Chapter 4 - Introduction to the Thumb Instruction Set

Abstract

This chapter introduces the Thumb instruction set. Thumb encodes a subset of the 32-bit ARM instructions into a 16-bit instruction set space. Since Thumb has higher performance than ARM on a processor with a 16-bit data bus, but lower performance than ARM on a 32-bit data bus, Thumb should be used for memory-constrained systems. Thumb has higher code density—the space taken up in memory by an executable program—than ARM. For memory-constrained embedded systems, for example, mobile phones and PDAs, code density is very important. Cost pressures also limit memory size, width, and speed. On average, a Thumb implementation of the same code takes up around 30% less memory than the equivalent ARM implementation. Even though the Thumb implementation uses more instructions, the overall memory footprint is reduced. Code density is the main driving force for the Thumb instruction set. The chapter provides a complete list of Thumb instructions available in the THUMBv2 architecture used in the ARMv5TE architecture.