Question

I have a library written in C mixed mixed with some assembly for ARM. It used to be compiled for armv6. Now I am trying to upgrade it to armv7. However, there is an interrupt handler which has the instruction stmdb sp!, {pc} which is not allowed in armv7. What would be an equivalent instruction on armv7? I tried str r15, [sp, #-4]! but that doesn’t work.

Answer 1

There’s specific limitations regarding the use of PC in the reg list for PUSH and POP instructions, depending on the operation mode, see:

ARM Instruction Set Reference, PUSH/POP

Specifically, in Thumb[2] there is no push {pc} operation (which is the same as saying there’s no stmfd sp!, {pc} operation – push is mapped to stmfd sp!).

So if you’re compiling your kernel code (why else do you have/need an interrupt handler if it’s not kernel code), check whether you’re compiling a Thumb-2 kernel.

That said, you’re saying you encountered a stmfd sp!,{pc} instruction – you’re sure that’s not a typo ? stmfd sp!,{lr} (and its sibling, ldmfd sp!, {pc} – here PC is present) is normal / fully legal and often required / encountered in both ARM mode and Thumb-2 mode. But that actually is the purpose of saving the program counter to the stack ? What can one do with that that wouldn’t be achievable in other ways ?