This document discusses microprogrammed control in CPUs. It explains that microprogrammed control uses a control memory to store microinstructions that specify sequences of micro-operations. Each microinstruction is a control word that determines which micro-operations will execute. By sequencing through different microinstructions stored in control memory, the CPU can perform complex instruction sets in a flexible manner compared to hardwired control logic. Advanced microprogrammed CPUs can dynamically load microprograms from auxiliary memory.

1. Introduction
Microprogrammed Control
Unit 3 - Microprogrammed Control
Abhineet Anand
Computer Science and Engg. Department
University of Petroleum and Energy Studies, Dehradun
November 25, 2012
Abhineet Anand Unit 3 - Microprogrammed Control

2. Introduction Control Memory
Microprogrammed Control Hardwired Controll
Control Memory
One of the Major functional part in a digital computer is
Central Processing Unit.
The main digital hardware functional unit of CPU are
control unit, ALU and registers.
Two methods for implementing control unit are:
Hardwired Control, and
Microprogrammed Control.
Abhineet Anand Unit 3 - Microprogrammed Control

3. Introduction Control Memory
Microprogrammed Control Hardwired Controll
Hardwired Control
Hardwired Control
The Key characteristics of hardwired control logic are high
speed operation, expensive, relatively complex, and no
flexibility of adding instruction.
Example Intel 8085, Motorola 6802, Zilog 80 and RISC
CPUs.
When the control signal are generated by hardware using
conventional logic design technique, the control unit is said
to be hardwired.
Abhineet Anand Unit 3 - Microprogrammed Control

4. Introduction
Introduction
Control Word
Microprogrammed Control
Micro Program
Introduction
Microprogrammed Control
The principal of microprogramming is an elegant and
systematic method for controlling the micro operation
sequences in a digital computer.
Examples: Intel 8080, Motorola 68000 and any CISC
CPUs.
The control function that specifies a micro-operation is a
binary variable, which is 1, for the corresponding
micro-operation to be executed.
Abhineet Anand Unit 3 - Microprogrammed Control

5. Introduction
Introduction
Control Word
Microprogrammed Control
Micro Program
Control Word
The control unit initiates a series of sequential steps of
micro-operation.
During any given time, certain micro-operation are to be
initiated, while other remain idle.
The control variable at any given time can be represented
by a string of 1’s and 0’s called a control word.
A control unit whose binary control variables are stored in
memory is called a microprogrammed control unit.
Abhineet Anand Unit 3 - Microprogrammed Control

6. Introduction
Introduction
Control Word
Microprogrammed Control
Micro Program
Micro Program
Each word in control memory contains within it
microinstruction.
The microinstruction specifies one or more micro-operation
for the system.
A sequence of micro-operation constitute a microprogram.
A more advanced development known as dynamic
microprogramming, which permits a microprogram to be
loaded initially from an auxiliary memory.
A memory that is part of a control unit is referred as a
control memory.
Abhineet Anand Unit 3 - Microprogrammed Control

7. Introduction
Introduction
Control Word
Microprogrammed Control
Micro Program
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Abhineet Anand Unit 3 - Microprogrammed Control