Semiconductor MainMemory
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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.