Unit 3: Processor
Content
3.1 Hardwired and Micro programmed
3.2 Arithmetic Logic Unit
3.3 Types of Registers
3.4 Bus Architecture
3.5 Instruction Execution
3.6 Addressing Modes (Immediate Addressing, Direct Addressing, Register Addressing,
Register Indirect Addressing, Indexed Addressing and Based-Indexed Addressing)
Specific Objective
•Explain hardwired implementation, micro-instruction sequencing and execution
as well as application of micro-programming.
•Elaborate Arithmetic and Logic unit; register types, bus architecture, instruction
sets and executions.
•Illustrate and explain different addressing modes with suitable examples.
3.1 Hardwired and Micro programmed
Design of Control Unit
Control unit generates timing and control signals for the operations of the computer. To
execute an instruction, the control unit of the CPU must generate the required control
signal in the proper sequence. There are two approaches used for generating the control
signals in proper sequence, which is given below:
Hardwired Control Unit
The control unit whose control signals are generated by the hardware through a sequence
of instruction is called a hardwired control Unit. It is implemented with the help of gates,
flip flops, decoders etc. in the hardware. The inputs to control unit are the instruction
register, flags, timing signals etc. This organization can be very complicated if we have to
make the control unit large. If the design has to be modified or changed, all the
combinational circuits have to be modified which is a very difficult task.
The advantage of hardwired control is that is very fast. The disadvantage is that the
instruction set and the control logic are directly tied together by special circuits that are
complex and difficult to design or modify. If someone designs a hardwired computer and
later decides to extend the instruction set, the physical components in the computer must
be changed. This is prohibitively expensive, because not only must new chips be
fabricated but also the old ones must be located and replaced.
The characteristics of hardwired control units are as follows:
•Hardwired control units are based on combinational circuits.
•Fixed logic circuits that correspond directly to the Boolean expressions are used to
generate the control signals.
•Hardwired control is faster than micro-programmed control.
•A controller that uses this approach can operate at high speed.
Micro-programmed Control Unit
Microprogramming is a second alternative for designing control unit of digital computer.
The control unit whose control signals are generated by the data stored in control memory
and constitutes a program on the small scale is called a micro-programmed control unit.
It is implemented by using programming approach.
In computer central processing units, micro-operations are detailed low-level instructions
used in some designs to implement complex machine instructions. Each micro-operation is
described in symbolic notation. This notation looks suspiciously like a programming
language. In fact it is a language, known as a microprogramming language.
The control variables at any given time can be represented by a string of 1's and 0's called
a control word (which can be programmed to perform various operations on the
component of the system). Each word in control memory contains within it a
microinstruction. A sequence of micro operations is carried out by executing a program
consisting of micro-instructions. A sequence of microinstructions constitutes a micro
program or firmware. In this organization any modifications or changes can be done by
updating the micro program in the control memory by the programmer. A micro-program
is midway between hardware and software. It is easier to design in firmware than
hardware, but it is more difficult to write a firmware program than a software program. A
memory that is part of a control unit is referred to as a control memory
The main advantages of the micro-programmed control are the fact that once the
hardware configuration is established; there should be no need for further hardware or
wiring changes. If we want to establish are different control sequence for the system, all
we need to do is specify different set microinstructions for control memory. The hardware
configuration should not be changed for different operations; the only thing that must be
changed is the micro-program residing in control memory.
Characteristics of micro programmed control units:
■The control signals associated with operations are stored in special memory units
inaccessible by the programmer as Control Words.
■Control signals 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.
