资料来源 : pyDict

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资料来源 : WordNet®

microcode
     n : (computer science) coded instructions that are stored
         permanently in read-only memory [syn: {firmware}]

资料来源 : Free On-Line Dictionary of Computing

microcode
     
         A technique for implementing the {instruction
        set} of a processor as a sequence of microcode instructions
        ("microinstructions"), each of which typically consists of a
        (large) number of bit fields and the address of the next
        microinstruction to execute.  Each bit field controls some
        specific part of the processor's operation, such as a gate
        which allows some {functional unit} to drive a value onto the
        {bus} or the operation to be performed by the {ALU}.  Several
        microinstructions will usually be required to fetch, decode
        and execute each {machine code} instruction
        ("{macroinstruction}").  The microcode may also be responsible
        for {polling} for hardware {interrupt}s between each
        macroinstruction.  Writing microcode is known as
        "microprogramming".
     
        Microcode may be classified as "horizontally encoded" or
        "vertically encoded".  Horizontal microcode is as described
        above where there is a fairly direct correspondence between
        the bit fields in a microinstruction and the control signals
        sent to the various parts of the CPU.  Not all combinations of
        bits will be valid (e.g. two units driving the bus at once).
        Vertical microcode is closer to {machine code} because a bit
        field value may pass through some intermediate combinatory
        logic which generates the actual control signals.  This allows
        a few bits of a microinstruction to determine several control
        signals and ensure that only valid combinations of those
        signals are generated (e.g. a field may be decoded to determine
        which unit drives the bus).  The disadvantage with vertical
        encoding is that the encoding is usually fixed and takes extra
        time compared with horizontal encoding which allows any
        combination of signals to be generated and takes no time to
        decode.
     
        The alternative to a microcoded processor is a {hard-wired}
        one where the control signals are generated directly from the
        bits of the {machine code} instruction.  This is more common
        in modern {RISC} architectures because it is faster.
     
        Microcode is usually stored in {ROM} chips though some
        processors (e.g. the {Orion}) use fast RAM, making them
        dynamically microprogrammable.
     
        (1996-11-26)