Microcode

2 definitions found

From WordNet (r) 2.0 [wn]:

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

From The Free On-line Dictionary of Computing (27 SEP 03) [foldoc]:



  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)