Thumb instruction set

The Arm architecture is a family of reduced instruction set computing (RISC) architectures for computer processors. 

The ARM instruction set is a set of 32-bit instructions providing comprehensive data-processing and control functions. It defines the supported instructions, data types, registers, etc. for managing a system containing an Arm processor (Central Processing Unit).

The Thumb instruction set was developed as a 16-bit instruction set with a subset of the functionality of the ARM instruction set. It provides significantly improved code density, at a cost of some reduction in performance. A processor executing Thumb instructions can change to executing ARM instructions for performance critical segments, in particular for handling interrupts.