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ARM System Developer's Guide by Chris Wright, Dominic Symes, Andrew Sloss

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CHAPTER 4

INTRODUCTION TO THE THUMB INSTRUCTION SET

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, use Thumb for memory-constrained systems.

Thumb has higher code density—the space taken up in memory by an executable program—than ARM. ...

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