The Thumb Extension (Thumb-1)

Thumb instructions are 16-bits wide


Code is more compact
2-operand operations
Only a subset of the registers can be accessed
Some "special" instructions are not supported.

BX and BLX (branch (and link) and exchange) switch mode
Bit 0 of the program counter is used as a selector
ARM instructions run at even addresses
Thumb instructions (appear to) run at odd addresses