ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

A1.2 The ARM and Thumb instruction sets
Introduction to the ARM Architecture
The ARM instruction set is a set of 32-bit instructions providing comprehensive data-processing and control
functions.
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.
In ARMv6T2, Thumb-2 technology is introduced. This technology makes it possible to extend the original
Thumb instruction set with many 32-bit instructions. The range of 32-bit Thumb instructions included in
ARMv6T2 permits Thumb code to achieve performance similar to ARM code, with code density better than
that of earlier Thumb code.
From ARMv6T2, the ARM and Thumb instruction sets provide almost identical functionality. For more
information, see Chapter A4 The Instruction Sets.
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