microinstruction format Related: Microinstruction Format - Computer O...
There are several formats for encoding the control signals in a control word. In this section, we will discuss the following three control word formats:
Horizontal Format, called Horizontal microcode
Vertical Format, called Vertical microcode
Field-encoded Format
Each of these formats provides a tradeoff in terms of the control store size and the speed of operation of the control unit.
Horizontal Microcode
In the horizontal format, each control signal is represented by a single bit in the control word. Thus, if the design has 500 control signals, this will require 500 bits in each control word to store the control bits. In this format, the control store looks horizontal in shape since the control words are wide.
The disadvantage of the horizontal format is that the size of the control store is large. However, it has the advantage of speed of operation as the control signals will be ready as soon as the control word is fetched from the control store.
Vertical Microcode
In the vertical microcode organization, the following steps are performed:
Identify the number of distinct control words in the design
Encode each distinct control word by assigning a unique n-bit code to it, where n is log2 (number of distinct control words)
Instead of storing the actual control signals that need to be generated, only the n-bit code is stored for each CW
Use a nx2n decoder to generate a decoded signal for each distinct control word
To generate the control signals, use an OR gate based on the decoded control word signals, for each control signal in the design.