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(

Answer 1

There are different official undefined instructions in [1] (look for UDF; . below can be replaced with any hex digit):

• 0xe7f...f. - ARM or A1 encoding (ARMv4T, ARMv5T, ARMv6, ARMv7, ARMv8)
• 0xde.. - Thumb or T1 encoding (ARMv4T, ARMv5T, ARMv6, ARMv7, ARMv8)
• 0xf7f.a... - Thumb2 or T2 encoding (ARMv6T2, ARMv7, ARMv8)
• 0x0000.... - ARMv8-A permanently undefined

There are others, but these are probably as future-proof as it gets.

As for generating the code that triggers it, you can just use .short or .word, like this:

• asm volatile (".short 0xde00\n"); /* thumb illegal instruction */
• asm volatile (".word 0xe7f000f0\n"); /* arm illegal instruction */
• asm volatile (".word 0xe7f0def0\n"); /* arm+thumb illegal instruction */

Note that the last one will decode to 0xdef0, 0xe7f0 in thumb context, and therefore also cause an undefined instruction exception.

[1] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.subset.architecture.reference/index.html

[2] DDI0487D_b_armv8_arm.pdf