Dynamic random-access memory

"DRAM" redirects here. For other uses, seeDram.

Transistorized memory, such as RAM, ROM, flash and cache sizes as well as file sizes are specified usingbinary meaningsfor K (1024¹), M (1024²), G (1024³), etc.

Dynamic random-access memory(dynamic RAMorDRAM) is a type ofrandom-accesssemiconductor memorythat stores eachbitof data in amemory cell,usually consisting of a tinycapacitorand atransistor,both typically based onmetal–oxide–semiconductor(MOS) technology. While most DRAM memory cell designs use a capacitor and transistor, some only use two transistors. In the designs where a capacitor is used, the capacitor can either be charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1. Theelectric chargeon the capacitors gradually leaks away; without intervention the data on the capacitor would soon be lost. To prevent this, DRAM requires an externalmemory refreshcircuit which periodically rewrites the data in the capacitors, restoring them to their original charge. This refresh process is the defining characteristic of dynamic random-access memory, in contrast tostatic random-access memory(SRAM) which does not require data to be refreshed. Unlikeflash memory,DRAM isvolatile memory(vs.non-volatile memory), since it loses its data quickly when power is removed. However, DRAM does exhibit limiteddata remanence.

Adiephotograph of theMicron TechnologyMT4C1024 DRAMintegrated circuit(1994). It has a capacity of 1megabitequivalent to$2^{20}$bits or128KiB.

Motherboardof theNeXTcubecomputer, 1990, with 64 MiB main memory DRAM (top left) and 256 KiB ofVRAM(lower edge, right of middle)

DRAM typically takes the form of anintegrated circuitchip, which can consist of dozens to billions of DRAM memory cells. DRAM chips are widely used indigital electronicswhere low-cost and high-capacitycomputer memoryis required. One of the largest applications for DRAM is themain memory(colloquially called the RAM) in moderncomputersandgraphics cards(where the main memory is called thegraphics memory). It is also used in many portable devices andvideo gameconsoles. In contrast, SRAM, which is faster and more expensive than DRAM, is typically used where speed is of greater concern than cost and size, such as thecache memoriesinprocessors.

The need to refresh DRAM demands more complicated circuitry and timing than SRAM. This is offset by the structural simplicity of DRAM memory cells: only one transistor and a capacitor are required per bit, compared to four or six transistors in SRAM. This allows DRAM to reach very highdensitieswith a simultaneous reduction in cost per bit. Refreshing the data consumes power and a variety of techniques are used to manage the overall power consumption.

DRAM had a 47% increase in the price-per-bit in 2017, the largest jump in 30 years since the 45% jump in 1988, while in recent years the price has been going down.In 2018, a "key characteristic of the DRAM market is that there are currently only three major suppliers —Micron Technology,SK HynixandSamsung Electronics"that are" keeping a pretty tight rein on their capacity ".There is alsoKioxia(previouslyToshibaMemory Corporation after 2017 spin-off) which doesn't manufacture DRAM. Other manufacturers make and sellDIMMs(but not the DRAM chips in them), such asKingston Technology,and some manufacturers that sellstacked DRAM(used e.g. in the fastestsupercomputerson theexascale), separately such asViking Technology.Others sell such integrated into other products, such asFujitsuinto its CPUs, AMD in GPUs, andNvidia,withHBM2in some of their GPU chips.

History

Precursors

A schematic drawing depicting the cross-section of the original one-transistor, one-capacitorNMOSDRAM cell. It was patented in 1968.

Thecryptanalyticmachine code-namedAquariusused atBletchley ParkduringWorld War IIincorporated 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') ".

In November 1965,Toshibaintroduced a bipolar dynamic RAM for itselectronic calculatorToscal BC-1411.In 1966, Tomohisa Yoshimaru and Hiroshi Komikawa from Toshiba applied for a Japanese patent of a memory circuit composed of several transistors and a capacitor, in 1967 they applied for a patent in the US.

The earliest forms of DRAM mentioned above usedbipolar transistors.While it offered improved performance overmagnetic-core memory,bipolar DRAM could not compete with the lower price of the then-dominant magnetic-core memory.Capacitors had also been used for earlier memory schemes, such as the drum of theAtanasoff–Berry Computer,theWilliams tubeand theSelectron tube.

Single MOS DRAM

In 1966, Dr.Robert Dennardinvented modern DRAM architecture in which there's a single MOS transistor per capacitor,at theIBM Thomas J. Watson Research Center,while he was working on MOS memory and was trying to create an alternative to SRAM which required six MOS transistors for eachbitof data. While examining the characteristics of MOS technology, he found it was capable of building capacitors, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor. This led to his development of the single-transistor MOS DRAM memory cell.He filed a patent in 1967, and was granted U.S. patent number3,387,286in 1968.MOS memory offered higher performance, was cheaper, and consumed less power, than magnetic-core memory.The patent describes the invention: "Each cell is formed, in one embodiment, using a single field-efiiect transistor and a single capacitor."

