The Thumb Instruction Set (T variants)

It is a subset of the ARM instruction set. The thumb instruction set has 16 bit instructions. i.e. the Thumb instructions are half the size of ARM instructions.

The Thumb instruction set has greater code density than the ARM instruction set.

The drawbacks of the Thumb instruction set are that it requires more instructions to complete the same task. Hence the ARM instruction set is best for maximizing the performance.

The Thumb instruction set does not include instructions required for exception handling.

Due to these drawbacks the Thumb instruction set is used in conjunction with a suitable version of the ARM instruction set.

The Thumb instruction set is not valid prior to version 4 of the ARM architecture. It is signified by letter T.

Thumb instruction set versions

There are two versions of the Thumb instruction set i.e. version 1 and version 2.

The version 1 of the Thumb instruction set is used in T variants of the ARM architecture version 4 and version 2 is used in T variants of the ARM architecture version 5.

The Thumb instruction set version 2 modifies the instructions to improve the efficiency of ARM interworking.

The Thumb instruction set tightens the definition of how flags are set by multiply instructions.

The Thumb instruction set includes the breakpoint instruction.

These modifications are related to the changes between ARM architecture versions 4 and 5.