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The Definitive Guide to the ARM Cortex-M3 by Joseph Yiu

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CHAPTER 3 Cortex-M3 Basics

Registers

As we’ve seen, the Cortex-M3 processor has registers R0–R15 and a number of special registers. R0–R12 are general purpose, but some of the 16-bit Thumb instructions can only access R0–R7 (low registers), whereas 32-bit Thumb-2 instructions can access all these registers. Special registers have predefined functions and can only be accessed by special register access instructions.

General-Purpose Registers R0–R7

The R0–R7 general-purpose registers are also called low registers. They can be accessed by all 16-bit Thumb instructions and all 32-bit Thumb-2 ...

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