Dynamic random access memory
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Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor. As real-world capacitors are not ideal and hence leak electrons, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory. Its advantage over SRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to six transistors in SRAM. This allows DRAM to reach very high density. Since DRAM loses its data when the power supply is removed, it is in the class of volatile memory devices.
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History
The first DRAM cell was invented in 1966 by Robert Dennard, a researcher at IBM's Thomas J. Watson Research Center. This cell worked like most modern DRAM cells in that data must be refreshed to restore the charge; the data was destroyed after a read operation and had to be rewritten. Although DRAM cells are ubiquitous today, Dennard was one of few people at the time who believed an effective DRAM cell could be developed.
Principle of operation
DRAM is usually arranged in a square array of one capacitor and transistor per cell. The illustrations above show a simple example with only 4 by 4 cells (modern DRAM can be thousands of cells in length/width). A read operation proceeds as follows: the row of the selected cell is activated, turning on the transistors and connecting the capacitors of that row to the sense lines. The sense lines lead to the sense amplifiers, which discriminate between a stored 0 or 1. The amplified value from the appropriate column is then selected and connected to the output. At the end of a read cycle, the row values must be restored to the capacitors, which were depleted during the read. This write is done by activating the row and connecting the values to be written to the sense lines, which charges the capacitors to the desired values. During a write to a particular cell, the entire row is read out, one value changed, and then the entire row is written back in, as illustrated in the figure to the right.
Typically, manufacturers specify that each row should be refreshed every 64 ms or less, according to the JEDEC standard. Refresh logic is commonly used with DRAMs to automate the periodic refresh. This makes the circuit more complicated, but this drawback is usually outweighed by the fact that DRAM is much cheaper and of greater capacity than SRAM. Some systems refresh every row in a tight loop that occurs once every 64 ms. Other systems refresh one row at a time -- for example, a system with 213 = 8192 rows would require a refresh rate of one row every 7.8 µs (64 ms / 8192 rows). Both methods require some sort of counter to keep track of which row is the next to be refreshed. Some DRAM chips include that counter; other kinds require external refresh logic to hold that counter. (Under some conditions, most of the data in DRAM can be recovered even if the DRAM has not been refreshed for several minutes [1].)
Errors and error correction
Electrical or magnetic interference inside a computer system can cause a single bit of DRAM to spontaneously flip to the opposite state. Some research has shown that the majority of one-off ("soft") errors in DRAM chips occur as a result of cosmic rays, which may change the contents of one or more memory cells, or interfere with the circuitry used to read/write them - there is some concern that as DRAM density increases further, and thus the components on DRAM chips get smaller; whilst at the same time operating voltages continue to fall, DRAM chips will be affected by such radiation more frequently - since lower energy particles will be able to change a memory cell's state. On the other hand, smaller cells make smaller targets, and moves to technologies such as SOI may make individual cells less susceptible and so counteract, or even reverse this trend.
This problem can be mitigated by using DRAM modules that include extra memory bits and memory controllers that exploit these bits. These extra bits are used to record parity or to use an ECC. Parity allows the detection of a single-bit error (actually, any odd number of wrong bits). The most common error correcting code, Hamming code, allows a single-bit error to be corrected and (in the usual configuration, with an extra parity bit) double-bit errors to be detected.
Error detection and correction in computer systems seems to go in and out of fashion. Seymour Cray famously said "parity is for farmers" when asked why he left this out of the CDC 6600. He included parity in the CDC 7600, and reputedly said "I learned that a lot of farmers buy computers." 486-era PCs often used parity. Pentium-era ones mostly did not. Wider memory busses make parity and especially ECC more affordable. Current microprocessor memory controllers generally support ECC but most non-server systems do not use these features. Even if they do, it isn't clear that the software layers do their part.
Memory controllers in most modern PCs can typically detect, and correct errors of a single bit per 64 bit "word" (the unit of bus transfer), and detect (but not correct) errors of two bits per 64 bit word. Some systems also 'scrub' the errors, by writing the corrected version back to memory. The BIOS in some computers, and operating systems such as Linux, allow counting of detected and corrected memory errors, in part to help identify failing memory modules before the problem becomes catastrophic. Unfortunately, most modern PCs are supplied with memory modules that have no parity or ECC bits.
