In computer processors, the instruction that the CPU reads from memory determines what the CPU will do. In the decoding stage, performed by circuitry known as the instruction decoder, the instruction is converted into signals that control other parts of the CPU.
The way the instruction is interpreted is defined by the instruction set architecture (ISA) of the CPU. Often, a group of bits (that is, a «field») within the instruction calls the opcode, indicating what operation is being performed, while the remaining fields usually provide supplemental information required for the operation, such as the operands. . Those operands can be specified as a constant value (called an immediate value), or as the location of a value that can be a processor register or a memory address, as determined by some addressing mode.
In some CPU designs the instruction decoder is implemented as a hardwired, unchangeable circuit. In others, a microprogram is used to translate instructions into sets of CPU configuration signals that are applied sequentially over multiple clock ticks. In some cases, the memory that stores the firmware is rewritable, allowing you to change the way the CPU decodes instructions.
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