Microprogrammed control

Also called: microprogramming, microcode, microprogrammed control unit

A control unit that stores each step's control signals as a microinstruction in a small ROM, stepping through them with a micro-PC.

In microprogrammed control, the control signals aren't computed by gates. They're stored. Each step of each instruction gets a microinstruction: a word holding one bit per control signal, plus information about which microinstruction comes next. The words live in a small ROM called the control store.

A register called the micro-PC points at the current microinstruction, just as the program counter points at the current instruction. Each clock cycle:

the microinstruction at the micro-PC drives the control signals;
the micro-PC moves on, to the next word or to a jump target. After decode, it jumps to the microprogram for the current opcode.

So a machine instruction like ADD is run by a short microprogram: a few microinstructions, one per step.

Trade-offs compared with hardwired control:

Easier to change: a new instruction is new words in the control store, plus an entry pointing its opcode there. The datapath may still need new wires.
Usually slower per step, because each step needs a ROM read.
Good for large, complex instruction sets. Today's x86 chips, for instance, run their most complicated instructions from microcode, and a firmware patch can change that microcode after the chip has shipped.

Worked example

Example

How wide is a microinstruction?

A control unit drives 14 control signals. Each microinstruction holds one bit per signal plus a next-address field that can point anywhere in a 64-word control store.

1.
Control bits: 14.
2.
Next address: 2⁶ = 64, so 6 bits.
3.
Width: 14 + 6 = 20 bits.
4.
The whole control store holds 64 × 20 = 1280 bits.

Common mistakes

Confusing the micro-PC with the program counter. One steps through microinstructions, the other through machine instructions.
Thinking microprogramming removes the need for datapath changes. A new transfer still needs a wire.
Assuming microprogrammed control is faster. It's usually more flexible but slower per step.

Practice Microprogrammed control

Interactive questions with instant feedback and a worked solution for every wrong answer.

Basic CPU / Computer Architecture lesson full course

Practice exams

Learn it step by step

Microprogrammed control is taught in Basic CPU / Computer Architecture.

Basic CPU / Computer Architecture →

All terms