Lecture 1
Computer Architecture
Introduction
Computer architecture deals with the functional behaviour of a computer system as viewed
by a programmer. This view includes aspects such as the sizes of data types ( e.g. using 16
binary digits to represent an integer), and the types of operations that are supported (like
addition, subtraction, and subroutine calls). Computer organization deals with structural
relationships that are not visible to the programmer, such as interfaces to peripheral devices,
the clock frequency, and the technology used for the memory. There is a concept of levels in
computer architecture. The basic idea is that there are many levels, or views, at which a
computer can be considered, from the highest level, where the user is running programs, or
using the computer, to the lowest level, consisting of transistors and wires. Between the high
and low levels are a number of intermediate levels.
Von Neumann model:
The von Neumann model consists of five major components as illustrated in Figure below:
The Input Unit provides instructions and data to the sys- tem, which are subsequently stored
in the
Memory Unit. The instructions and data are processed by the Arithmetic and Logic Unit
(ALU) under the direction of the Control Unit. The results are sent to the Output Unit. The
ALU and Control unit are frequently referred to collectively as the central processing unit
(CPU). Most commercial computers can be decomposed into these five basic units.
Lecture 1
The System Bus Model
Although the von Neumann model prevails in modern computers, it has been streamlined.
Figure 1-3 shows the system bus model of a computer system. This
Most important to the system bus model, the communications among the components are by
means of a shared pathway called the system bus, which is made up of the data bus (which
carries the information being transmitted), the address bus (which identifies where the
information is being sent), and the control bus (which describes aspects of how the
information is being sent, and in
What manner). There is also a power bus for electrical power to the components, which is
not shown, but its presence is understood. Some architecture may also have a separate I/O
bus.
Levels of Machines
As with any complex system, the computer can be viewed from a number of perspectives, or
levels, from the highest “user” level to the lowest, transistor level. Each of these levels
represents an abstraction of the computer. Perhaps one of the reasons for the enormous
success of the digital computer is the extent to which these levels of abstraction are separate,
or independent from one another.
Lecture 1
Control unit :
The function of the control unit in a digital computer is to initiate
sequences of micro operations that are available in a given system is
finite. The complexity of the digital system is derived from the
number sequences of micro operation.
Hardwired control unit
The control unit that using conventional logic design techniques to
generate control signal is called hardware micro control unit
Micro programmed control unit
Is a second alternative for designing the control unit of a digital
computer? The principle of micro programming is an elegant and
systematic method for controlling the micro operation sequences in a
digital computer. The control unit initiates a series of sequential
steps of micro operations. During any given time, certain micro
operation is to be initiated, while others remain idle.
Lecture 1
Control word
Are the control variables that can be represented by a string of ones
and zeros, which can be programmed to perform various operations
on the components of the systems?
We can be defining the micro programmed control unit as the control
unit whose binary control variables are stored in memory.
Each word in control memory contains within it a micro instruction,
which the micro instruction specifies one or more micro operations
for the system. A sequence of micro instructions constitutes a micro
program.
Control memory
A computer that employs a micro programmed control unit will have
two separate memories: main memory and control memory. The
main memory is available to the user for storing the programs. The
contents of the main memory may alter when the data are
manipulated and every time that the program is changed. Control
memory is a memory which is a part of control unit. There are two
types of control memory there are:
1- Read Only Memory (ROM)
The content of the words in ROM are fixed and cannot be altered by
simple programming since no writing capacity i.e available in the
rom words in ROM are made permanent during the hardware
production of the unit. The content of the word in ROM at agiven
address specifies a micro instruction.
Lecture 1
2- Dynamic memory
The dynamic micro programming permits a micro program to be
loaded initially from an auxiliary memory such as magnetic disk.
Control unit that use dynamic microprogramming employs a writable
control memory, which can be used for writing to change the micro
program.
Micro programmed control organization
In contrast, the control memory holds a fixed micro program that
cannot be altered by the occasional user. The micro program consists
of micro instructions that specify various internal control signals for
execution of register micro operations. Each machine instruction
initiates a series of micro instruction in control memory. These micro
instructions generate the micro operations to fetch the instruction
from main memory to evaluate the effective address, to execute the
operation specified by the instruction, and to return control to the
fetch phase in order to repeat the cycle for the next instruction. The
general configuration of a micro programmed control unit is
demonstrated in the block diagram shown below:
Next address
generator
"sequencer"
Control address
register
Control memory
"Rom"
Next address information
Fig 1 ( Micro programming Control Organization)
Control data
register
Lecture 1
The control memory is assumed to be a ROM, within which all control
information is permanently stored. The control memory address
register holds the micro instruction, and the control data register
holds the micro instruction read from memory.
The micro instruction contains a control word that specifies one or
more micro operations for the data processor, once these operations
are executed, the control must determine the next address. The
location of the next instruction may be the one next in sequence, or it
may be located somewhere else in control memory, while the micro
operations are being executed, the next address is computed in the
next address generator circuit and then transferred in the control
address register to read the next micro instruction. Thus the micro
instruction contains bits for initiating micro operation in the data
processor part and bits that determine the address sequence for the
control memory.
Sequencer
The next address generator is sometimes called a micro program
sequencer, as it determines the address sequence that is read from
control memory.
Pipeline register
The data register is sometimes called “pipeline", it allows the
execution of the micro operations specifies by the control word
simultaneously with the generation of the next micro instruction.