MOS DRAM chips were commercialized in 1969 by Advanced Memory Systems, Inc ofSunnyvale, CA.This 1024 bit chip was sold toHoneywell,Raytheon,Wang Laboratories,and others. The same year, Honeywell askedIntelto make a DRAM using a three-transistor cell that they had developed. This became the Intel 1102 in early 1970.However, the 1102 had many problems, prompting Intel to begin work on their own improved design, in secrecy to avoid conflict with Honeywell. This became the first commercially available DRAM, theIntel 1103,in October 1970, despite initial problems with low yield until the fifth revision of themasks.The 1103 was designed by Joel Karp and laid out by Pat Earhart. The masks were cut by Barbara Maness and Judy Garcia.MOS memory overtook magnetic-core memory as the dominant memory technology in the early 1970s.

The first DRAM with multiplexed row and columnaddress lineswas theMostekMK4096 4 Kbit DRAM designed by Robert Proebsting and introduced in 1973. This addressing scheme uses the same address pins to receive the low half and the high half of the address of the memory cell being referenced, switching between the two halves on alternating bus cycles. This was a radical advance, effectively halving the number of address lines required, which enabled it to fit into packages with fewer pins, a cost advantage that grew with every jump in memory size. The MK4096 proved to be a very robust design for customer applications. At the 16 Kbit density, the cost advantage increased; the 16 Kbit Mostek MK4116 DRAM,introduced in 1976, achieved greater than 75% worldwide DRAM market share. However, as density increased to 64 Kbit in the early 1980s, Mostek and other US manufacturers were overtaken by Japanese DRAM manufacturers, which dominated the US and worldwide markets during the 1980s and 1990s.

Early in 1985,Gordon Mooredecided to withdraw Intel from producing DRAM. By 1986, many, but not all, United States chip makers had stopped making DRAMs.Micron Technology and Texas Instruments continued to produce them commercially, and IBM produced them for internal use.

In 1985, when 64K DRAM memory chips were the most common memory chips used in computers, and when more than 60 percent of those chips were produced by Japanese companies, semiconductor makers in the United States accused Japanese companies ofexport dumpingfor the purpose of driving makers in the United States out of the commodity memory chip business. Prices for the 64K product plummeted to as low as 35 cents apiece from $3.50 within 18 months, with disastrous financial consequences for some U.S. firms. On 4 December 1985 the US Commerce Department's International Trade Administration ruled in favor of the complaint.

Synchronous dynamic random-access memory(SDRAM) was developed bySamsung.The first commercial SDRAM chip was the Samsung KM48SL2000, which had a capacity of 16Mb,and was introduced in 1992.The first commercialDDR SDRAM(double data rateSDRAM) memory chip was Samsung's 64Mb DDR SDRAM chip, released in 1998.

Later, in 2001, Japanese DRAM makers accused Korean DRAM manufacturers of dumping.

In 2002, US computer makers made claims ofDRAM price fi xing.

Principles of operation

The principles of operation for reading a simple 4$\times$4 DRAM array

Basic structure of a DRAM cell array

DRAM is usually arranged in a rectangular array of charge storage cells consisting of one capacitor and transistor per data bit. The figure to the right shows a simple example with a four-by-four cell matrix. Some DRAM matrices are many thousands of cells in height and width.

The long horizontal lines connecting each row are known as word-lines. Each column of cells is composed of two bit-lines, each connected to every other storage cell in the column (the illustration to the right does not include this important detail). They are generally known as the+and−bit lines.

Asense amplifieris essentially a pair of cross-connectedinvertersbetween the bit-lines. The first inverter is connected with input from the + bit-line and output to the − bit-line. The second inverter's input is from the − bit-line with output to the + bit-line. This results inpositive feedbackwhich stabilizes after one bit-line is fully at its highest voltage and the other bit-line is at the lowest possible voltage.

Operations to read a data bit from a DRAM storage cell

1. The sense amplifiers are disconnected.
2. The bit-lines are precharged to exactly equal voltages that are in between high and low logic levels (e.g., 0.5 V if the two levels are 0 and 1 V). The bit-lines are physically symmetrical to keep the capacitance equal, and therefore at this time their voltages are equal.
3. The precharge circuit is switched off. Because the bit-lines are relatively long, they have enoughcapacitanceto maintain the precharged voltage for a brief time. This is an example ofdynamic logic.
4. The desired row's word-line is then driven high to connect a cell's storage capacitor to its bit-line. This causes the transistor to conduct, transferringchargefrom the storage cell to the connected bit-line (if the stored value is 1) or from the connected bit-line to the storage cell (if the stored value is 0). Since the capacitance of the bit-line is typically much higher than the capacitance of the storage cell, the voltage on the bit-line increases very slightly if the storage cell's capacitor is discharged and decreases very slightly if the storage cell is charged (e.g., 0.54 and 0.45 V in the two cases). As the other bit-line holds 0.50 V there is a small voltage difference between the two twisted bit-lines.
5. The sense amplifiers are now connected to the bit-lines pairs. Positive feedback then occurs from the cross-connected inverters, thereby amplifying the small voltage difference between the odd and even row bit-lines of a particular column until one bit line is fully at the lowest voltage and the other is at the maximum high voltage. Once this has happened, the row isopen(the desired cell data is available).
6. All storage cells in the open row are sensed simultaneously, and the sense amplifier outputs latched. A column address then selects which latch bit to connect to the external data bus. Reads of different columns in the same row can be performed without arow opening delaybecause, for the open row, all data has already been sensed and latched.
7. While reading of columns in an open row is occurring, current is flowing back up the bit-lines from the output of the sense amplifiers and recharging the storage cells. This reinforces (i.e. refreshes) the charge in the storage cell by increasing the voltage in the storage capacitor if it was charged to begin with, or by keeping it discharged if it was empty. Note that due to the length of the bit-lines there is a fairly long propagation delay for the charge to be transferred back to the cell's capacitor. This takes significant time past the end of sense amplification, and thus overlaps with one or more column reads.
8. When done with reading all the columns in the current open row, the word-line is switched off to disconnect the storage cell capacitors (the row is closed) from the bit-lines. The sense amplifier is switched off, and the bit-lines are precharged again.

