1. Introduction to Computing: Lecture 4
Computer Systems

2. Overview This Lecture covers:
To learn about different memory devices used in our computers.
To learn about the major part of our computer, CPU and its components as well.

3. Memory Devices
The device, which is used to store the data, instructions and information before, during and after the processing by Arithmetic Logic Unit (ALU).
It is actually a work area (physically a collection of integrated circuits) within the computer, where the CPU stores the data and instructions. It is also known as a Main/Primary/Internal memory.

4. Memory Devices It is of the following three types:-
Read Only Memory (ROM)
Random Access Memory (RAM)
Complementary Metal Oxide Semiconductor Memory (CMOS)

5. Memory: ROM
Non-volatile chips inside which data or programs are stored.
Whenever the computer is switched on, it searches for the required information or instructions. The memory which has these essential instructions is known as Read Only Memory (ROM).

6. Memory: ROM
This memory is permanent and not erased when the system is switched off.
The memory capacity of ROM varies from 64 KB to 256 KB etc.
ROM contains a number of programs (set of instructions). The most important program of ROM is the Basic Input Output System (BIOS). Which activates the hardware.

7. Memory: ROM
There are many types of ROM available for microcomputers like Mask ROM, PROM, EPROM, EEPROM and EAPROM.
Mask ROM : Mask ROM is the basic ROM chip. In this type of ROM, the information is stored at the time of its manufacturing. So, it cannot be altered or erased later on.

8. Memory: ROM
PROM : PROM stands for Programmable Read Only Memory. In this type of ROM, the information is stored by the programmers after its manufacturing. It also cannot be altered or erased later on.
EPROM : EPROM stands for Erasable PROM. It is similar to PROM, but its information can be erased later on by ultra violet light and it can be reprogrammed.

9. Memory: ROM
EEPROM : EEPROM stands for Electrically EPROM. It is similar to EPROM, but its information can be erased by using an high voltage current.
EAPROM : EAPROM stands for Electrically Alterable PROM. As compared to EPROM & EEPROM, the information stored in EAPROM can be altered later.

10. Memory: RAM
RAM (random access memory) is the computer’s main memory and is used to temporarily storage programs and data with which it is working.
RAM is volatile (erased when the power to the PC goes off).
RAM comes in a variety of types, speeds, and size.

11. Memory: RAM Types of RAM:
Dynamic RAM (DRAM): The information stored in DRAM has to be refreshed after every few milliseconds, otherwise it is erased. DRAM has higher storage capacity and is cheaper than Static RAM.
Static RAM (SRAM): The information stored in Static RAM need not be refreshed, but it remains stable as long as power supply is provided. SRAM is costly but has higher speed than DRAM.

12. Memory: CMOS
CMOS (Complementary Metal Oxide Semiconductor Memory) memory is used to store the system configuration, date, time and other important data.
When the computer is switched on, BIOS matches the information of CMOS with the peripheral devices and displays error in case of mismatching.

13. Processing Unit: CPU
CPU is the main component or “brain” of a computer, which performs all the processing of input data.
Its function is to fetch, decode and then execute the instructions stored in the main memory of a computer.
In microcomputers, the CPU is built on a single chip or integrated circuit (IC) and is called as a Microprocessor.

14. Processing Unit: CPU
The CPU follows a set of steps-called a machine cycle-for each instruction it carries out.
By using a technique called pipelining, many CPUs can process more than one instruction at a time.
The machine cycle includes two smaller cycles:
During the instruction cycle, the CPU fetches a command or data from memory and decodes it for the CPU.
During the execution cycle, the CPU executes the instruction, and may store the instruction's result in memory.

16. Processing Unit: CPU
The CPU consists of the following distinct parts:-
Arithmetic Logic Unit (ALU)
Control Unit (CU)
Registers
Buses
Clock

17. Processing Unit: CPU Arithmetic Logic Unit (ALU):
The arithmetic and logic unit of CPU is responsible for all arithmetic operations like addition, subtraction, multiplication and division as well as logical operations such as less than, equal to and greater than.
Control Unit (CU):
The control unit is responsible for controlling the transfer of data and instructions among other units of a computer. It is considered as the “ Central nervous System ” of computer, as it manages and coordinates all the units of the computer.

18. ALU Operations List Arithmetic Operations Logical + Add
 equal to ( and not equal to)
 Subtract
> greater than, (and not greater than)
x Multiply
< less than (and not less than)
 Divide
 greater than or equal to
(and not greater than or equal to)
^ Raise by a power
 less than or equal to
(and not less than or equal to)

19. Processing Unit: CPU Registers:
Registers are small high speed circuits (memory locations) which are used to store data, instructions and memory addresses temporarily (memory location numbers). Registers can be divided into various categories depending upon their functions:-
Memory Address Register (MAR)
Memory Buffer register (MBR)
Program Control Register (PC)
Accumulator Register (AR)
Instruction register (IR)
Input/output Register (I/O R)

20. Processing Unit: CPU Buses:
Data is stored as a unit of eight bits in a register. Each bit is transferred from one register to another by means of a separate wire. This group of eight wires, which is used as a common way to transfer data between registers is known as a bus.
Bus can be of three major types:-
Data bus is used to move data.
Address bus is used to move address or memory location.
Control bus is used to send control signal between various components of a computer.

22. Processing Unit: CPU Clock:
Clock is another important component of CPU. Which measures and allocates a fixed time slots for processing each and every micro-operation (smallest functional operation). In simple terms, CPU allocated one or more clock cycles to complete a micro-operation. CPU executes the instructions in synchronization with the clock pulse.
The clock speed of CPU is measured in terms of Mega Hertz (MHZ) or millions of cycles per second.
CPU speed is also measured in terms of Millions of instructions Per Second (MIPS) or Millions of Floating Point Operations Per Second (MFLOPS).

24. Ports
External devices—such as those used for input and output—are connected to the system by ports on the back of the computer.
PCs feature a number of built-in ports, which are ready to accept devices such as a printer, mouse, keyboard, phone line, microphone and speakers, and others.
Most computers come with a serial port and a parallel port. A serial port transmits one bit of data at a time; a parallel port transmits data one byte at a time.

25. Expansion Slots & Boards
If the PC does not have a port for an external device, you can install an expansion board into one of the empty expansion slots.
A board provides the correct port for the new device, and connects the device to the CPU by way of the computer's expansion bus.
Newer bus technologies such as Universal Serial Bus (USB) and IEEE 1394 enable many devices to be connected to one port.
Small Computer System Interface (SCSI) is an older standard for extending the bus to multiple devices through a single port.

27. Summary
Inside the system unit; memory units
How the CPU works