Question

Explain the organisations of micro-programmed control unit in detail?

Answer 1

Micro-programmed Control Unit –

The control signals associated with operations are stored in special memory units inaccessible by the programmer as Control Words.Control signals are generated by a program are similar to machine language programs.Micro-programmed control unit is slower in speed because of the time it takes to fetch microinstructions from the control memory.

Some Important Terms –



1) Control Word : A control word is a word whose individual bits represent various control signals.
2) Micro-routine : A sequence of control words corresponding to the control sequence of a machine instruction constitutes the micro-routine for that instruction.
3) Micro-instruction : Individual control words in this micro-routine are referred to as micro instructions.
4) Micro-program : A sequence of micro-instructions is called a micro-program, which is stored in a ROM or RAM called a Control Memory (CM).
5) Control Store : the micro-routines for all instructions in the instruction set of a computer are stored in a special memory called the Control Store.



Types of Micro-programmed Control Unit –Based on the type of Control Word stored in the Control Memory (CM), it is classified into two types :

1. Horizontal Micro-programmed control Unit :
The control signals are represented in the decoded binary format that is 1 bit/CS. Example: If 53 Control signals are present in the processor than 53 bits are required. More than 1 control signal can be enabled at a time.

It supports longer control word.It is used in parallel processing applications.
* It allows higher degree of parallelism. If degree is n, n CS are enabled at a time.
* It requires no additional hardware(decoders). It means it is faster than Verical Microprogrammed.

2. Vertical Micro-programmed control Unit :
The control signals re represented in the encoded binary format. For N control signals- Log2(N) bits are required.

* It supports shorter control words.It supports easy implementation of new conrol signals therefore it is more flexible.
* It allows low degree of parallelism i.e., degree of parallelism is either 0 or 1.
* Requires an additional hardware (decoders) to generate control signals, it implies it is slower than horizontal microprogrammed.