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SEMICONDUCTOR MAIN MEMORY

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ORGANISATION

The basic element of a semiconductor memory is the memory cellThey exhibit two stable states,

which can be used to represent binary 1 and 0.

They are capable of being written into (at least once), to set the state.

They are capable of being read to sense the state.

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SEMICONDUCTORMEMORY

TYPES

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MEMORY TYPE

CATEGORY

ERASUREWRITE

MECHANISM

VOLATILITY

Random Access Memory (RAM)

Read-write memory

Electrically, byte level

Electrically Volatile

Read Only Memory (ROM) Read-only

memoryNot possible

Masks

Non Volatile

Programmable ROM (PROM)

Electrically

Erasable PROM (EPROM)

Read-mostly memory

UV light, chip level

Electrically Erasable PROM(EEPROM)

Electrically, byte level

Flash Memory

Electrically, byte level

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Random Access Memory-RAM Most common semiconductor memory. Possible both to read data from the

memory and to write new data into the memory easily and rapidly.

Reading and writing are accomplished through the use of electrical signals.

Volatile, that is it must be provided with a constant power supply. If the power is interrupted, then the data are lost.

Can be used only as temporary storage.

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TYPES OF RAM

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1. Dynamic RAM-DRAM Made with cells that store data as charge

on capacitors. The presence or absence of charge in a

capacitor is interpreted as a binary 1 or 0. Require periodic charge refreshing to

maintain data storage. The term dynamic refers to this tendency

of the stored charge to leak away, even with power continuously applied.

Essentially an analog device.

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2. Static RAM-SRAM Binary values are stored using

traditional flip-flop logic-gate configurations

A static RAM will hold its data as long as power is supplied to it.

SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis.

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SRAM versus DRAM

SRAM DRAM

1. It has long life2. Digital Device3. There is no need to

refresh4. Faster5. Used as cache memory6. Large size7. Expensive8. High power

consumption

1. It has short data lifetime2. Analog Device3. Need to be refreshed

continuously4. Slower as compared to

SRAM5. Used as RAM-main

memory.6. Lesser in size7. Less expensive8. Less power consumption

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Read Only Memory-ROM Contains a permanent pattern of data that

cannot be changed. Nonvolatile; that is, no power source is

required to maintain the bit values in memory. Not possible to write new data into it. Applications:

Micropramming Library subroutines for frequently wanted

functions System programs Function tables

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TYPES OF ROM

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1. PROM When only a small number of ROMs with a

particular memory content is needed, a less expensive alternative is the programmable ROM (PROM).

Nonvolatile and may be written into only once.

Writing process is performed electrically and may be performed by a supplier or customer at a time later than the original chip fabrication.

Provide flexibility and convenience.

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• The difference between a PROM and a ROM (read-only memory) is that a PROM is manufactured as blank memory, whereas a ROM is programmed during the manufacturing process. • To write data onto a PROM chip, you need a special device called a PROM programmer or PROM burner.

PROM versus ROM

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Read-mostly Memory

EPROM EEPROM Flash Memory

When read operations are far more frequent than write operations but for which nonvolatile storage is required, then read-mostly memory is used.

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2. EPROM Read and written electrically. Before a write operation, all the storage

cells must be erased to the same initial state by exposure of the packaged chip to ultraviolet radiation.

Can be altered multiple times Non Volatile. More expensive than PROM Has the advantage of the multiple update

capability.

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3. EEPROM More commonly used. Can be written into at any time without

erasing prior contents; only the byte or bytes addressed are updated.

The write operation takes considerably longer than the read operation.

Combines the advantage of non-volatility with the flexibility of being updatable in place, using ordinary bus control, address, and data lines.

More expensive. Less dense, supporting fewer bits per chip.

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3. Flash Memory So named because of the speed with which it

can be reprogrammed. First introduced in the mid-1980s, Intermediate between EPROM and EEPROM in

both cost and functionality. Like EEPROM, flash memory uses an electrical

erasing technology. In addition, it is possible to erase just blocks of

memory rather than an entire chip. Like EPROM, flash memory uses only one

transistor per bit, and so achieves the high density of EPROM.

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Thank you.