Micro- programmed Control Unit

8.1 Micro- programmed Control Unit :
In micro programmed control, the control signals associated with operations are stored in special memory units inaccessible by the programmer as control words. A control word is a microinstruction that specifies one or more micro operations. A sequence of microinstructions is called a micro program, which is stored in a ROM or RAM called a control memory (CM).
A micro program consists of a sequence of micro instructions. A microinstruction is a vector of bits, where each bit is a control signal, condition code, or the address of the next microinstruction. Microinstructions are fetched from CM the same way program instructions are fetched from main memory:

When an instruction is fetched from memory, the op-code field of the instruction will determine which microprogram is to be executed. In other words, the op-code is mapped to a microinstruction address in the control memory.
8.2 How to generate Control Words?
Microinstructions can be classified as horizontal or vertical.Individual bits in horizontal microinstructions correspond to individual control lines.Horizontal microinstructions are long and allow maximum parallelism since each bit controls a single control line. This method requires arranging micro operations and registers (taking into consideration that there are two situations for each a register in/out) according to a particular sequence.