To write to memory

Writing to a DRAM cell

To store data, a row is opened and a given column's sense amplifier is temporarily forced to the desired high or low-voltage state, thus causing the bit-line to charge or discharge the cell storage capacitor to the desired value. Due to the sense amplifier's positive feedback configuration, it will hold a bit-line at stable voltage even after the forcing voltage is removed. During a write to a particular cell, all the columns in a row are sensed simultaneously just as during reading, so although only a single column's storage-cell capacitor charge is changed, the entire row is refreshed (written back in), as illustrated in the figure to the right.

Refresh rate

Main article:Memory refresh

See also:§ Security

Typically, manufacturers specify that each row must be refreshed every 64 ms or less, as defined by theJEDECstandard.

Some systems refresh every row in a burst of activity involving all rows every 64 ms. Other systems refresh one row at a time staggered throughout the 64 ms interval. For example, a system with 2¹³= 8,192 rows would require a staggeredrefresh rateof one row every 7.8 μs which is 64 ms divided by 8,192 rows. A few real-time systems refresh a portion of memory at a time determined by an external timer function that governs the operation of the rest of a system, such as thevertical blanking intervalthat occurs every 10–20 ms in video equipment.

The row address of the row that will be refreshed next is maintained by external logic or acounterwithin the DRAM. A system that provides the row address (and the refresh command) does so to have greater control over when to refresh and which row to refresh. This is done to minimize conflicts with memory accesses, since such a system has both knowledge of the memory access patterns and the refresh requirements of the DRAM. When the row address is supplied by a counter within the DRAM, the system relinquishes control over which row is refreshed and only provides the refresh command. Some modern DRAMs are capable of self-refresh; no external logic is required to instruct the DRAM to refresh or to provide a row address.

Under some conditions, most of the data in DRAM can be recovered even if the DRAM has not been refreshed for several minutes.

Memory timing

Main article:Memory timings

Many parameters are required to fully describe the timing of DRAM operation. Here are some examples for two timing grades of asynchronous DRAM, from a data sheet published in 1998:

	"50 ns"	"60 ns"	Description
tRC	84 ns	104 ns	Random read or write cycle time (from one full /RAS cycle to another)
tRAC	50 ns	60 ns	Access time: /RAS low to valid data out
tRCD	11 ns	14 ns	/RAS low to /CAS low time
tRAS	50 ns	60 ns	/RAS pulse width (minimum /RAS low time)
tRP	30 ns	40 ns	/RAS precharge time (minimum /RAS high time)
tPC	20 ns	25 ns	Page-mode read or write cycle time (/CAS to /CAS)
tAA	25 ns	30 ns	Access time: Column address valid to valid data out (includes addresssetup timebefore /CAS low)
tCAC	13 ns	15 ns	Access time: /CAS low to valid data out
tCAS	8 ns	10 ns	/CAS low pulse width minimum

Thus, the generally quoted number is the minimum /RAS low time. This is the time to open a row, allowing the sense amplifiers to settle. Note that the data access for a bit in the row is shorter, since that happens as soon as the sense amplifier has settled, but the DRAM requires additional time to propagate the amplified data back to recharge the cells. The time to read additional bits from an open page is much less, defined by the /CAS to /CAS cycle time. The quoted number is the clearest way to compare between the performance of different DRAM memories, as it sets the slower limit regardless of the row length or page size. Bigger arrays forcibly result in larger bit line capacitance and longer propagation delays, which cause this time to increase as the sense amplifier settling time is dependent on both the capacitance as well as the propagation latency. This is countered in modern DRAM chips by instead integrating many more complete DRAM arrays within a single chip, to accommodate more capacity without becoming too slow.

When such a RAM is accessed by clocked logic, the times are generally rounded up to the nearest clock cycle. For example, when accessed by a 100 MHz state machine (i.e. a 10 ns clock), the 50 ns DRAM can perform the first read in five clock cycles, and additional reads within the same page every two clock cycles. This was generally described as"5-2-2-2"timing, as bursts of four reads within a page were common.

When describing synchronous memory, timing is described by clock cycle counts separated by hyphens. These numbers representt_CL-t_RCD-t_RP-t_RASin multiples of the DRAM clock cycle time. Note that this is half of the data transfer rate whendouble data ratesignaling is used. JEDEC standard PC3200 timing is3-4-4-8with a 200 MHz clock, while premium-priced high performance PC3200 DDR DRAM DIMM might be operated at2-2-2-5timing.

