Unit-2

The Computer Memory System and There Characteristic

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Memory Heirarchy

The Memory Heirarchy:

Characteristics of Computer Memory System:

1. Location: a) Processor

b) Internal

c) External

2. Capacity: a) Wordsize(1 word = 1 byte or 8 bits)

3. Units of Transfer – The unit of transfer is equal to the no. of data lines into an out of the memory module.

4. Access Method :a) Sequential Access

b) Direct Access

c) Random Access

d) Associative Access

5. Physical Type: a) Semiconductor

b) Magnetic

c) Optical

d) Magneto – Optical(M-O drive like magnetic but read by optically)

6. Physical Characteristic: a) Volatile/Non-Volatile

b) Erasable/Non-Erasable

7. Organization: a) Inboard Memoryb) Outboard Memory

c) Offline Memory

Semi – Conductor Main Memory:

It is used by semi-conductor technology for making memory chips. Semi-conductor are devices whose property lies b/w conductor & insulator silicons and germenium are generally used for semi-conductor devices. Semi-conductors were used for making high speed CPU Registers.

<Diagram>

The basic element of a semi-conductor memory is the memory cell most commonly the cell has three functional terminal capable of carrying an electric signal.

<Diagram>

Memory Cell Operations

A memory cell using Control Switch

RAM – A RAM must be provided with a constant power supply. If a power is interrupted then the data is lost i.e.(RAM is a volatile memory), they can be used only as temporary storage the two traditional form of RAM used in computer are SRAM and DRAM.

DRAM – A Dynamic RAM is made with cells that share data as charge on capacitor. The presence or absense of charge in a capacitor is interpretated as a binary 1 or 0 because capacitor have a natural tendency to discharge refreshing to maintain Data Storage(thus Dynamic RAM can be required extra circuitory). The DRAM cell is used to store a single bit(0 or 1), it is essential for analog device.

<Diagram>

DRAM Cell Structure

SRAM – A static RAM is a digital device in a SRAM. Binary values are stored using traditional flip-flop logic gate configuration. A static RAM will hold its data as long as power is supplied to it. Figure is a typical SRAM structure for an individual cell.

Four transistors(T1, T2, T3, T4) are cross connected in an arrangement that produce a stable logic state. In logic state (1), point C1 is high and C2 is low. In this state, T1 and T4 are off and T2 and T3 are on. In logic state (0), point C1 is low and C2 is high. In this state T1 and T4 are on and T2 and T3 are off. Both states are stable as long as the direct current voltage(DC) is applied. Unlike the DRAM no refreshing is needed to retain data.

As in Dynamic RAM the static RAM address line is used to open or close a switch the address line controls a switch. The address line controls two transistors (T5 and T6). When a signal is applied to this line the two transistors are switch on allowing a read or write operations.

<Diagram>

SRAM vs DRAM:

Both static and dynamic RAM are volatile i.e. must continuously supply to the memory to preserve the bit values. A dynamic memory cell is simpler and smaller then static memory cell. A Dynamic RAM requires the supporting refresh circuitory. A final point is that SRAM are somewhat faster than DRAM because of there relative characteristics, a SRAM is used for ____ memory and DRAM is used for main memory.

Types of ROM:

Chip Packaging: