Programmatically cause Undefined Instruction exception

Question

I want to cause an ARM Cortex-M3 Undefined Instruction exception for the test of my test fixture. The IAR compiler supports this with inline assembly like this:

asm("udf.w #0");

Unfortunately the GNU CC inline assembler does not know this opcode for the NXP LPC177x8x. It writes the diagnostic:

ccw3kZ46.s:404: Error: bad instruction `udf.w #0'

How can I create a function that causes a Undefined Instruction exception?

Answer 1

Building on Masta79's answer:

There is a "permanently undefined" encoding listed in the ARMv7-M architecture reference manual - ARM DDI 0403D (documentation placeholder, registration required). The encoding is 0xf7fXaXXX (where 'X' is ignored). Of course instruction fetches are little-endian, so (without testing):

asm volatile (".word 0xf7f0a000\n");

should yield a guaranteed undefined instruction on any ARMv7-M or later processor.