	PC-3200 (DDR-400)				PC2-6400 (DDR2-800)				PC3-12800 (DDR3-1600)				Description
	Typical		Fast		Typical		Fast		Typical		Fast
	cycles	time	cycles	time	cycles	time	cycles	time	cycles	time	cycles	time
tCL	3	15 ns	2	10 ns	5	12.5 ns	4	10 ns	9	11.25 ns	8	10 ns	/CAS low to valid data out (equivalent totCAC)
tRCD	4	20 ns	2	10 ns	5	12.5 ns	4	10 ns	9	11.25 ns	8	10 ns	/RAS low to /CAS low time
tRP	4	20 ns	2	10 ns	5	12.5 ns	4	10 ns	9	11.25 ns	8	10 ns	/RAS precharge time (minimum precharge to active time)
tRAS	8	40 ns	5	25 ns	16	40 ns	12	30 ns	27	33.75 ns	24	30 ns	Row active time (minimum active to precharge time)

Minimum random access time has improved fromt_RAC= 50 ns tot_RCD+t_CL= 22.5 ns,and even the premium 20 ns variety is only 2.5 times better compared to the typical case (~2.22 times better).CAS latencyhas improved even less, fromt_CAC= 13 nsto 10 ns. However, the DDR3 memory does achieve 32 times higher bandwidth; due to internal pipelining and wide data paths, it can output two words every 1.25 ns(1600Mword/s),while the EDO DRAM can output one word pert_PC= 20 ns (50 Mword/s).

Timing abbreviations

tCL– CAS latency tCR– Command rate tPTP– precharge to precharge delay tRAS– RAS active time tRCD– RAS to CAS delay tREF– Refresh period tRFC– Row refresh cycle time tRP– RAS precharge

tRRD– RAS to RAS delay tRTP– Read to precharge delay tRTR– Read to read delay tRTW– Read to write delay tWR– Write recovery time tWTP– Write to precharge delay tWTR– Write to read delay tWTW– Write to write delay

Memory cell design

See also:Memory cell (computing)

Each bit of data in a DRAM is stored as a positive or negative electrical charge in a capacitive structure. The structure providing the capacitance, as well as the transistors that control access to it, is collectively referred to as aDRAM cell.They are the fundamental building block in DRAM arrays. Multiple DRAM memory cell variants exist, but the most commonly used variant in modern DRAMs is the one-transistor, one-capacitor (1T1C) cell. The transistor is used to admit current into the capacitor during writes, and to discharge the capacitor during reads. The access transistor is designed to maximize drive strength and minimize transistor-transistor leakage (Kenner, pg. 34).

The capacitor has two terminals, one of which is connected to its access transistor, and the other to either ground or V_CC/2. In modern DRAMs, the latter case is more common, since it allows faster operation. In modern DRAMs, a voltage of +V_CC/2 across the capacitor is required to store a logic one; and a voltage of -V_CC/2 across the capacitor is required to store a logic zero. The electrical charge stored in the capacitor is measured incoulombs.For a logic one, the charge is:${\textstyle Q={V_{CC} \over 2}\cdot C}$,whereQis the charge in coulombs andCis the capacitance infarads.A logic zero has a charge of:${\textstyle Q={-V_{CC} \over 2}\cdot C}$.

Reading or writing a logic one requires the wordline is driven to a voltage greater than the sum of V_CCand the access transistor's threshold voltage (V_TH). This voltage is calledV_CCpumped(V_CCP). The time required to discharge a capacitor thus depends on what logic value is stored in the capacitor. A capacitor containing logic one begins to discharge when the voltage at the access transistor's gate terminal is above V_CCP.If the capacitor contains a logic zero, it begins to discharge when the gate terminal voltage is above V_TH.

Capacitor design

Up until the mid-1980s, the capacitors in DRAM cells were co-planar with the access transistor (they were constructed on the surface of the substrate), thus they were referred to asplanarcapacitors. The drive to increase both density and, to a lesser extent, performance, required denser designs. This was strongly motivated by economics, a major consideration for DRAM devices, especially commodity DRAMs. The minimization of DRAM cell area can produce a denser device and lower the cost per bit of storage. Starting in the mid-1980s, the capacitor was moved above or below the silicon substrate in order to meet these objectives. DRAM cells featuring capacitors above the substrate are referred to asstackedorfolded platecapacitors. Those with capacitors buried beneath the substrate surface are referred to astrenchcapacitors. In the 2000s, manufacturers were sharply divided by the type of capacitor used in their DRAMs and the relative cost and long-term scalability of both designs have been the subject of extensive debate. The majority of DRAMs, from major manufactures such asHynix,Micron Technology,Samsung Electronicsuse the stacked capacitor structure, whereas smaller manufacturers such Nanya Technology use the trench capacitor structure (Jacob, pp. 355–357).

