Editing Microcode (section)

===Microprograms===

Microprograms consist of series of microinstructions, which control the CPU at a very fundamental level of hardware circuitry. For example, a single typical ''horizontal'' microinstruction might specify the following operations:
* Connect register 1 to the ''A'' side of the [[Arithmetic logic unit|ALU]]
* Connect register 7 to the ''B'' side of the ALU
* Set the ALU to perform [[Two's complement|two's-complement]] addition
* Set the ALU's carry input to zero
* Store the result value in register 8
* Update the condition codes from the ALU status flags (''negative'', ''zero'', ''overflow'', and ''carry'')
* Microjump to a given [[program counter|μPC]] address 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 [[IBM System/360 Model 85|360/85]] with an emulator feature. Microprograms are carefully designed and optimized for the fastest possible execution, as a slow microprogram would result in a slow machine instruction and degraded performance for related application programs that use such instructions.