What are the types of microinstructions available?

A microinstruction is one word stored in the control memory. It holds the control signals for one step. There are two main types.

• Horizontal microinstruction: each control signal has its own bit. The word is long, but many signals can be active together, so it is fast.
• Vertical microinstruction: the bits are encoded into small fields. A decoder gives the signals. The word is short, but decoding makes it slower.

Some books also mention a third type, the nanoinstruction, used in a two-level control store.