Engr. Sobuj Kumar RayFaculty

Dept. of EEE

Read Only Memory - ROM is a special type of memory that contains instructions that are activated each time the computer is turned on.   These instructions are set at the time the computer is assembled and cannot be changed.

ROM can only be "read", not written to. ROM chips are considered as Von-volatile memory. This means that the contents of the memory chip are retained even when the power is turned off.

One of the most important uses of the ROM chip is to store a series of programs that provide basic control of the computer. These programs together are called the "Basic Input /Output System" or BIOS.

These "BIOS" programs control the computer's startup processes and other basic components such as the keyboard, monitor/display, disk drives, etc.  The BIOS is always on a ROM Chip.

The ROM may be in chips that look something like this: ROM MEMORY is reserved for use by operating systems (DOS, Win95, NT, etc.), and hardware devices, BIOS, CMOS, etc. You cannot use this memory for applications such as MS-WORD, buffers, etc.

During system operation the ROM BIOS controls communications between the CPU, the Operating System and installed hardware.

The version of the ROM BIOS can be a key factor in whether your computer is compatible with certain hardware devices.

Fig: ROM(Read Only Memory)

Fig: ROM(Read Only Memory)

PROM (``Programmable Read Only Memory'')PROMs are blank chips which have nothing recorded on them. Once instructions or data are recorded into the chip by special programming device, the PROM chip permanently stores the information like ROM.

The programming of the PROM chip is generally done by the manufacturer of computer system.

Fig: PROM(Programmable Read Only Memory)

EPROM (“Erasable Programmable Read Only Memory”)EPROM chips can be programmed, as the PROM chips. But they can be erased and reprogrammed by a special programming device.  

Fig: EPROM (``Erasable Programmable Read Only Memory'')

EEPROM (``Electrically Erasable Programmable Read Only Memory'') EEPROM is similar to EPROM except that it is erased by applying electrical pulses to the chip.

EPROM (``Erasable Programmable Read Only Memory'')

EEPROM (``Electrically Erasable Programmable Read Only Memory'')

Random Access Memory (RAM) This form of memory is used to store data and application programs. The memory is read-write, and volatile

Random Access Memory is actually a set of miniaturized circuits which represents the working memory of the computer. This is where application programs (software) is loaded and then executed. Unlike a human, the computer requires electricity to keep the information within RAM.

  

Each time the computer is turned off, all information within RAM is lost. For this reason RAM is called volatile memory. To save instructions for later use requires a storage device.

RAM is divided into multiple parts (locations, not types): Conventional Memory (location 0 thru 640 KB)

High (Upper) Memory (641 thru 1 MB) Extended Memory (addresses above 1 MB) Expanded Memory (above 1 MB).

The larger the RAM the better. A typical modern computer system may have the capacity to temporarily store between 64,000,000 and 128,000,000 bytes of memory space. (A Byte is one character, so you can say 32-64 million characters)

There are TWO main types of RAM used in computer systems today:DynamicThis memory is based on capacitor technology, and requires the contents of each storage cell within the chip to be periodically refreshed (about every 4ms).

It consumes very little power, but suffers from slow access times. Another advantage is the large capacity offered by this technology per chip.

StaticThis memory is based on transistor technology, and does not require refreshing. It consumes more power (thus generates more heat) than the dynamic type, and is significantly faster.

It is often used in high speed computers or as cache memory. Another disadvantage is that the technology uses more silicon space per storage cell than dynamic memory, thus chip capacities are a lot less than dynamic chips.

  Advantage Disadvantage

Dynamic RAM

CheaperLow PowerHigh Density

SlowerNeeds refreshing

Static RAM Faster

No need to refresh

More ExpensiveConsumes More PowerLow Density

Cache memory:Cache memory is high speed memory which interfaces between the processor and the system memory.

Dynamic memory is used to implement large memory systems in modern computers. This is due to features like low power consumption, high chip densities and low cost.

Fig : Cache Memory

Dynamic memory is however slow, and cannot keep up with modern fast processors. When a processor requests data from a memory chip, it expects to receive that data within a specific time.

Dynamic memory is currently too slow to keep up with processors running faster. It is too expensive to use static memory in place of dynamic memory.

To use slow dynamic memory with a fast processor requires an extra hardware subsystem (called cache memory) which fits between the processor and the memory subsystem.

