It is the implementation of a hardware CPU with some help from software rather than building the entire CPU as hard wired logic. Hence, these are the hardware level instructions which can directly control the microprocessor. A single machine code may translate into many microcode instructions on the microprocessor level which may map to a particular field of a larger instruction like a single bit-field in the ALU or a complete instruction depending on the type of microprocessor. It is generally written during the design of a microprocessor in the ROM and is generally not program-addressable.
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