Page 138 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 8: Memory

and memory. That is, the rate at which data can be fetched from memory is about 10 times slower Notes
than the rate at which CPU can process data. Obviously, the overall performance of a processor
can be improved greatly by minimizing the memory-processor speed mismatch. Cache memory
(pronounced “cash” memory) is commonly used for this purpose. It is an extremely fast, small
memory between CPU and main memory whose access time is closer to the processing speed of
CPU. It acts as a high-speed buffer between CPU and main memory and is used to temporarily
store very active data and instructions during processing. Since cache memory is faster than main
memory, processing speed is improved by making the data and instructions needed for current
processing available in the cache. Cache memory is an extremely fast and small memory between
CPU and main memory. Its access time is closer to the processing speed of CPU. It acts as a high-
speed buffer between CPU and main memory and is used to temporarily store very active data
and instructions during processing.

Figure 8.4: Cache Memory

The EPROM was invented by Dov Frohman of Intel in 1971, who was awarded
US patent 3660819 in 1972.

8.5 Electrically Erasable Programmable Read Only Memory (EEPROM)

The EEPROM stands for electrically erasable programmable read only Memory. An EEPROM is
like an EPROM chip since, it can be written in or programmed more than once. Unlike the EPROM
chip, however, an EEPROM chip need not be taken out of the computer or electronic device of
which it is part when a new program or data needs to be written on it.
Selective programing can be done to an EEPROM chip. The user can alter the value of certain cells
without needing to erase the programming on other cells. Thus, sections of data can be erased
and replaced without needing to alter the rest of the chip’s programming.
Data stored in an EEPROM chip is permanent, at least until the user decides to erase and replace
the information it contains. Furthermore, the data stored in an EEPROM chip is not lost even
when power is turned off.

8.5.1 EEPROM Structure
The EEPROM chip is physically similar to the EPROM chip. It is also composed of cells with
two transistors. The floating gate is separated from the control gate by a thin oxide layer. Unlike
the EPROM chip, however, the EEPROM chip’s oxide layer is much thinner. In EEPROM chips,
the insulating layer is only around 1 nanometre thick whereas in EPROM chips, the oxide layer
is around 3 nanometres thick. The thinner oxide layer means lower voltage requirements for
initiating changes in cell value.

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