Error detection and correction depends on an expectation of the kinds of errors that occur. Implicitly, we have assumed that the failure of each bit in a word of memory is independent and hence that two simultaneous errors are improbable. This used to be the case when memory chips were one bit wide (typical in the first half of the 1980s). Now many bits are in the same chip. This weakness does not seem to be widely addressed; one exception is Chipkill.
A reasonable rule of thumb is to expect one bit error, per month, per gigabyte of memory. Actual error rates vary widely.[ecc]
Variations
Fast Page Mode DRAM (FPM)
Fast page mode DRAM is also called FPM DRAM, Page mode DRAM, Fast page mode memory, or Page mode memory.
In page mode, a row of the DRAM can be kept "open", so that successive reads or writes within the row do not suffer the delay of precharge and accessing the row. This increases the performance of the system when reading or writing bursts of data.
Static column is a variant of page mode in which the column address does not need to be strobed in.
Nibble mode is another variant in which four sequential locations within the row can be accessed.
Video DRAM (VRAM)
VRAM is a dual-ported version of DRAM formerly used in graphics adaptors. It is now almost obsolete, having been superseded by SDRAM and SGRAM. VRAM has two paths (or ports) to its memory array that can be used simultaneously.
The first port, the DRAM port, is accessed as with plain DRAM. The second port, the video port, is read-only, and is dedicated to feeding a fast stream of data to the display. To use the video port, the controller first uses the DRAM port to select the row of the memory array that is to be displayed. The VRAM then copies that entire row to an internal shift-register. The controller can then continue to use the DRAM port for drawing objects on the display. Meanwhile, the controller feeds a clock called the shift clock (SCLK) to the VRAM's video port. Each SCLK pulse causes the VRAM to deliver the next item of data, in strict address order, from the shift-register to the video port. For simplicity, the graphics adapter is usually designed so that the contents of a row, and therefore the contents of the shift-register, corresponds to a complete horizontal line on the display.
Window RAM (WRAM)
Window RAM or WRAM is an obsolete type of semiconductor computer memory that was designed to replace video RAM (VRAM) in graphics adapters. It was developed by Samsung and also marketed by Micron Technology, but had only a short market life before being superseded by SDRAM and SGRAM.
WRAM has a dual-ported dynamic RAM structure similar to that of VRAM, with one parallel port and one serial port, but has extra features to enable fast block copies and block fills (so-called window operations). It was often clocked at 50 MHz. It has a 32-bit wide host port to enable optimal data transfer in PCI and VESA Local Bus systems. Typically WRAM was 50% faster than VRAM, but with costs 20% lower. It is sometimes erroneously called Windows RAM, because of confusion with the Microsoft Windows operating systems, to which it is unrelated.
It was used by Matrox on both their MGA Millennium and Millennium II graphics cards.
Extended Data Out (EDO) DRAM
EDO DRAM is similar to Fast Page Mode DRAM with the additional feature that a new access cycle can be started while keeping the data output of the previous cycle active. This allows a certain amount of overlap in operation (pipelining), allowing somewhat improved speed. It was 5% faster than Fast Page Mode DRAM, which it began to replace in 1993.
Single-cycle EDO has the ability to carry out a complete memory transaction in one clock cycle. Otherwise, each sequential RAM access within the same page takes two clock cycles instead of three, once the page has been selected. EDO's speed and capabilities allowed it to somewhat replace the then-slow L2 caches of PCs. It created an opportunity to reduce the immense performance loss associated with a lack of L2 cache, while making systems cheaper to build. This was also good for notebooks due to difficulties with their limited form factor, and battery life limitations. Performance was still lost though, and an EDO system with L2 cache was tangibly faster than the older FPM/L2 combination.
Single-cycle EDO DRAM became very popular on video cards towards the end of the 1990s. It was very low cost, yet nearly as efficient for performance as the far more costly VRAM.
EDO was sometimes referred to as Hyper Page Mode.
Burst EDO (BEDO) DRAM
An evolution of the former, Burst EDO DRAM, could process four memory addresses in one burst, for a maximum of 5-1-1-1, saving an additional three clocks over optimally designed EDO memory. It was done by adding an address counter on the chip to keep track of the next address. BEDO also added a pipelined stage allowing page-access cycle to be divided into two components. During a memory-read operation, the first component accessed the data from the memory array to the output stage (second latch). The second component drove the data bus from this latch at the appropriate logic level. Since the data is already in the output buffer, faster access time is achieved (up to 50% for large blocks of data) than with traditional EDO.
