Memory and Its Management

8/8/2019 Memory and Its Management

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INDEX

1. About Computer Memory

2. Computer Memory Hierarchy

3. Classification of Memory

4. Primary MemoryI. R/WM

a) SRAM

b) DRAM

II. ROM

a) Permanent ROM

b) Erasable ROM

5. Secondary Memory


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Memory refers to the state information of a computing

system, as it is kept active in some physical structure.

By design, the term memory refers to the temporary state

devices, where as the term storage is reserved for permanent

data.

Computer memory refers to the physical devices used tostore data or programs (sequences of instruction) on a

temporary or permanent basis for used in an electronic digital

computer.

Computers represent information in binary code, written as

sequences of 0s and 1s. Each binary digit (or Bit) may be

stored by any physical system that can be in either of two stable

states, to represent 0 and 1.


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Memory

PrimaryMemory

Read/Write

Memory

R/WM

Static R/WM

Zero Power

Non-volatileRAM

DynamicR/WM

IntegratedRAM

Read-Only

Memory

ROM

ErasableMemory

EPROM

EE-PROM

FlashMemory

PermanentMemory

Masked ROM

PROM

Semi-

random

Access

Disks

Floppy

Hard

CD-ROM

Zip Disk

Serial

Access

Magnetic Tape

CCD

Storage Memory

Secondary

Storage

Backup

Storage


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The memory is classified into 2 main types :

1.Primary Memory

2.Secondary Memory


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Primary Memory

Primary memory is used by the microprocessor in executing and storing

programs.

Egs. of primary memory are R/WM and ROM.

This memory should be able to respond fast enough to keep up with the

execution speed of the microprocessor.

Hence it should be random access memory, meaning the microprocessor

should be able to access the information from any register with the same

speed (independent of its place in the chip).

The size of the memory chip is specified in terms of bits.

Primary memory is classified into:

1. R/WM (Read/Write Memory)

2. ROM (Read Only Memory)


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R/WM (Read/Write Memory)

As its name suggest, the microprocessor can write into or read

from this memory; it is popularly known as RAM (Random AccessMemory).

It is volatile in nature, meaning when power is turned off, all

the content are destroyed.

It is fast, no extra mechanism is required and it has limited

storage capacity.

It is further classified into SRAM (Static RAM) and DRAM(Dynamic RAM).


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SRAM (Static Random Access Memory)

It is built up of flip-flops.

It stores bit as voltage.

It has six transistors per cell.

It is high-speed and low density in nature.

It is costly compared to DRAM.

It is popularly known as CACHE MEMORY.


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DRAM (Dynamic Random Access Memory)

It is made up of MOS gates.

It stores bits as charge.

It is high density and low cost.

It requires extra circuitry for recharging (dis-advantage).

It is preferable for greater than 8K memory.


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ROM (Read Only Memory)

It is non-volatile memory it retains stored information even when the power is

turned off.

This memory is used for programs and data that need not to be altered.

As its name suggests, the information can be read-only which means once a bit

pattern is stored, it is permanent or at least semi-permanent.

It is classified into Permanent Rom and Erasable Rom.


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Permanent Rom

Permanent Rom is further classified into:

1. Masked Rom2. PROM

1. Masked Rom:

In this Rom, a bit pattern is permanently recorded by the masking and

metallization process.

Memory manufacturers are equipped to do this process.

It is expensive and specialized process, but economic for large production

quantities.

2. PROM (Programmable Read Only Memory) :

This memory has nichrome or poly-silicon wires arranged in a matrix.

This memory can be programmed by user with a special PROM Programmer

that selectively burns the fuses according to bit pattern to be stored.

Information stored is permanent.


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Erasable ROM

Erasable Rom is further classified into:

1. EPROM2. EEPROM

3. Flash Memory

1. EPROM (Erasable Programmable Read Only Memory):

It Is made up of floating gate like FET.

Entire chip is needed to be erased if you want to alter data. UV rays are passed approx. 15-20 minutes to erase entire EPROM.

Register or cluster level erasing is not possible.

Consumes time to erase and cannot be programmed partially or erased (dis-

advantage).


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2. EEPROM (Electrically Erasable Programmable Read Only Memory):

It can be partially programmed easily than EPROM.

It removes all the dis-advantages of EPROM.

It takes 10 millisecond to erase the data

without separating it from the circuit. Register level erasing is possible.

Costlier than EPROM (dis-advantage).

3. Flash Memory:

It is costlier than EPROM but cheaper than EEPROM.

It has all the advantages of EPROM.

It is considered as popular and extended version of EEPROM.

Deletion is possible at clock or sector level.


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Secondary Memory

It is used for unlimited and permanent storage.

It has high storage capacity.

It is non-volatile in nature.

It is measured in bytes.

It is further classified into:1. Random Access (Eg. Disks)

2. Serial Access (Eg. Magnetic Tapes)

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