Q. Explain about Micro-instruction Formats?

Now let's focus on format of a micro-instruction. The two widely used formats employed for micro-instructions are vertical and horizontal. In horizontal micro-instruction every bit of micro-instruction signifies a control signal that directly controls a single bus line or occasionally a gate in the machine. Though the length ofsuch a micro-instruction can be hundreds of bits. A typical horizontal micro-instruction with its related fields is demonstrated in Figure below.

(a)   Horizontal Micro-instruction

(b)   Vertical Micro-instructions

Figure: Micro- instruction Formats

In a vertical micro-instruction several similar control signals can be encoded in a few micro-instruction bits. For example for 16 ALU operations that may need 16 individual control bits in horizontal micro-instruction only 4 encoded bits are required in vertical micro-instruction. In the same way in a vertical micro-instruction only 3 bits are required to select one of eight registers. Though these encoded bits need to be passed from the respective decoders to get the individual control signals. This is shown in figure above.

Generally a horizontal control unit is faster however requires wider instruction words while vertical control units though; require a decoder are shorter in length. Most of systems employ neither purely vertical nor purely horizontal micro-instructions figure (c) above.