# Linux Kernel Module Cheat

#### 30.5.2. ARM UDF instruction

Guaranteed undefined! Therefore raise illegal instruction signal. Used by GCC `__builtin_trap` apparently: https://stackoverflow.com/questions/16081618/programmatically-cause-undefined-instruction-exception

- userland/arch/arm/udf.S

- userland/arch/aarch64/udf.S

Why GNU GAS 2.29 does not have a mnemonic for it in A64 because it is very recent: shows in ARMv8 architecture reference manual db but not `ca`.