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2 definitions found
 for microcode
From WordNet (r) 3.0 (2006) :

      n 1: (computer science) coded instructions that are stored
           permanently in read-only memory [syn: firmware,

From The Free On-line Dictionary of Computing (30 December 2018) :

      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 interrupts between each
     macroinstruction.  Writing microcode is known as
     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
     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.

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