Dynamic random-access memory

Dynamic random-access memory (DRAM) is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1. Since even "nonconducting" transistors always leak a small amount, the capacitors will slowly discharge, and the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to static random-access memory (SRAM) and other static types of memory. Unlike flash memory, DRAM is volatile memory (vs. non-volatile memory), since it loses its data quickly when power is removed. However, DRAM does exhibit limited data remanence.

DRAM is widely used in digital electronics where low-cost and high-capacity memory is required. One of the largest applications for DRAM is the main memory (colloquially called the "RAM") in modern computers; and as the main memories of components used in these computers such as graphics cards (where the "main memory" is called the graphics memory). In contrast, SRAM, which is faster and more expensive than DRAM, is typically used where speed is of greater concern than cost, such as the cache memories in processors.

The advantage of DRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to four or six transistors in SRAM. This allows DRAM to reach very high densities. The transistors and capacitors used are extremely small; billions can fit on a single memory chip. Due to the dynamic nature of its memory cells, DRAM consumes relatively large amounts of power, with different ways for managing the power consumption.^[1]

HistoryEdit

The cryptanalytic machine code-named "Aquarius" used at Bletchley Park during World War II incorporated a hard-wired dynamic memory. Paper tape was read and the characters on it "were remembered in a dynamic store. ... The store used a large bank of capacitors, which were either charged or not, a charged capacitor representing cross (1) and an uncharged capacitor dot (0). Since the charge gradually leaked away, a periodic pulse was applied to top up those still charged (hence the term 'dynamic')".^[2]

The Toshiba Toscal BC-1411 electronic calculator, which was introduced in either 1965^[3][4] or November 1966,^[5] used a form of DRAM built from discrete components.^[4] In 1966, dynamic RAM was developed by Dr. Robert Dennard at the IBM Thomas J. Watson Research Center. He was granted U.S. patent number 3,387,286 in 1968. Capacitors had been used for earlier memory schemes such as the drum of the Atanasoff–Berry Computer, the Williams tube and the Selectron tube.

VersionsEdit

While the fundamental DRAM cell and array has maintained the same basic structure (and performance) for many years, there have been many different interfaces for communicating with DRAM chips. When one speaks about "DRAM types", one is generally referring to the interface that is used.

DRAM can be divided into asynchronous and synchronous DRAM. In addition, graphics DRAM is specially designed for graphics tasks, and can be asynchronous or synchronous DRAM in nature. Pseudostatic RAM (PSRAM) have an architecture and interface that closely mimics the operation and interface of static RAM. Lastly, 1T DRAM uses a capacitorless design, as opposed to the usual 1T/1C (one transistor/one capacitor) designs of conventional DRAM.

Asynchronous DRAMEdit

Principles of operationEdit

An asynchronous DRAM chip has power connections, some number of address inputs (typically 12), and a few (typically one or four) bidirectional data lines. There are four active-low control signals:

• RAS, the Row Address Strobe. The address inputs are captured on the falling edge of RAS, and select a row to open. The row is held open as long as RAS is low.
• CAS, the Column Address Strobe. The address inputs are captured on the falling edge of CAS, and select a column from the currently open row to read or write.
• WE, Write Enable. This signal determines whether a given falling edge of CAS is a read (if high) or write (if low). If low, the data inputs are also captured on the falling edge of CAS.
• OE, Output Enable. This is an additional signal that controls output to the data I/O pins. The data pins are driven by the DRAM chip if RAS and CAS are low, WE is high, and OE is low. In many applications, OE can be permanently connected low (output always enabled), but it can be useful when connecting multiple memory chips in parallel.

This interface provides direct control of internal timing. When RAS is driven low, a CAS cycle must not be attempted until the sense amplifiers have sensed the memory state, and RAS must not be returned high until the storage cells have been refreshed. When RAS is driven high, it must be held high long enough for precharging to complete.

Although the DRAM is asynchronous, the signals are typically generated by a clocked memory controller, which limits their timing to multiples of the controller's clock cycle.

RAS Only Refresh (ROR)Edit

Classic asynchronous DRAM is refreshed by opening each row in turn.

The refresh cycles are distributed across the entire refresh interval in such a way that all rows are refreshed within the required interval. To refresh one row of the memory array using RAS Only Refresh, the following steps must occur:

1. The row address of the row to be refreshed must be applied at the address input pins.
2. RAS must switch from high to low. CAS must remain high.
3. At the end of the required amount of time, RAS must return high.

This can be done by supplying a row address and pulsing RAS low; it is not necessary to perform any CAS cycles. An external counter is needed to iterate over the row addresses in turn.^[6]

CAS before RAS refresh (CBR)Edit

For convenience, the counter was quickly incorporated into the DRAM chips themselves. If the CAS line is driven low before RAS (normally an illegal operation), then the DRAM ignores the address inputs and uses an internal counter to select the row to open. This is known as CAS-before-RAS (CBR) refresh. This became the standard form of refresh for asynchronous DRAM, and is the only form generally used with SDRAM.