Although BEDO DRAM showed additional optimization over EDO, by the time it was available, the market had made a significant investment towards synchronous DRAM, or SDRAM [2], even though BEDO RAM was technically superior to SDRAM.
Multibank DRAM (MDRAM)
Multibank RAM applies the interleaving technique for main memory to second level cache memory to provide a cheaper and faster alternative to SRAM. The chip splits its memory capacity into small blocks of 256 kB and allows operations to two different banks in a single clock cycle.
This memory was primarily used in graphic cards with Tseng Labs ET6x00 chipsets, and was made by MoSys. Boards based upon this chipset often used the unusual RAM size configuration of 2.25 MB, owing to MDRAM's ability to be implemented in various sizes more easily. This size of 2.25 MB allowed 24-bit color at a resolution of 1024x768, a very popular display setting in the card's time.
Synchronous Graphics RAM (SGRAM)
SGRAM is a specialized form of SDRAM for graphics adaptors. It adds functions such as bit masking (writing to a specified bit plane without affecting the others) and block write (filling a block of memory with a single colour). Unlike VRAM and WRAM, SGRAM is single-ported. However, it can open two memory pages at once, which simulates the dual-port nature of other video RAM technologies.
SGRAM and SDRAM became the most popular types of DRAM at the end of the 1990s, and well into the first decade of the 2000s.
Synchronous Dynamic RAM (SDRAM)
Single Data Rate (SDR) SDRAM is a synchronous form of DRAM.
Direct Rambus DRAM (DRDRAM)
Double Data Rate (DDR) SDRAM
Double data rate (DDR) SDRAM was a later development of SDRAM, used in PC memory from 2000 onwards. DDR2 SDRAM is a minor enhancement on DDR-SDRAM that mainly affords higher clock speeds and somewhat deeper pipelining.
Quad Data Rate (QDR) SDRAM
Pseudostatic RAM (PSRAM)
PSRAM is dynamic RAM with built-in refresh and address-control circuitry to make it behave similarly to static RAM (SRAM). It combines the high density of DRAM with the ease of use of true SRAM.
Some DRAM components have a "self-refresh mode". While this involves much of the same logic that is needed for pseudo-static operation, this mode is often equivalent to a standby mode. It is provided primarily to allow a system to suspend operation of its DRAM controller to save power without losing data stored in DRAM, not to allow operation without a DRAM controller as is the case with PSRAM.
Spelling and Pronunciation
There is quite a lot of argument about the correct pronunciation of DRAM. Most professional people pronounce it "Dee Ram". But other people occasionally pronounce it as one syllable, like "Dram", maybe because they have only seen it written down.
Because it is an acronym, DRAM should always be spelled in all caps. Both "Dram" and "DRam" are incorrect spellings, but these are occasionally used by writers or editors unfamiliar with the term.
Both spelling and pronunciation reflect the fact that the word is a partial acronym (or perhaps more correctly, an initialism), which was derived from the older acronym 'RAM' by the addition of the prefix 'D' to distinguish it from SRAM, VRAM and so on.
For other words derived from RAM, a more general pronunciation rule is that 'RAM' is always pronounced as single word, while any letters preceding it are spelled out. So, for example, DDR SDRAM is pronounced 'Dee Dee Ar Es Dee Ram'
See also
External links
- Basic DRAM operation has some interesting historical trend charts of cell size and DRAM density -- but they only go to 1995. Anyone have more recent data?
- Back to Basics - Memory, part 3
- Benefits of Chipkill-Correct ECC for PC Server Main Memory - A 1997 discussion of SDRAM reliability - some interesting information on "soft errors" from cosmic rays, especially with respect to Error-correcting code schemes
- a Tezzaron Semiconductor Soft Error White Paper 1994 litterature review of memory error rate measurements.
- Soft errors' impact on system reliability - Ritesh Mastipuram and Edwin C Wee, Cypress Semiconductor, 2004
- Scaling and Technology Issues for Soft Error Rates - A Johnston - 4th Annual Research Conference on Reliability Stanford University, October 2000
- Challenges and future directions for the scaling of dynamic random-access memory (DRAM) - J. A. Mandelman, R. H. Dennard, G. B. Bronner, J. K. DeBrosse, R. Divakaruni, Y. Li, and C. J. Radens, IBM 2002
- http://arstechnica.com/paedia/r/ram_guide/ram_guide.part1-2.html
