Date: February 23, 2001
From: barefaced Semiconductor memory is an Intel creation. Many early semiconductor
numbers were based upon the Intel numbering system. 21xx was Static memory (SRAM or Static
Random Access Memory). 27xx was EPROM (Erasable Programmable Read Only Memory). 28xx was
EEPROM (Electrically Erasable Programmable ROM), 80xx were microprocessors, 82xx were
processor peripherals, 87xx were processors containing EPROM - single chip computers. As
Intel left the memory business to concentrate on processors, others took over and modified
the numbering systems. Basically, the last digits (xx in Intel's numbers) suggest the
memory size.
Static memory remembers as long as power was available. EPROM always remember but must
be written to (modified) externally. EEPROM always remember, can be written to and read
from, like RAM, but can only be written to a limited number of times. Dynamic memory (DRAM
or Dynamic Random Access Memory) will forget if power is removed and if not refreshed
every so many milliseconds.
DRAM is the base core memory in PC today because more memory locations can be stored in
one chip. DRAMs use less transistors to remember each bit, but must be constantly
refreshed and are not as fast as SRAMs.
SRAMs are used for cache because they are faster and don't require refreshing.
EPROM is where the BIOS was stored. Newer processors now use EEPROMs to store BIOS,
code for the modem computer, etc so that you can upgrade your BIOS and modem software.
Those are the basic semiconductor memory structures and their early history.
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