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Figure 13.10 ARM Instruction Formats
2. Thumb has only a subset of the operations in the full instruction set and uses only a 2-bit opcode field,
plus a 3-bit type field. Savings: 2 bits.
3. The remaining savings of 9 bits comes from reductions in the operand specific cations. For example,
Thumb instructions reference only registers r0 through r7, so only 3 bits are required for register
references, rather than 4 bits. Immediate values do not include a 4-bit rotate field.
Figure 13.11 Examples of Use of ARM Immediate Constants
The ARM processor can execute a program consisting of a mixture of Thumb instructions and 32-bit ARM
instructions.
A bit in the processor control register determines which type of instruction is currently being executed.
Figure 13.12 shows an example. The figure shows both the general format and a specific instance of an
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