Computer system


















Computer System

Book I: computer system fundamentals.
Chapter 1: INTRODUCTION TO COMPUTER.
Question 1. What is a computer?
A computer may be defined as a machine which accepts data from an input device, processes it by performing arithmetical and logic operations in accordance with a program of instructions and returns the results through an output unit.
A computer is basically an electronic machine operating on current.
Question 2. Components of a Computer system?
A computer system comprises of the following components:
Central Processing Unit (CPU).
CPU is the heart of the whole sys
CPU consists of the :
control unit (CU)
arithmetic logic unit (ALU)
accumulator (ACC)
program counter (PC)
instruction register (IR)
memory address register (MAR)
memory data register (MDR)
status register (SR)
general purpose register
The function of each components of CPU:
Control unit:
control and co_ordinate all hardware functions of the CS.
examine and decode all program instructions to the computer and initiate their execution by sending the appropriate signals.
ALU:
performs all arithmetic and logic comparision two values functions required by computer.
ACC:
holds the first operand of the temporary result of the ALU.
PC:
contains the add of the next instruction to be excuted.
IR:
contains the current instruction to be executed.
Main memory
MAR:
holds the address location to or from which data is to be transferred
MDR:
contains the data to be written to or read out of the addressed location.
SR:
keeps track of the status of the accumalator.
General Purpose Register:
for general purpose procedures.
Please refer to diagram for an illustratin of the basic components of the CPU.

CPU

Control unit
Arithmetic Logic Unit
Accumulator
Program Couter to main
Instruction Register memory
Memory Address Register
Memory Data Register
Status Register
General Purpose Register


Basic components of a CPU.




Control Unit

Input Unit ALU Output Unit

Main Memory



Backing Storage


Control signals
Data flow
Components of a CS.
Input units
Used to enter data( raw unprocessed facts) and instructions to the computer.
Output units
Used for delevering the processed result from the computer in useful form.
Backing storage units
Backing storage units need for high capacity data storage devices that can store data in a more permanent form for later retrieral, updating and referencing.
Backing storage is also called secondary storage external storage and auxiliary storage.
Chapter 2: MICOPROCESSOR.
Question 1. Cache Memory?
Cache memory is a small amount of very fast store with faster access time than the main memory.
Cache memory is used to temporaryty store data instructions that are likely to be retrieved many times, thus speeds up the processing of data.
Sits between main storage and the processor acting as holding area through which all data and instructions pass.
Old data in the cache memory is over written by new then cache is full.
Question 2. Virtual Memory?
Virtual memory makes use of both the main memory and backing store.
In a virtual memory sys, each user has the illusion that his program is in the main memory all the time.
The sys maintains this illusion by keeping some of the “unused” portion of the program’s code and data on a backing store device which is usually magnetic disk
The movement of the unused portion from the backing store to the mian memory is transparent to the users.
Please refer to diagram for virtual memory.

Backing Store Main Memory




























Virtual Memory

Chapter 3: BATCH/ ONLINE AND REAL TIME PROCESSING SYSTEM.
Question 1. Batch Processing System?
Def: Computer processing does not begin until all the input data has been collected and grouped together called Batched Generally data is accumulated for a certain period of time or unitl a certain quantity.
Ads:
Response time is not critical.
Need to process large volumn of data.
Computer efficiency is more important than response time.
Dis:
Time between recording and processing of source document is long
Rereen normally required if errors are encountered.
Data is not current.
Error correction is more difficult.
Question 2. Online Processing System?
Def: Inputs data enters the computer directly as soon as it is being transacted. There information will be processed immediately and updated into the master file.
Ads:
Enter availability of information for decision making.
More accurate data capture.
Schedules suits user.
Dis:
CPU time is used less efficiently.
Random arrival of transactions, terminal operator process each transaction separately.
More expensive than batch processing.
Question 3. Real Time Processing System?
Def: One which controls the environment by receiving data processing them and returning results sufficiently quickly to affect the functioning of the environment at that time.
Ads:
Response time is very critical and sufficient quick.
Dis:
Expensive hardware & software.
Very complex in terms of hardware & software.
Chapter 4: PRINTERS AND TERMINALS.
Question 1. Classification of printers?
Classifying printers according to speed.
Serial printers
Slow printers that print one character at a time.
Eg: Dot matrix printers
Daisywheel printers
Line printers
Medium to high speed printers that can print in excess of 2000 lines per minute.
Eg: Chain Printers
Band Printers
Drum Printers
Calssifying printers according to method of printing
Impact printers
Use hammers or prints to strike a print rebbon in order to form the character on the paper.
Non impact printers
Use more silent methods of printing.
Eg: Thermal printers
Ink Jet printers
Lazers printers
Classifying printers according to print quality
Kinds of quality printers
Draft quality
Near letter quality(NLQ)
Letter quality
Graphic quality
Question 2. Describe some types of printer?
According to speed:
Dot matrix printer
Serial impact printers that can print draft, near letter quality and a limited amount of graphics.
The print resolution is generally lower than lazer printers.
Daisywheel printers
Are serial impact printers, the speed of a daisywheel printer is slow(20-55 characters per second), noisy in operation.
The print head has the letters arranged at the end of spokes round a central hub.
Chain printers
The chains printers has its characters set rapidly rotating on a print chain.
Band printers
The band printer has rotating scalloped steel band.
Drum printers
Are line printers, the print character are raised in bands around a heavy metal drum which rotates at very high speed.
The print hammers strike the paper and a print ribbon against an apropriate character on the line. An entire line of the same character is printed on one rotation of the drum.
Thermal printers
Uses special heat sensitive paper and a matrix of print wires that become hot when exposed to an electric current. The heated wires come into close contact with the paper, burning the image of the character onto it.
The more advanced thermal printers are using thermal transfer printing.
They have a special heat sensitive ribbon and a print head with wires that become hot when a currents is applied.
The heat from the print wires causes the ink from the ribbon to fuse to a piece of regular paper.
Inl Jet Printers
The ink jet prints by using a small droplet generator to break special inks into tiny drops, which are then forced towards a paper supply.
Lazer printers
Using a photoconductive drum.
A lazer is then used to write the image of the character onto the drum.
After exposure to the lazer, the drum rotates through a developing station, picks up toner and transfers it to the paper.
The character is fused onto the paper by heat.
Ion deposition printers
Ions are created in a cavity, and directed electrically through an orifice onto the dielectric surface of a rotating cylinder.
The required characters are formed as an electric charge image on the cylinders surface.
Toner is the applied to the charged image and transferred to the paper on which it is transfixed by pressure(cold fusion).
Electrostatic printers
Letterheads and logos are created electrostatically from a changeable metal cylinder.
Magnetic printers
A drum in the printer has a surface that can be coated with sows of tiny spots of magnetion by means of thousands of minute recording heads.
As the drum rotates it becomes covered with these magnetic spots so as to from a latent image of the page to be printed.
Dry ink particles are brought into contact with the drum’s surface and these adthere to the magnetised spots. The ink was then pressunal on to the surface and subsequently transferred onto the paper.
Question 3. Characteristics of a page printers?
Speed
Characters sets
Copies
Intelligence
Output
Chapter 5: DATA STORAGE MEDIA.
Question 1. Data storage Requirements Characteristics?
Low access time: fast speed
Storage capacity: much enough
Interchangeability: can be change easily
Security: safe enough
Transfer rate: fast enough
Cost: economic
Question 2. Magnetic disks?
This comprises a drive unit onto which one or perhaps two magnetic disk cartridges are loaded.
The dri