Thumb set’s 16-bit instruction

The key idea behind Thumb is that of a super-reduced instruction set. Essentially, the

ARM7TDMI-S processor has two instruction sets:

• The standard 32-bit ARM set.

• A 16-bit Thumb set.

The Thumb set’s 16-bit instruction length allows it to approach twice the density of

standard ARM code while retaining most of the ARM’s performance advantage over a

traditional 16-bit processor using 16-bit registers. This is possible because Thumb code

operates on the same 32-bit register set as ARM code.

Thumb code is able to provide up to 65 % of the code size of ARM, and 160 % of the

performance of an equivalent ARM processor connected to a 16-bit memory system.

The particular flash implementation in the LPC2141/42/44/46/48 allows for full speed

execution also in ARM mode. It is recommended to program performance critical and

short code sections (such as interrupt service routines and DSP algorithms) in ARM

mode. The impact on the overall code size will be minimal but the speed can be increased

by 30 % over Thumb mode.