The capacitor in the stacked capacitor scheme is constructed above the surface of the substrate. The capacitor is constructed from an oxide-nitride-oxide (ONO) dielectric sandwiched in between two layers of polysilicon plates (the top plate is shared by all DRAM cells in an IC), and its shape can be a rectangle, a cylinder, or some other more complex shape. There are two basic variations of the stacked capacitor, based on its location relative to the bitline—capacitor-over-bitline (COB) and capacitor-under-bitline (CUB). In a former variation, the capacitor is underneath the bitline, which is usually made of metal, and the bitline has a polysilicon contact that extends downwards to connect it to the access transistor's source terminal. In the latter variation, the capacitor is constructed above the bitline, which is almost always made of polysilicon, but is otherwise identical to the COB variation. The advantage the COB variant possesses is the ease of fabricating the contact between the bitline and the access transistor's source as it is physically close to the substrate surface. However, this requires the active area to be laid out at a 45-degree angle when viewed from above, which makes it difficult to ensure that the capacitor contact does not touch the bitline. CUB cells avoid this, but suffer from difficulties in inserting contacts in between bitlines, since the size of features this close to the surface are at or near the minimum feature size of the process technology (Kenner, pp. 33–42).

The trench capacitor is constructed by etching a deep hole into the silicon substrate. The substrate volume surrounding the hole is then heavily doped to produce a buried n⁺plate and to reduce resistance. A layer of oxide-nitride-oxide dielectric is grown or deposited, and finally the hole is filled by depositing doped polysilicon, which forms the top plate of the capacitor. The top of the capacitor is connected to the access transistor's drain terminal via a polysilicon strap (Kenner, pp. 42–44). A trench capacitor's depth-to-width ratio in DRAMs of the mid-2000s can exceed 50:1 (Jacob, p. 357).

Trench capacitors have numerous advantages. Since the capacitor is buried in the bulk of the substrate instead of lying on its surface, the area it occupies can be minimized to what is required to connect it to the access transistor's drain terminal without decreasing the capacitor's size, and thus capacitance (Jacob, pp. 356–357). Alternatively, the capacitance can be increased by etching a deeper hole without any increase to surface area (Kenner, pg. 44). Another advantage of the trench capacitor is that its structure is under the layers of metal interconnect, allowing them to be more easily made planar, which enables it to be integrated in a logic-optimized process technology, which have many levels of interconnect above the substrate. The fact that the capacitor is under the logic means that it is constructed before the transistors are. This allows high-temperature processes to fabricate the capacitors, which would otherwise be degrading the logic transistors and their performance. This makes trench capacitors suitable for constructingembedded DRAM(eDRAM) (Jacob, p. 357). Disadvantages of trench capacitors are difficulties in reliably constructing the capacitor's structures within deep holes and in connecting the capacitor to the access transistor's drain terminal (Kenner, pg. 44).

Historical cell designs

First-generation DRAM ICs (those with capacities of 1 Kbit), of which the first was theIntel 1103,used a three-transistor, one-capacitor (3T1C) DRAM cell. By the second-generation, the requirement to reduce cost by fitting the same amount of bits in a smaller area led to the almost universal adoption of the 1T1C DRAM cell, although a couple of devices with 4 and 16 Kbit capacities continued to use the 3T1C cell for performance reasons (Kenner, p. 6). These performance advantages included, most significantly, the ability to read the state stored by the capacitor without discharging it, avoiding the need to write back what was read out (non-destructive read). A second performance advantage relates to the 3T1C cell has separate transistors for reading and writing; the memory controller can exploit this feature to perform atomic read-modify-writes, where a value is read, modified, and then written back as a single, indivisible operation (Jacob, p. 459).

Proposed cell designs

The one-transistor, zero-capacitor (1T, or 1T0C) DRAM cell has been a topic of research since the late-1990s.1T DRAMis a different way of constructing the basic DRAM memory cell, distinct from the classic one-transistor/one-capacitor (1T/1C) DRAM cell, which is also sometimes referred to as1T DRAM,particularly in comparison to the 3T and 4T DRAM which it replaced in the 1970s.

In 1T DRAM cells, the bit of data is still stored in a capacitive region controlled by a transistor, but this capacitance is no longer provided by a separate capacitor. 1T DRAM is a capacitorless bit cell design that stores data using the parasitic body capacitance that is inherent tosilicon on insulator (SOI)transistors. Considered a nuisance in logic design, thisfloating body effectcan be used for data storage. This gives 1T DRAM cells the greatest density as well as allowing easier integration with high-performance logic circuits since they are constructed with the same SOI process technologies.

Refreshing of cells remains necessary, but unlike with 1T1C DRAM, reads in 1T DRAM are non-destructive; the stored charge causes a detectable shift in thethreshold voltageof the transistor.Performance-wise, access times are significantly better than capacitor-based DRAMs, but slightly worse than SRAM. There are several types of 1T DRAMs: the commercializedZ-RAMfrom Innovative Silicon, the TTRAMfrom Renesas and theA-RAMfrom theUGR/CNRSconsortium.

Array structures

Self-aligned storage node locations simplify the fabrication process in modern DRAM.

DRAM cells are laid out in a regular rectangular, grid-like pattern to facilitate their control and access via wordlines and bitlines. The physical layout of the DRAM cells in an array is typically designed so that two adjacent DRAM cells in a column share a single bitline contact to reduce their area. DRAM cell area is given asnF²,wherenis a number derived from the DRAM cell design, andFis the smallest feature size of a given process technology. This scheme permits comparison of DRAM size over different process technology generations, as DRAM cell area scales at linear or near-linear rates with respect to feature size. The typical area for modern DRAM cells varies between 6–8 F².

