microcode
microprogramming

    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
   "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)