Abstract :
Random-access
memory (RAM ) is a form of computer data storage.
A random-access memory device allows data items
to be read and written in approximately the same amount of time, regardless of
the order in which data items are accessed.[1] In contrast,
with other direct-access data storage media such as hard disks, CD-RWs, DVD-RWs and the older drum memory, the time required
to read and write data items varies significantly depending on their physical
locations on the recording medium, due to mechanical limitations such as media rotation
speeds and arm movement delays.
Today, random-access
memory takes the form of integrated circuits. RAM is
normally associated with volatile types
of memory (such as DRAM memory modules),
where stored information is lost if power is removed, although many efforts have
been made to develop non-volatile RAM chips. Other types of non-volatile memory
exist that allow random access for read operations,
but either do
not allow write operations or have limitations on them. These include most types
of
ROM and a type
of flash memory called NOR-Flash.
Types of RAM
• Static
RAM
• Dynamic
RAM
The two
main forms of modern RAM are static RAM
(SRAM) and dynamic RAM (DRAM).
In SRAM, a bit of data is stored using the state of a
six transistor memory cell.
This form of RAM is more expensive
to produce, but is generally faster and requires less power than DRAM and, in
modern computers, is often used as cache memory for the CPU.
DRAM stores a bit of data using a transistor and capacitor pair, which together
comprise a DRAM memory cell.
The capacitor holds a high or low charge (1 or 0, respectively), and the transistor
acts as a switch that lets the control circuitry on the chip read the capacitor's
state of charge or change it. As this form of memory is less expensive to produce
than static RAM, it is the predominant form of computer memory used in modern computers.
Both static
and dynamic RAM are considered volatile, as their state is lost or reset when
power is removed from the system. By contrast, read-only memory (ROM) stores
data by permanently enabling or disabling selected transistors, such that the memory
cannot be altered. Writeable variants of ROM (such as EEPROM and flash memory) share properties
of both ROM and
RAM, enabling
data to persist without
power and to be updated without requiring special equipment. These persistent
forms of semiconductor ROM include USB flash
drives, memory cards for cameras and portable devices, etc. ECC memory (which can be either
SRAM or DRAM) includes special circuitry to detect and/or correct random faults
(memory errors) in the stored data, using parity bits or error correction code.
In general,
the term RAM refers solely to solid-state memory devices (either DRAM or SRAM),
and more specifically the main memory in most computers. In optical storage, the
term DVD- RAM is somewhat of a misnomer
since, unlike CD-RW or
DVD-RW it does not need to
be erased before reuse. Nevertheless a DVD-RAM behaves much like a hard disc drive
if somewhat slower.
Other uses of
RAM
In addition
to serving as temporary storage and working space for the operating system and applications,
RAM is used in numerous other ways.
Virtual memory
Most modern
operating systems employ a method of extending RAM capacity, known as "virtual
memory". A portion of the computer's hard drive is set aside for
a paging file or a scratch partition, and the combination of physical RAM and the
paging file form the system's total memory. (For example, if a computer has 2 GB
of RAM and a 1 GB page file, the operating system has 3 GB total memory available
to it.) When the system runs low on physical memory, it can "swap" portions of RAM to the paging
file to make room for new data, as well as to read previously swapped information
back into RAM. Excessive use of this mechanism results in thrashing and
generally hampers overall system performance, mainly because hard drives are far
slower than RAM.
RAM disk
Software can
"partition" a portion of a computer's RAM, allowing it to act as a much
faster hard drive that is called a RAM disk.
A RAM disk loses the stored data when the computer is shut down, unless memory is
arranged to have a standby battery source.
Shadow RAM
Sometimes,
the contents of a relatively slow ROM chip are copied to read/write memory to
allow for shorter access times. The ROM chip is then disabled while the initialized
memory locations are switched in on the same block of addresses (often write-protected).
This process, sometimes called shadowing, is fairly common in both computers
and embedded systems.
As a common
example, the BIOS in
typical personal computers often has an option called “use shadow BIOS” or similar.
When enabled, functions relying on data from the BIOS’s ROM will instead use DRAM
locations (most can also toggle shadowing of video card ROM or other ROM sections).
Depending on the system, this may not result in increased performance, and may cause
incompatibilities. For example, some hardware may be inaccessible to the operating system if shadow
RAM is used. On some systems the benefit may be hypothetical because the BIOS is
not used after booting in favor of direct hardware access. Free memory is reduced
by the size of the shadowed ROMs.
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