Calling ARM assembly from C, GCC (bare metal)

gcc arm qemu thumb armv6
singpolyma picture singpolyma · Jan 22, 2012 · Viewed 8.6k times · Source

I am trying to do some bare-metal programming in ARM with GCC and testing on QEMU. Whenever I call into an ARM label from C, my program hangs. I have a simple example of code that shows the problem at https://gist.github.com/1654392 -- when I call activate() in that code, it hangs.

I have observed with objdump that when I do a bl from assembly to C code (as from _start) it is generating a small wrapper that switches to thumb instructions. It seems that the C code is all being generated in thumb instructions, but all my assembly is being generated in ARM (32-bit) instructions. I cannot figure out why this is or how to fix it.

Answer

Sasa picture Sasa · Apr 20, 2013

In order to call an ARM mode function defined in assembly from a THUMB mode function defined in C, you need to define a symbol in assembly as a function, and the tools (Linaro gcc) will produce a blx instruction instead of bl.

Example:

@ Here, we suppose that this part of code is inside of .code 32

.type fn, %function

fn:
   mov  pc, lr

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