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Example of writable volatile random-access memory: Synchronous Dynamic RAM modules, primarily used as main memory in personal computers, workstations, and servers.
1 Megabit chip – one of the last models developed by VEB Carl Zeiss Jena in 1989
Knowledge Bank
Ram
Ram
RAM -- Random Access Memory
DRAM -- Dynamic RAM
Dynamic random access memory (DRAM) is the most common kind of random access memory (RAM) for personal computers and workstations. Memory is the network of electrically-charged points in which a computer stores quickly accessible data in the form of 0s and 1s. Random access means that the PC processor can access any part of the memory or data storage space directly rather than having to proceed sequentially from some starting place. DRAM is dynamic in that, unlike static RAM (SRAM), it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. Static RAM does not need refreshing because it operates on the principle of moving current that is switched in one of two directions rather than a storage cell that holds a charge in place. Static RAM is generally used for cache memory, which can be accessed more quickly than DRAM.
DRAM stores each bit in a storage cell consisting of a capacitor and a transistor. Capacitors tend to lose their charge rather quickly; thus, the need for recharging. A variety of other RAM interfaces to the computer exist, such as EDO RAM and SDRAM.
RDRAM -- Rambus DRAM
If you are using RDRAM, make sure that all memory sockets of a channel are filled with either a memory chip or a continuity module.
RDRAM often has to be installed in pairs of the same type of memory chips.
RDRAM devices may be configured into single-, dual- or quad-channel RIMM modules. For dual-channel or quad-channel (4-channel) RDRAM chipsets and motherboards, memory module upgrades should be in matched pairs. For instance, to add 512 MByte of memory into a dual or 4-channel system, two matched 256 MByte modules should be inserted.
32-bit RIMM modules, such as RIMM 4200, 4800, and 6400 modules, can be upgraded singly on dual channel systems.
Q: DIMM (Dual Inline Memory Module) or RIMM (Rambus Inline Memory Module)?
A: RIMM is the trademarked name for a Direct Rambus memory module.
RDRAM is available only in RIMM packages (such as 184-pin RIMMs).
RIMMs look similar to DIMMs, but have a different pin count. (Typically DIMMs are 168-pin, while RIMMs are 184-pin.) RIMMs transfer data in 16-bit chunks.
For more information, see the Rambus F.A.Q.
Example of RDRAM speeds include PC 800 (PC-800 or PC800) and PC 1066 (PC-1006 or PC1066).
Dual Channel RDRAM -- Dual-channel PC800 RDRAM, as found in Intel's 850 Pentium 4 chipset, provides a higher bandwidth than even PC2700 DDR-SDRAM (DDR333).
[view chart]
SDRAM -- Synchronous DRAM (JEDEC SDRAM)
SDRAM (synchronous DRAM) is a generic name for various kinds of dynamic random access memory (DRAM) that are synchronized with the clock speed that the microprocessor is optimized for. This tends to increase the number of instructions that the processor can perform in a given time. The speed of SDRAM is rated in MHz rather than in nanoseconds (ns). This makes it easier to compare the bus speed and the RAM chip speed. You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.
Example: PC-133 CL2 SDRAM = 133MHz (SDRAM, PC133 • CL=2 • Unbuffered • Non-parity • 133MHz • 3.3V )
DDR SDRAM -- Double Data Rate SDRAM
DDR SDRAM is synchronous dynamic RAM (SDRAM) that can theoretically improve memory clock speed to at least 200 MHz*. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling output. It's expected that a number of Socket 7 chipset makers will support this form of SDRAM.
When released DDR SDRAM memory was about twice as expensive as conventional SDRAM memory.
*Synchronous DRAM speed is measured in MHz rather than nanoseconds (ns). You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.
Examples:
PC-4000 = DDR-500
PC-3200 = DDR-400 *
PC-2700 = DDR-333 (DDR333)*
PC-2100 = DDR-266 (DDR266)
PC133
PC100 = PC-100 DDR RAM was sometimes called PC-1600 SDRAM because of its data bandwidth (transfer capacity) of 1.6GB per second.
*comes in 184-pin DIMM and 200-pin SODIMM formats
The fiirst official platform with DDR SDRAM support was released in October 2000.
DDR2 SDRAM -- Double Data Rate SDRAM
Examples:
DDR2 PC2-3200 = DDR2-400
DDR2 PC2-4300 = DDR2-533
DDR2 PC2-3200 = DDR2-400
DDR2 PC2-4300 = DDR2-533
Random-access memory
From Wikipedia, the free encyclopedia
"RAM" redirects here. For the Daft Punk album, see Random Access Memories. For other uses of the word, see Ram (disambiguation).
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This article has multiple issues. Please help improve it or discuss these issues on the talk page.
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| Early stage NVRAM |
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Random-access memory (RAM /ræm/) is a form of computer data storage. A random-access device allows stored data to be accessed directly in any random order. In contrast, other data storage media such as hard disks, CDs, DVDs and magnetic tape, as well as early primary memory types such as drum memory,
read and write data only in a predetermined order, consecutively,
because of mechanical design limitations. Therefore the time to access a
given data location varies significantly depending on its physical
location.