The horizontal wire, the wordline, is connected to the gate terminal of every access transistor in its row. The vertical bitline is connected to the source terminal of the transistors in its column. The lengths of the wordlines and bitlines are limited. The wordline length is limited by the desired performance of the array, since propagation time of the signal that must transverse the wordline is determined by theRC time constant.The bitline length is limited by its capacitance (which increases with length), which must be kept within a range for proper sensing (as DRAMs operate by sensing the charge of the capacitor released onto the bitline). Bitline length is also limited by the amount of operating current the DRAM can draw and by how power can be dissipated, since these two characteristics are largely determined by the charging and discharging of the bitline.

Bitline architecture

Sense amplifiersare required to read the state contained in the DRAM cells. When the access transistor is activated, the electrical charge in the capacitor is shared with the bitline. The bitline's capacitance is much greater than that of the capacitor (approximately ten times). Thus, the change in bitline voltage is minute. Sense amplifiers are required to resolve the voltage differential into the levels specified by the logic signaling system. Modern DRAMs use differential sense amplifiers, and are accompanied by requirements as to how the DRAM arrays are constructed. Differential sense amplifiers work by driving their outputs to opposing extremes based on the relative voltages on pairs of bitlines. The sense amplifiers function effectively and efficient only if the capacitance and voltages of these bitline pairs are closely matched. Besides ensuring that the lengths of the bitlines and the number of attached DRAM cells attached to them are equal, two basic architectures to array design have emerged to provide for the requirements of the sense amplifiers: open and folded bitline arrays.

Open bitline arrays

The first generation (1 Kbit) DRAM ICs, up until the 64 Kbit generation (and some 256 Kbit generation devices) had open bitline array architectures. In these architectures, the bitlines are divided into multiple segments, and the differential sense amplifiers are placed in between bitline segments. Because the sense amplifiers are placed between bitline segments, to route their outputs outside the array, an additional layer of interconnect placed above those used to construct the wordlines and bitlines is required.

The DRAM cells that are on the edges of the array do not have adjacent segments. Since the differential sense amplifiers require identical capacitance and bitline lengths from both segments, dummy bitline segments are provided. The advantage of the open bitline array is a smaller array area, although this advantage is slightly diminished by the dummy bitline segments. The disadvantage that caused the near disappearance of this architecture is the inherent vulnerability tonoise,which affects the effectiveness of the differential sense amplifiers. Since each bitline segment does not have any spatial relationship to the other, it is likely that noise would affect only one of the two bitline segments.

Folded bitline arrays

The folded bitline array architecture routes bitlines in pairs throughout the array. The close proximity of the paired bitlines provide superiorcommon-modenoise rejection characteristics over open bitline arrays. The folded bitline array architecture began appearing in DRAM ICs during the mid-1980s, beginning with the 256 Kbit generation. This architecture is favored in modern DRAM ICs for its superior noise immunity.

This architecture is referred to asfoldedbecause it takes its basis from the open array architecture from the perspective of the circuit schematic. The folded array architecture appears to remove DRAM cells in alternate pairs (because two DRAM cells share a single bitline contact) from a column, then move the DRAM cells from an adjacent column into the voids.

The location where the bitline twists occupies additional area. To minimize area overhead, engineers select the simplest and most area-minimal twisting scheme that is able to reduce noise under the specified limit. As process technology improves to reduce minimum feature sizes, the signal to noise problem worsens, since coupling between adjacent metal wires is inversely proportional to their pitch. The array folding and bitline twisting schemes that are used must increase in complexity in order to maintain sufficient noise reduction. Schemes that have desirable noise immunity characteristics for a minimal impact in area is the topic of current research (Kenner, p. 37).

Future array architectures

Advances in process technology could result in open bitline array architectures being favored if it is able to offer better long-term area efficiencies; since folded array architectures require increasingly complex folding schemes to match any advance in process technology. The relationship between process technology, array architecture, and area efficiency is an active area of research.

Row and column redundancy

The first DRAMintegrated circuitsdid not have any redundancy. An integrated circuit with a defective DRAM cell would be discarded. Beginning with the 64 Kbit generation, DRAM arrays have included spare rows and columns to improve yields. Spare rows and columns provide tolerance of minor fabrication defects which have caused a small number of rows or columns to be inoperable. The defective rows and columns are physically disconnected from the rest of the array by a triggering aprogrammable fuseor by cutting the wire by a laser. The spare rows or columns are substituted in by remapping logic in the row and column decoders (Jacob, pp. 358–361).

Error detection and correction

Main articles:RAM parityandECC memory

Electrical or magnetic interference inside a computer system can cause a single bit of DRAM tospontaneously flipto the opposite state. The majority of one-off ( "soft") errors in DRAM chips occur as a result ofbackground radiation,chieflyneutronsfromcosmic raysecondaries, which may change the contents of one or more memory cells or interfere with the circuitry used to read/write them.

The problem can be mitigated by usingredundantmemory bits and additional circuitry that use these bits to detect and correct soft errors. In most cases, the detection and correction are performed by thememory controller;sometimes, the required logic is transparently implemented within DRAM chips or modules, enabling the ECC memory functionality for otherwise ECC-incapable systems.The extra memory bits are used to recordparityand to enable missing data to be reconstructed byerror-correcting code(ECC). Parity allows the detection of all single-bit errors (actually, any odd number of wrong bits). The most common error-correcting code, aSECDED 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.

