0, r1 /
... ...
bx lr --- pop {pc}
Instructions that have been overwritten should be executed first thing in your subroutine (not per se, but unless you know exactly what you're doing just assume that is true). For GBA games those instructions will most likely be THUMB code, so one 16-bit instruction that has been overwritten with the push instruction and then two more that had to make room for the single 32-bit branch and link instruction.
In your subroutine at 0xC0DE:
Code:
mov r0, #0xA
mov r1, #0xB
add r0, r0, r1
# Injection code here, but don't forget to push/pop registers you want to use!
bx lr
I've fixed your patch locally to test, and verified this approach will solve your issues. Good luck!
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Psionic Roshambo:
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Xdqwerty:
