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Apr 27, 2003
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Random-access memory (RAM) is computer data storage which allows access to any element in it in an equal amount of time. Access time to any data does not depend upon prior accesses, location of storage, or other factors. While many storage devices provide this, the term RAM specifically refers to solid-state memory devices with this function which are both readable and writeable.
The distinction comes from the use of the term "memory", in this case referring to a monolithic memory device, as opposed to a circuit or some other system of components which together act as data storage.
The term was originally used to distinguish a class of memory devices from other solid state memories such as [wiki]ROM[/wiki]s, [wiki]CCD[/wiki]s, and [wiki]shift register[/wiki]s.
Since that time, the term has been applied to other forms of storage which are not strictly RAM. These include devices which do not provide uniform access to data elements within them, such as page-mode and burst-mode RAM, as well as storage such as DVD-RAM which uses the term RAM to differentiate its features from other forms of DVD storage.
[h="2"]DRAM vs. SRAM[/h]
The major division between traditional RAM types is [wiki="Dynamic RAM"]DRAM[/wiki] versus [wiki="Static RAM"]SRAM[/wiki]. SRAM, or Static RAM, stores its data indefinitely while the device has power. DRAM, or Dynamic RAM will only maintain the state of its storage cells for a short period of time before they are lost, unless a "refresh" restores the charge on memory cells in a charged state. This requires dedicated circuitry outside the memory device, and may interfere with system access to the memory during refresh.
DRAM memory cells require fewer components than those of static memories, so for the same [wiki]process technology[/wiki], greater memory densities can be achieved with DRAM than with SRAM. This means that the price per bit for DRAM memory is usually a fraction of that for static memories in contemporary technology.
SRAM is simpler to incorporate in a design than DRAM, as it requires no refresh circuitry. Also, SRAM usually requires less power for both operation during accesses and data retention when not being accessed than contemporary DRAM.
[h="2"]Use in Vintage Systems[/h]
RAM typically forms the working data store of a computer system. In most mass produced commercial designs, the main memory will consist of Dynamic RAM. Static RAM is used in many of these systems as data or instruction caches external to the [wiki]microprocessor[/wiki]. RAM is also often incorporated in other devices within the system, notably [wiki]real time clock[/wiki]s. In [wiki]PC AT[/wiki] and related systems, the "CMOS RAM" was SRAM in such a device, used to maintain information about the system's configuration, including number and type of integrated peripherals.
[h="2"]Example Devices[/h]
  • Fairchild 4100, introduced 1970, 256 bit (256 x 1) bipolar TTL SRAM manufactured for use in the [wiki]Fairchild[/wiki] Illiac IV computer. 16 pin DIP package, 100nS access time, operating power ~20mW/bit.
  • Intel 3101, released 1969, 64 bit (16 x 4) bipolar Schottky TTL DRAM, first commodity RAM device. 16 pin DIP package, 60nS access time, operating power 8mW/bit.
  • Intel 1101, released 1970, 256 bit (256 x 1) static p-channel MOS. 16 pin DIP package, 1500nS access time, operating power 2.5mW/bit.
  • Intel 1103, released 1970, 1024 bit (1024 x 1) dynamic p-channel MOS. 18 pin 0.4" DIP package, 300nS access time, operating power 0.45mW/bit.
  • Intel 2102, released 1971, 1024 bit (1024 x 1) static n-channel MOS. 18 pin 0.4" DIP package, 1000nS access time, operating power 0.35mW/bit.