Recent studies give widely varying error rates with over seven orders of magnitude difference, ranging from10⁻¹⁰−10⁻¹⁷error/bit·h,roughly one bit error, per hour, per gigabyte of memory to one bit error, per century, per gigabyte of memory.The Schroeder et al. 2009 study reported a 32% chance that a given computer in their study would suffer from at least one correctable error per year, and provided evidence that most such errors are intermittent hard rather than soft errors and that trace amounts of radioactive material that had gotten into the chip packaging were emitting Alpha particles and corrupting the data.A 2010 study at the University of Rochester also gave evidence that a substantial fraction of memory errors are intermittent hard errors.Large scale studies on non-ECC main memory in PCs and laptops suggest that undetected memory errors account for a substantial number of system failures: the 2011 study reported a 1-in-1700 chance per 1.5% of memory tested (extrapolating to an approximately 26% chance for total memory) that a computer would have a memory error every eight months.

Security

Data remanence

Main article:Data remanence

Although dynamic memory is only specified andguaranteedto retain its contents when supplied with power and refreshed every short period of time (often64 ms), the memory cellcapacitorsoften retain their values for significantly longer time, particularly at low temperatures.Under some conditions most of the data in DRAM can be recovered even if it has not been refreshed for several minutes.

This property can be used to circumvent security and recover data stored in the main memory that is assumed to be destroyed at power-down. The computer could be quickly rebooted, and the contents of the main memory read out; or by removing a computer's memory modules, cooling them to prolong data remanence, then transferring them to a different computer to be read out. Such an attack was demonstrated to circumvent popular disk encryption systems, such as theopen sourceTrueCrypt,Microsoft'sBitLocker Drive Encryption,andApple'sFileVault.This type of attack against a computer is often called acold boot attack.

Memory corruption

See also:§ Operations to read a data bit from a DRAM storage cell

Dynamic memory, by definition, requires periodic refresh. Furthermore, reading dynamic memory is a destructive operation, requiring a recharge of the storage cells in the row that has been read. If these processes are imperfect, a read operation can causesoft errors.In particular, there is a risk that some charge can leak between nearby cells, causing the refresh or read of one row to cause adisturbance errorin an adjacent or even nearby row. The awareness of disturbance errors dates back to the first commercially available DRAM in the early 1970s (theIntel 1103). Despite the mitigation techniques employed by manufacturers, commercial researchers proved in a 2014 analysis that commercially availableDDR3DRAM chips manufactured in 2012 and 2013 are susceptible to disturbance errors.The associated side effect that led to observed bit flips has been dubbedrow hammer.

Packaging

Memory module

Main article:Memory module

Dynamic RAM ICs can be packaged in molded epoxy cases, with an internal lead frame for interconnections between thesilicon dieand the package leads. The originalIBM PCdesign used ICs, including those for DRAM, packaged indual in-line packages(DIP), soldered directly to the main board or mounted in sockets. As memory density skyrocketed, the DIP package was no longer practical. For convenience in handling, several dynamic RAM integrated circuits may be mounted on a single memory module, allowing installation of 16-bit, 32-bit or 64-bit wide memory in a single unit, without the requirement for the installer to insert multiple individual integrated circuits. Memory modules may include additional devices for parity checking or error correction. Over the evolution of desktop computers, several standardized types of memory module have been developed. Laptop computers, game consoles, and specialized devices may have their own formats of memory modules not interchangeable with standard desktop parts for packaging or proprietary reasons.

Embedded

Main article:eDRAM

DRAM that is integrated into an integrated circuit designed in a logic-optimized process (such as anapplication-specific integrated circuit,microprocessor,or an entiresystem on a chip) is calledembedded DRAM(eDRAM). Embedded DRAM requires DRAM cell designs that can befabricatedwithout preventing the fabrication of fast-switching transistors used in high-performance logic, and modification of the basic logic-optimized process technology to accommodate the process steps required to build DRAM cell structures.

Versions

Since the fundamental DRAM cell and array has maintained the same basic structure for many years, the types of DRAM are mainly distinguished by the many different interfaces for communicating with DRAM chips.

Asynchronous DRAM

The original DRAM, now known by theretronymasynchronous DRAMwas the first type of DRAM in use. From its origins in the late 1960s, it was commonplace in computing up until around 1997, when it was mostly replaced bysynchronous DRAM.In the present day, manufacture of asynchronous RAM is relatively rare.

Principles of operation

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 fouractive-lowcontrol signals:

• RAS,the Row Address Strobe. The address inputs are captured on the falling edge ofRAS,and select a row to open. The row is held open as long asRASis low.
• CAS,the Column Address Strobe. The address inputs are captured on the falling edge ofCAS,and select a column from the currently open row to read or write.
• WE,Write Enable. This signal determines whether a given falling edge ofCASis a read (if high) or write (if low). If low, the data inputs are also captured on the falling edge ofCAS.
• 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 ifRASandCASare low,WEis high, andOEis low. In many applications,OEcan be permanently connected low (output always enabled), but switchingOEcan be useful when connecting multiple memory chips in parallel.

