Cortex M3上的非拇指ARM代码

Question

I've got some ARM code that I'm trying to run on the Cortex M3. The majority of my code in written in Thumb, compiled from C - but for some functions I'd like to be able to run normal ARM code (as I understand it, this is possible on the M3?).

So...

Original C code and assembly:

int donothing(int a) {
  return a;
}


00000068 <donothing>:
  68:   e52db004    push    {fp}        ; (str fp, [sp, #-4]!)
  6c:   e28db000    add fp, sp, #0
  70:   e24dd00c    sub sp, sp, #12
  74:   e50b0008    str r0, [fp, #-8]
  78:   e51b3008    ldr r3, [fp, #-8]
  7c:   e1a00003    mov r0, r3
  80:   e28bd000    add sp, fp, #0
  84:   e8bd0800    ldmfd   sp!, {fp}
  88:   e12fff1e    bx  lr

Compiled using arm-none-eabi-gcc -mfloat-abi=soft -nostdinc -nostdlib

I branch to this with blx r4 - which should exchange if the base address&3 is 0, which it is.

Stepping through this in GDB, it HardFaults as soon as it gets to the line shown, even though the address contains the right data.

(gdb) p/x *0x2000934c 
$2 = 0xe52db004

The code (with BLX) works perfectly for Thumb code though...

Any idea what's going on? Thanks!

Answer 1

Cortex-M processors do not support ARM mode instructions. They only support Thumb-2, which includes a mixture of 16 and 32-bit instructions. There is no way to run ARM instructions on a Cortex-M.