The Thumb Extension (ARM7 = ARMv4)

Thumb instructions are 16-bits wide
The idea is to "expand" instructions on the fly
Pro: It took very little logic in the core
Pro: Code is much more compact
Con: Only a subset of the registers can be accessed
Con: 2-operand operations (ARM has 3-operand ops)
Con: No conditional ops nor some other interesting features

BX and BLX (branch (and link) and exchange) switch mode