All memory accesses by the processor are fed through the cache system. The cache system starts off by trying to read as much data as possible from the dynamic memory subsystem.

It stores this data in its own high speed static memory (or cache). When the cache system can respond to the processor request, its called a cache hit.  

If the cache system cannot service the processor request, its called a cache miss.  

2.Secondary Storage:# It stores several programs, documents, data bases etc.

# The programs that run on the computer are first transferred to the primary memory before it is actually run.

# Whenever the results are saved, again they get stored in the secondary memory.# The secondary memory is slower and cheaper than the primary memory. Some of the commonly used secondary memory devices are Hard disk, CD, DVD etc.

Central Processing UnitThe control unit and ALU of the computer are together known as the Central Processing Unit (CPU). The CPU is like brain performs the following functions:1) It performs all calculations.2) It takes all decisions.

3) It controls all units of the computer.

A PC may have CPU-IC such as Intel 8088, 80286, 80386, 80486, Celeron, Pentium, Pentium Pro, Pentium II, Pentium III, Pentium IV, Dual Core, and AMD etc. 

The Arithmetic Logic Unit The arithmetic/logic unit (ALU) contains the electronic circuitry that executes all arithmetic and logical operations.

The arithmetic/logic unit can perform four kinds of arithmetic operations, or mathematical calculations: addition, subtraction, multiplication, and division.

As its name implies, the arithmetic/logic unit also performs logical operations. A logical operation is usually a comparison. The unit can compare numbers, letters, or special characters. The computer can then take action based on the result of the comparison. This is a very important capability.

It is by comparing that a computer is able to tell, for instance, whether there are unfilled seats on airplanes, whether charge- card customers have exceeded their credit limits, and whether one candidate for Congress has more votes than another.

Logical operations can test some conditions: Equal-to condition. In a test for this condition, the arithmetic/logic unit compares two values to determine if they are equal. For example: If the number of tickets sold equals the number of seats in the auditorium, then the concert is declared sold out.

Less-than condition. To test for this condition, the computer compares values to determine if one is less than another. For example: To calculate tax money, if employee income of a company is less than $30000, employee no needs to pay any tax.

Greater-than condition. In this type of comparison, the computer determines if one value is greater than another. For example: If the hours a person worked this week are greater than 40, then multiply every extra hour by 1.5 times the usual hourly wage to compute overtime pay.

A computer can simultaneously test for more than one condition. In fact, a logic unit can usually discern six logical relationships: equal to, less than, greater than, less than or equal to, greater than or equal to, and not equal.

The symbols that let you define the type of comparison you want the computer to perform are called relational operators. The most common relational operators are the equal sign(=), the less-than symbol(<), and the greater-than symbol(>).

Control Unit:It controls all other units in the computer. The control unit instructs the input unit, where to store the data after receiving it from the user. It controls the flow of data and instructions from the storage unit to ALU.

It also controls the flow of results from the ALU to the storage unit. The control unit is generally referred as the central nervous system of the computer that control and synchronizes it’s working. 

Registers: Temporary Storage Areas Registers are temporary storage areas for instructions or data. They are not a part of memory; rather they are special additional storage locations that offer the advantage of speed.

Registers work under the direction of the control unit to accept, hold, and transfer instructions or data. The control unit uses a data storage register -as a temporary, convenient place to store what is used in transactions.

Computers usually assign special roles to certain registers, including these registers: An accumulator, which collects the result of computations.

An address register, which keeps track of where a given instruction or piece of data is stored in memory. Each storage location in memory is identified by an address, just as each house on a street has an address.

A storage register, which temporarily holds data taken from or about to be sent to memory.

Input Unit

Computers need to receive data and instruction in order to solve any problem. Therefore, we need to input the data and instructions into the computers. The input unit consists of one or more input devices.

Keyboard is the one of the most commonly used input device. Other commonly used input devices are the mouse, floppy disk drive, magnetic tape, etc. All the input devices perform the following functions.

1) Accept the data and instructions from the outside world.

2) Convert it to a form that the computer can understand.

3) Supply the converted data to the computer system for further processing.

Some input devices are: KeyboardMouseScannerMicrophoneBar code reader

Output UnitThe output unit of a computer provides the information and results of a computation to outside world. Printers, Visual Display Unit (VDU) are the commonly used output devices.

      

Other commonly used output devices are floppy disk drive, hard disk drive, and magnetic tape drive.

   

Some output devices are: MonitorPrinterSpeaker