Semiconductor memory
Semiconductor memory technology is an essential element of today’s electronics.
Normally based around semiconductor technology, memory is used in any equipment
that uses a processor of one form or another.
With the rapid growth in the requirement for semiconductor
memories there have been a number of technologies and types of memory that have
emerged. Names such as ROM, RAM, EPROM, EEPROM, Flash memory, DRAM, SRAM,
SDRAM, and the very new MRAM can now be seen in the electronics literature.
Each one has its own advantages and area in which it may be used.
Types
of semiconductor memory
Electronic semiconductor memory technology
can be split into two main types or categories, according to the way in which
the memory operates:
RAM – Random Access Memory
Random Access Memory (RAM)is the best known form of computer memory.
The Read and write (R/W) memory of a computer is called RAM. The User
can write information to it and read information from it.
The RAM is a volatile memory, it
means information written to it can be accessed as long as power is on. As soon
as the power is off, it cannot be accessed. so this mean RAM computer memory
essentially empty. RAM holds data and processing instructions temporarily until
the CPU needs it. Scratchpad storage in
memory space is used for the temporary storage of
data.
ROM – Read Only
Memory
Read only memory (ROM) is an example of nonvolatile
memory. ROM is a class of storage medium used in
computers and other electronic devices. Read Only Memory (ROM), also
known as firmware, is an integrated
circuit programmed with specific data when it is manufactured.
The instructions for starting the computer are housed on Read only memory chips.
Semiconductor
Memory Technologies
There is a large variety of types of ROM and
RAM that are available. These arise from the variety of applications and also
the number of technologies available. This means that there is a large number
of abbreviations or acronyms and categories for memories ranging from Flash to
MRAM, PROM to EEPROM, and many more:
PROM:
This stands for Programmable Read Only Memory.
It is a semiconductor memory which can only
have data
written to it once – the data written to it is permanent.
These memories are bought in a blank format and they are programmed using a
special PROM programmer. Typically a PROM will
consist of an array of fuse able links some of which are “blown” during
the programming process to provide the required data pattern.
The PROM stores its data as a charge
on a capacitor. There is a charge storage capacitor for each
cell and this can be read repeatedly as required. However it is found that
after many years the charge may leak away
and the data may be lost. Nevertheless, this type of semiconductor memory used
to be widely used in applications where a form of ROM was required, but where
the data needed to be changed periodically, as in a development environment, or
where quantities were low.
EPROM:
This is an Erasable Programmable Read Only Memory.
This form of semiconductor memory can
be programmed and then erased at a later time. This is normally achieved by
exposing the silicon to ultraviolet light. To
enable this to happen there is a circular window in the package of the EPROM to
enable the light to reach the silicon of the chip. When the PROM is in use,
this window is normally covered by a label, especially when the data may need
to be preserved for an extended period.
EEPROM:
This is an Electrically Erasable Programmable
Read Only Memory. Data can be written to it and it can be erased
using an electrical voltage. This is typically applied to an erase
pin on the chip. Like other types of PROM, EEPROM retains the
contents of the memory even when the power is turned off. Also like other types
of ROM, EEPROM is not as fast as RAM.
EEPROM memory cells are made from
floating-gate MOSFETS (known as FGMOS)
Flash memory: Flash memory may be
considered as a development of EEPROM technology. Data can be written to
it and it can be erased, although only in blocks, but data can be read on an
individual cell basis. To erase and re-programmed areas of the chip,
programming voltages at levels that are available within electronic equipment are used. It is also non-volatile, and this makes it particularly useful. As a result Flash memory
is widely used in many applications including memory cards for digital cameras, mobile phones, computer memory
sticks and many other applications.
Flash memory stores data in an array of memory cells. The memory cells are made from floating-gate MOSFETS (known as FGMOS). These FG MOSFETs (or
FGMOS in short) have the ability to store an electrical charge for extended
periods of time (2 to 10 years) even without a connecting to a power supply.
DRAM:
Dynamic RAM is a form of random access memory. DRAM uses a capacitor to store each bit of
data, and the level of charge on each capacitor determines whether that bit is
a logical 1 or 0. However these capacitors
do not hold their charge indefinitely, and therefore the data needs to be refreshed periodically. As a result of this
dynamic refreshing it gains its name of being a dynamic RAM. DRAM is the form
of semiconductor memory that is often used
in equipment including personal computers and workstations where it forms the
main RAM for the computer.