Hidden refreshEdit

Given support of CAS-before-RAS refresh, it is possible to deassert RAS while holding CAS low to maintain data output. If RAS is then asserted again, this performs a CBR refresh cycle while the DRAM outputs remain valid. Because data output is not interrupted, this is known as hidden refresh.^[7]

Page mode DRAMEdit

Page mode DRAM is a minor modification to the first-generation DRAM IC interface which improved the performance of reads and writes to a row by avoiding the inefficiency of precharging and opening the same row repeatedly to access a different column. In Page mode DRAM, after a row was opened by holding RAS low, the row could be kept open, and multiple reads or writes could be performed to any of the columns in the row. Each column access was initiated by asserting CAS and presenting a column address. For reads, after a delay (t_CAC), valid data would appear on the data out pins, which were held at high-Z before the appearance of valid data. For writes, the write enable signal and write data would be presented along with the column address.^[8]

Page mode DRAM was later improved with a small modification which further reduced latency. DRAMs with this improvement were called fast page mode DRAMs (FPM DRAMs). In page mode DRAM, CAS was asserted before the column address was supplied. In FPM DRAM, the column address could be supplied while CAS was still deasserted. The column address propagated through the column address data path, but did not output data on the data pins until CAS was asserted. Prior to CAS being asserted, the data out pins were held at high-Z. FPM DRAM reduced t_CAC latency.^[9]

Static column is a variant of fast page mode in which the column address does not need to be stored in, but rather, the address inputs may be changed with CAS held low, and the data output will be updated accordingly a few nanoseconds later.^[9]

Nibble mode is another variant in which four sequential locations within the row can be accessed with four consecutive pulses of CAS. The difference from normal page mode is that the address inputs are not used for the second through fourth CAS edges; they are generated internally starting with the address supplied for the first CAS edge.^[9]

Extended data out DRAM (EDO DRAM)Edit

EDO DRAM, sometimes referred to as Hyper Page Mode enabled 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 performance. It was 5% faster than FPM DRAM, which it began to replace in 1995, when Intel introduced the 430FX chipset that supported EDO DRAM.

To be precise, EDO DRAM begins data output on the falling edge of CAS, but does not stop the output when CAS rises again. It holds the output valid (thus extending the data output time) until either RAS is deasserted, or a new CAS falling edge selects a different column address.

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 performance 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. 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.

Burst EDO DRAM (BEDO DRAM)Edit

An evolution of EDO DRAM, 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 pipeline stage allowing page-access cycle to be divided into two parts. During a memory-read operation, the first part accessed the data from the memory array to the output stage (second latch). The second part drove the data bus from this latch at the appropriate logic level. Since the data is already in the output buffer, quicker 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 superior to SDRAM in some ways, the latter technology quickly displaced BEDO.

Synchronous dynamic RAM (SDRAM) Edit

Main article: Synchronous dynamic random-access memory

SDRAM significantly revises the asynchronous memory interface, adding a clock (and a clock enable) line. All other signals are received on the rising edge of the clock.

The /RAS and /CAS inputs no longer act as strobes, but are instead, along with /WE, part of a 3-bit command:

The /OE line's function is extended to a per-byte "DQM" signal, which controls data input (writes) in addition to data output (reads). This allows DRAM chips to be wider than 8 bits while still supporting byte-granularity writes.

Many timing parameters remain under the control of the DRAM controller. For example, a minimum time must elapse between a row being activated and a read or write command. One important parameter must be programmed into the SDRAM chip itself, namely the CAS latency. This is the number of clock cycles allowed for internal operations between a read command and the first data word appearing on the data bus. The "Load mode register" command is used to transfer this value to the SDRAM chip. Other configurable parameters include the length of read and write bursts, i.e. the number of words transferred per read or write command.

The most significant change, and the primary reason that SDRAM has supplanted asynchronous RAM, is the support for multiple internal banks inside the DRAM chip. Using a few bits of "bank address" which accompany each command, a second bank can be activated and begin reading data while a read from the first bank is in progress. By alternating banks, an SDRAM device can keep the data bus continuously busy, in a way that asynchronous DRAM cannot.

Single data rate synchronous DRAM (SDR SDRAM)Edit

See also: SDR SDRAM

Single data rate SDRAM (sometimes known as SDR) is a synchronous form of DRAM.

Double data rate synchronous DRAM (DDR SDRAM)Edit

Main articles: DDR SDRAM, DDR2 SDRAM, DDR3 SDRAM, and DDR4 SDRAM

Double data rate SDRAM (DDR) was a later development of SDRAM, used in PC memory beginning in 2000. Subsequent versions are numbered sequentially (DDR2, DDR3, etc.). DDR SDRAM internally performs double-width accesses at the clock rate, and uses a double data rate interface to transfer one half on each clock edge. DDR2 and DDR3 increased this factor to 4× and 8×, respectively, delivering 4-word and 8-word bursts over 2 and 4 clock cycles, respectively. The internal access rate is mostly unchanged (200 million per second for DDR-400, DDR2-800 and DDR3-1600 memory), but each access transfers more data.

