Chapter 6

Thumb Instruction Set


  • Presenting Thumb
  • What is Thumb used for?
  • What cores run Thumb?
  • What are the advantages of Thumb?
  • How to switch between ARM and Thumb
  • Writing for Thumb

Many of the most popular 32-bit processors for mobile devices use Reduced Instruction Set Computer (RISC) technology. Unlike Complete Instruction Set Computer processors (CISC), Reduced Instruction Set Computer engines generally execute each instruction in a single cycle, often resulting in faster program execution using the same clock speed.

Increased performance, however, comes at a price: a RISC processor typically needs more memory than a CISC does to store the same program. To achieve the same results as a single CISC instruction, RISC engines often require two, three, or more simpler instructions. For most embedded devices, memory constraints are more important than execution speed, so reducing code size is important.

In 1995, ARM released the Thumb instruction set, used for the first time on the ARM7TDMI core. Thumb instructions are denser than their ARM counterparts, being 16-bits long in the original Thumb extension. All Thumb instructions map directly to ARM instructions, but to save space, the instructions were simplified.

Thumb was introduced not only for the denser code, but also for devices that did not have full 32-bit wide memory access. One of the first devices to use Thumb, the Game Boy Advance, had little memory that was accessible in 32-bits; most of the ...

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