Hi,

Peter is right, you must clear carry before subb command.
But I think there could be some other reason for hanging. You must check the codes after you used command subb.
You may have not returned properly from some subroutine or Interrupt and cpu program counter is pointing to beyond the actual code limits, or reading some non-related codes.

In simulator you might not have checked your full code. check the full code, you will find the clue.

Rauf