Direct Rambus DRAM (DRDRAM)Edit

Main article: RDRAM

Direct RAMBUS DRAM (DRDRAM) was developed by Rambus.

Reduced Latency DRAM (RLDRAM)Edit

Main article: RLDRAM

Reduced Latency DRAM is a high performance double data rate (DDR) SDRAM that combines fast, random access with high bandwidth, mainly intended for networking and caching applications.

Graphics RAMEdit

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Samsung VRAM

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Samsung WRAM

MoSys MD908.png

thumb|MoSys MDRAM

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IBM SGRAM

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Infineon DDR SGRAM

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Qimonda GDDR3 SDRAM

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These are asynchronous and synchronous DRAMs designed for graphics-related tasks such as texture memory and framebuffers, and can be found on video cards.

Video DRAM (VRAM)Edit

Main article: VRAM

VRAM is a dual-ported variant of DRAM that was once commonly used to store the frame-buffer in some graphics adaptors.

Window DRAM (WRAM)Edit

WRAM is a variant of VRAM that was once used in graphics adaptors such as the Matrox Millennium and ATI 3D Rage Pro. WRAM was designed to perform better and cost less than VRAM. WRAM offered up to 25% greater bandwidth than VRAM and accelerated commonly used graphical operations such as text drawing and block fills.^[10]

Multibank DRAM (MDRAM)Edit

Multibank DRAM is a type of specialized DRAM developed by MoSys. It is constructed from small memory banks of 256 KB, which are operated in an interleaved fashion, providing bandwidths suitable for graphics cards at a lower cost to memories such as SRAM. MDRAM also allows operations to two banks in a single clock cycle, permitting multiple concurrent accesses to occur if the accesses were independent. MDRAM was primarily used in graphic cards, such as those featuring the Tseng Labs ET6x00 chipsets. Boards based upon this chipset often had the unusual capacity of 2.25 MB because of MDRAM's ability to be implemented more easily with such capacities. A graphics card with 2.25 MB of MDRAM had enough memory to provide 24-bit color at a resolution of 1024×768—a very popular setting at the time.

Synchronous graphics RAM (SGRAM) Edit

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.

Graphics double data rate SDRAM (GDDR SDRAM)Edit

Main article: GDDR

Graphics double data rate SDRAM (GDDR SDRAM) is a type of specialized DDR SDRAM designed to be used as the main memory of graphics processing units (GPUs). GDDR SDRAM is distinct from commodity types of DDR SDRAM such as DDR3, although they share some core technologies. Their primary characteristics are higher clock frequencies for both the DRAM core and I/O interface, which provides greater memory bandwidth for GPUs. As of 2016, there are five successive generations of GDDR: GDDR2, GDDR3, GDDR4, GDDR5, and GDDR5X.

Pseudostatic RAM (PSRAM) Edit

PSRAM or PSDRAM 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. PSRAM (made by Numonyx) is used in the Apple iPhone and other embedded systems such as XFlar Platform.^[11]

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, rather not to allow operation without a separate DRAM controller as is the case with PSRAM.

An embedded variant of PSRAM is sold by MoSys under the name 1T-SRAM. It is technically DRAM, but behaves much like SRAM. It is used in Nintendo GameCube and Wii video game consoles.

See alsoEdit

• DRAM price fixing
• Flash memory
• List of device bit rates
• Memory bank
• Memory geometry

ReferencesEdit

Further readingEdit

• Brent Keeth, R. Jacob Baker, Brian Johnson, Feng Lin. (2008). DRAM Circuit Design: Fundamental and High-Speed Topics. John Wiley & Sons.
• Bruce Jacob, Spencer W. Ng, David T. Wang (2008). Memory Systems: Cache, DRAM, Disk. Morgan Kaufmann Publishers.

External linksEdit

• DRAM density and speed trends has some interesting historical trend charts of DRAM density and speed from 1980.
• 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
• Tezzaron Semiconductor Soft Error White Paper 1994 literature review of memory error rate measurements.
• 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
• Ars Technica: RAM Guide
• David Tawei Wang (2005). Modern DRAM Memory Systems: Performance Analysis and a High Performance, Power-Constrained DRAM-Scheduling Algorithm. PhD thesis, University of Maryland, College Park. http://www.ece.umd.edu/~blj/papers/thesis-PhD-wang--DRAM.pdf. Retrieved 2007-03-10.  A detailed description of current DRAM technology.
• Multi-port Cache DRAM — MP-RAM
• What every programmer should know about memory by Ulrich Drepper