Chapter 5. Instruction Set
Background of ARM and Thumb Instruction Set
The early ARM processors use a 32-bit instruction set called the ARM instructions. The 32-bit ARM instruction set is powerful and provides good performance, but at the same time it often requires larger program memory when compared to 8-bit and 16-bit processors. This was and still is an issue, as memory is expensive and could consume a considerable amount of power.
In 1995, ARM introduced the ARM7TDMI processor, adding a new 16-bit instruction set called the Thumb instruction set. The ARM7TDMI supports both ARM instructions and Thumb instructions, and a state-switching mechanism is used to allow the processor to decide which instruction decode scheme should be used (Figure 5.1 ...
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