Computers as Components, 3rd Edition

3.4 Co-Processors

CPU architects often want to provide flexibility in what features are implemented in the CPU. One way to provide such flexibility at the instruction set level is to allow co-processors, which are attached to the CPU and implement some of the instructions. For example, floating-point arithmetic was introduced into the Intel architecture by providing separate chips that implemented the floating-point instructions.

To support co-processors, certain opcodes must be reserved in the instruction set for co-processor operations. Because it executes instructions, a co-processor must be tightly coupled to the CPU. When the CPU receives a co-processor instruction, the CPU must activate the co-processor and pass it the relevant instruction. ...