Microcode - Overview


The elements composing a microprogram exist on a lower conceptual level than a normal application program. Each element is differentiated by the "micro" prefix to avoid confusion: microinstruction, microassembler, microprogrammer, microarchitecture, etc.

The microcode usually does not reside in the main memory, but in a special high speed memory, called the control store. It might be either read-only or read-write memory. In the latter case the microcode would be loaded into the control store from some other storage medium as part of the initialization of the CPU, and it could be altered to correct bugs in the instruction set, or to implement new machine instructions.

Microprograms consist of series of microinstructions. These microinstructions control the CPU at a very fundamental level of hardware circuitry. For example, a single typical microinstruction might specify the following operations:

  • Connect Register 1 to the "A" side of the ALU
  • Connect Register 7 to the "B" side of the ALU
  • Set the ALU to perform two's-complement addition
  • Set the ALU's carry input to zero
  • Store the result value in Register 8
  • Update the "condition codes" with the ALU status flags ("Negative", "Zero", "Overflow", and "Carry")
  • Microjump to MicroPC nnn for the next microinstruction

To simultaneously control all processor's features in one cycle, the microinstruction is often wider than 50 bits, e.g., 128 bits on a 360/85 with an emulator feature. Microprograms are carefully designed and optimized for the fastest possible execution, since a slow microprogram would yield a slow machine instruction which would in turn cause all programs using that instruction to be slow.

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