Many ARM cores provide two instruction sets, ARM and Thumb.

Thumb is a hybrid 16/32-bit instruction set

No Thumb instructions can be conditionally executed

Thumb code is always slower than the equivalent ARM code

Some routines take more instructions in Thumb code than in the equivalent ARM code

The Thumb instruction set can access the Advanced SIMD "NEON" instructions

Thumb code is always more power-efficient than equivalent ARM code