Today, random-access memory takes the form of integrated circuits. Strictly speaking, modern types of DRAM are not random access, as data is read in bursts, although the name DRAM / RAM has stuck. However, many types of SRAM, ROM, OTP, and NOR flash are still random access even in a strict sense. RAM is normally associated with volatile types of memory (such as DRAM memory modules),
where its stored information is lost if the power is removed. Many
other types of non-volatile memory are RAM as well, including most types
of ROM and a type of flash memory called NOR-Flash. The first RAM modules to come into the market were created in 1951 and were sold until the late 1960s and early 1970s.
Contents
History
Early computers used relays, or delay lines for "main" memory functions. Ultrasonic delay lines could only reproduce data in the order it was written. Drum memory
could be expanded at low cost but retrieval of non-sequential memory
items required knowledge of the physical layout of the drum to optimize
speed. Latches built out of vacuum tube triodes, and later, out of discrete transistors,
were used for smaller and faster memories such as random-access
register banks and registers. Such registers were relatively large,
power-hungry and too costly to use for large amounts of data; generally
only a few hundred or few thousand bits of such memory could be
provided.
The first practical form of random-access memory was the Williams tube starting in 1947. It stored data as electrically charged spots on the face of a cathode ray tube.
Since the electron beam of the CRT could read and write the spots on
the tube in any order, memory was random access. The capacity of the
Williams tube was a few hundred to around a thousand bits, but it was
much smaller, faster, and more power-efficient than using individual
vacuum tube latches.
Magnetic-core memory
was invented in 1947 and developed up until the mid-1970s. It became a
widespread form of random-access memory, relying on an array of
magnetized rings. By changing the sense of each ring's magnetization,
data could be stored with one bit stored per ring. Since every ring had a
combination of address wires to select and read or write it, access to
any memory location in any sequence was possible.
Magnetic core memory was the standard form of memory system until
displaced by solid-state memory in integrated circuits, starting in the
early 1970s. Robert H. Dennard invented dynamic random-access memory
(DRAM) in 1968; this allowed replacement of a 4 or 6-transistor latch
circuit by a single transistor for each memory bit, greatly increasing
memory density at the cost of volatility. Data was stored in the tiny
capacitance of each transistor, and had to be periodically refreshed in a
few milliseconds before the charge could leak away.
Prior to the development of integrated read-only memory (ROM) circuits, permanent (or read-only) random-access memory was often constructed using diode matrices driven by address decoders, or specially wound core rope memory planes.
Types of RAM
The three main forms of modern RAM are static RAM (SRAM), dynamic RAM (DRAM) and phase-change memory (PRAM). In SRAM, a bit of data is stored using the state of a flip-flop.
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 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.
Memory hierarchy
Main article: Memory hierarchy
One can read and over-write data in RAM. Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems and virtual memory or swap space
on a hard drive. This entire pool of memory may be referred to as "RAM"
by many developers, even though the various subsystems can have very
different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media
or a tape is variable. The overall goal of using a memory hierarchy is
to obtain the higher possible average access performance while
minimizing the total cost of the entire memory system (generally, the
memory hierarchy follows the access time with the fast CPU registers at
the top and the slow hard drive at the bottom).
In many modern personal computers, the RAM comes in an easily upgraded form of modules called memory modules
or DRAM modules about the size of a few sticks of chewing gum. These
can quickly be replaced should they become damaged or when changing
needs demand more storage capacity. As suggested above, smaller amounts
of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system.
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
Main article: 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
Main article: 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.[1]
Recent developments
Several new types of non-volatile RAM, which will preserve data while powered down, are under development. The technologies used include carbon nanotubes and approaches utilizing the magnetic tunnel effect. Amongst the 1st generation MRAM, a 128 KiB (128 × 210 bytes) magnetic RAM (MRAM) chip was manufactured with 0.18 µm technology in the summer of 2003. In June 2004, Infineon Technologies unveiled a 16 MiB (16 × 220 bytes) prototype again based on 0.18 µm technology. There are two 2nd generation techniques currently in development: Thermal Assisted Switching (TAS)[2] which is being developed by Crocus Technology, and Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working.[3] Nantero built a functioning carbon nanotube memory prototype 10 GiB (10 × 230
bytes) array in 2004. Whether some of these technologies will be able
to eventually take a significant market share from either DRAM, SRAM, or
flash-memory technology, however, remains to be seen.
Since 2006, "solid-state drives"
(based on flash memory) with capacities exceeding 256 gigabytes and
performance far exceeding traditional disks have become available. This
development has started to blur the definition between traditional
random-access memory and "disks", dramatically reducing the difference
in performance.
Some kinds of random-access memory, such as "EcoRAM", are specifically designed for server farms, where low power consumption is more important than speed.[4]
Memory wall
The "memory wall" is the growing disparity of speed between CPU and
memory outside the CPU chip. An important reason for this disparity is
the limited communication bandwidth beyond chip boundaries. From 1986 to
2000, CPU
speed improved at an annual rate of 55% while memory speed only
improved at 10%. Given these trends, it was expected that memory latency
would become an overwhelming bottleneck in computer performance.[5]
CPU speed improvements slowed significantly partly due to major
physical barriers and partly because current CPU designs have already
hit the memory wall in some sense. Intel summarized these causes in a 2005 document.[6]
“First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat... Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.”
The RC delays in signal transmission were also noted in Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures which projects a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly shows a slowdown in performance improvements in recent processors. However, Intel's Core 2 Duo processors (codenamed Conroe) showed a significant improvement over previous Pentium 4 processors; due to a more efficient architecture, performance increased while clock rate actually decreased.[citation needed]
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