Disadvantage: Need to refresh the capacitor charge every once in
two milliseconds
SRAM:
Static Random Access Memory. This form of semiconductor memory gains its name from the fact that, unlike DRAM, the data does not need to be refreshed dynamically. It is able to support
faster read and write times than DRAM (typically 10 ns against 60 ns for DRAM),
and in addition its cycle time is much shorter because it does not
need to pause between accesses. However it consumes more power, is less dense
and more expensive than DRAM. As a result of this it is normally used for
caches, while DRAM is used as the main semiconductor memory
technology.
SDRAM:
Synchronous DRAM. This form of
semiconductor memory can run at faster speeds than conventional DRAM. It is synchronized
to the clock of the processor and is capable of keeping two sets of memory addresses open simultaneously.
By transferring data alternately from one set of
addresses, and then the other, SDRAM cuts down on the delays associated with
non-synchronous RAM, which must close one address bank before opening the next.
MRAM:
This is Magneto-resistive RAM, or Magnetic RAM. It is a non-volatile RAM memory technology that uses magnetic charges to store data instead of electric charges. Unlike
technologies including DRAM, which require a constant flow of electricity to
maintain the integrity of the data, MRAM retains data even when the power is
removed. An additional advantage is that it only requires low power for active
operation. As a result this technology could become a major player in the
electronics industry now that production processes have been developed to
enable it to be produced.
Just For Information
History of Computer Memory
• In 1932 Gustav Tauschek invents drum
memory in Austria.
• 1936 Konrad Zuse applies for a patent
for his mechanical memory to be used in his computer.
This computer memory is based on sliding
metal parts.
• 1939 Helmut Schreyer invents a
prototype memory using neon lamps.
• 1942 The Atanasoff-Berry Computer has
60 50-bit words of memory in the form of capacitors mounted on two revolving
drums. For secondary memory it uses punch cards.
• 1947 Frederick Viehe of Los Angeles,
applies for a patent for an invention that uses magnetic core memory. Magnetic
drum memory is independently invented by several people.
• 1949 Jay Forrester conceives the idea
of magnetic core memory as it is to become commonly used, with a grid of wires
used to address the cores. The first practical form manifests in
1952-53 and renders obsolete previous types of computer memory.
• 1950 Ferranti Ltd. completes the first
commercial computer with 256 40-bit words of main memory and 16K words of
drum memory. Only eight were sold.
• 1951 Jay Forrester files a patent for
matrix core memory.
• 1952 The EDVAC computer is completed
with 1024 44-bit words of ultrasonic memory. A core memory module is added to
the ENIAC computer.
• 1955 An Wang was issued U.S. patent
#2,708,722 with 34 claims for magnetic memory core.
• 1966 Hewlett-Packard releases their
HP2116A real-time computer with 8K of memory. The newly formed Intel starts
sell a semiconductor chip with 2,000 bits of memory.
• 1968 USPTO grants patent 3,387,286 to
IBM’s Robert Dennard for a one-transistor DRAM cell. DRAM stands for Dynamic
RAM (Random Access Memory) or Dynamic Random Access Memory. DRAM
will become the standard memory chip for personal computers replacing
magnetic core memory.
• 1969 Intel begin as chip designers and
produce a 1 KB RAM chip, the largest memory chip to date. Intel soon switch to
being notable designers of computer microprocessors.
• 1970 Intel releases the 1103 chip, the
first generally available DRAM memory chip.
• 1971 Intel releases the 1101 chip, a
256-bit programmable memory, and the 1701 chip, a 256-byte erasable read-only
memory (EROM).
• 1974 Intel receives a U.S. patent for a
“memory system for a multi chip digital computer”.
• 1975 Personal consumer computer Altair
released, it uses Intel’s 8-bit 8080 processor and includes 1 KB of memory.
Later in the same year, Bob Marsh manufacturers the first Processor
Technology’s 4 KB memory boards for the Altair.
• 1984 Apple Computers releases the Macintosh personal
computer. It is the first computer that came with 128KB of memory.
The one-megabyte memory chip is developed.
