Thumb is a subset of the ARM instruction set encoded in 16-bit-wide instructions.

• Requires 70% of the space of ARM code.
• Uses 40% more instructions than equivalent ARM code.

A CPU has Thumb support if it has a T in its name, or it is architecture v6 or later.

With 32-bit memory:

ARM code is 40% faster than Thumb code.

With 16-bit memory:

Thumb code is 45% faster than ARM code.

Uses 30% less external memory power than ARM code.

Thumb is not a complete architecture: you can’t have a Thumb-only CPU.

Some of the limitations of Thumb mode include:

• Conditional execution only exists for branch instructions.
• Data processing ops use a two-address format, as opposed to ARM’s three-address format.
• Its instruction encodings are less regular than ARM’s.

Thumb uses the same register set as ARM — but only R0-R7.