This interface provides direct control of internal timing. WhenRASis driven low, aCAScycle must not be attempted until the sense amplifiers have sensed the memory state, andRASmust not be returned high until the storage cells have been refreshed. WhenRASis 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

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 usingRASonly refresh (ROR), the following steps must occur:

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

This can be done by supplying a row address and pulsingRASlow; it is not necessary to perform anyCAScycles. An external counter is needed to iterate over the row addresses in turn.In some designs, the CPU handled RAM refresh, among these theZilog Z80is perhaps the best known example, hosting a row counter in aprocessor register,R, and including internal timers that would periodically poll the row at R and then increment the value in the register. Refreshes were interleaved with common instructions like memory reads.In other systems, especiallyhome computers,refresh was often handled by the video circuitry as it often had to read from large areas of memory, and performed refreshes as part of these operations.

CAS before RAS refresh

For convenience, the counter was quickly incorporated into the DRAM chips themselves. If theCASline is driven low beforeRAS(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 asCAS-before-RAS(CBR) refresh. This became the standard form of refresh for asynchronous DRAM, and is the only form generally used with SDRAM.

Hidden refresh

Given support ofCAS-before-RASrefresh, it is possible to deassertRASwhile holdingCASlow to maintain data output. IfRASis then asserted again, this performs a CBR refresh cycle while the DRAM outputs remain valid. Because data output is not interrupted, this is known ashidden refresh.

Page mode DRAM

Page mode DRAMis 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 holdingRASlow, 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 assertingCASand 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.

Page mode DRAM was in turn later improved with a small modification which further reduced latency. DRAMs with this improvement were calledfast page mode DRAMs(FPM DRAMs). In page mode DRAM,CASwas asserted before the column address was supplied. In FPM DRAM, the column address could be supplied whileCASwas still deasserted. The column address propagated through the column address data path, but did not output data on the data pins untilCASwas asserted. Prior toCASbeing asserted, the data out pins were held at high-Z. FPM DRAM reducedt_CAClatency.Fast page mode DRAM was introduced in 1986 and was used with Intel 80486.

Static columnis 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 withCASheld low, and the data output will be updated accordingly a few nanoseconds later.

Nibble modeis another variant in which four sequential locations within the row can be accessed with four consecutive pulses ofCAS.The difference from normal page mode is that the address inputs are not used for the second through fourthCASedges; they are generated internally starting with the address supplied for the firstCASedge.

Extended data out DRAM

A pair of 32MBEDO DRAM modules

Extended data out DRAM (EDO DRAM) was invented and patented in the 1990s byMicron Technologywho then licensed technology to many other memory manufacturers.EDO RAM, sometimes referred to ashyper page modeenabled 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 is up to 30% faster than FPM DRAM,which it began to replace in 1995 whenIntelintroduced the430FX chipsetwith EDO DRAM support. Irrespective of the performance gains, FPM and EDO SIMMs can be used interchangeably in many (but not all) applications.

To be precise, EDO DRAM begins data output on the falling edge ofCASbut does not stop the output whenCASrises again. It holds the output valid (thus extending the data output time) until eitherRASis deasserted, or a newCASfalling 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 created an opportunity to reduce the immense performance loss associated with a lack of L2 cache in low-cost, commodity PCs. This was also good for notebooks due to difficulties with their limited form factor, and battery life limitations. Additionally, for systems with an L2 cache, the availability of EDO memory improved the average memory latency seen by applications over earlier FPM implementations.

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

An evolution of EDO DRAM, burst EDO DRAM (BEDO DRAM), could process four memory addresses in one burst, for a maximum of5-1-1-1,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.Even though BEDO RAM was superior to SDRAM in some ways, the latter technology quickly displaced BEDO.

Synchronous dynamic RAM

Main article:Synchronous dynamic random-access memory

Synchronous dynamic RAM (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.

TheRASandCASinputs no longer act as strobes, but are instead, along withWE,part of a 3-bit command controlled by a new active-low strobe,chip selectorCS:

SDRAM Command summary

CS	RAS	CAS	WE	Address	Command
H	x	x	x	x	Command inhibit (no operation)
L	H	H	H	x	No operation
L	H	H	L	x	Burst Terminate: stop a read or write burst in progress.
L	H	L	H	Column	Read from currently active row.
L	H	L	L	Column	Write to currently active row.
L	L	H	H	Row	Activate a row for read and write.
L	L	H	L	x	Precharge (deactivate) the current row.
L	L	L	H	x	Auto refresh: refresh one row of each bank, using an internal counter.
L	L	L	L	Mode	Load mode register: address bus specifies DRAM operation mode.

TheOEline'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 theCAS 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. TheLoad mode registercommand 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 ofbank addressthat accompany each command, a second bank can be activated and begin reading datawhile 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

See also:SDR SDRAM

Single data rate SDRAM (SDR SDRAM or SDR) is the original generation of SDRAM; it made a single transfer of data per clock cycle.

Double data rate synchronous DRAM

Main articles:DDR SDRAM,DDR2 SDRAM,DDR3 SDRAM,DDR4 SDRAM,andDDR5 SDRAM

Thedieof a Samsung DDR-SDRAM 64-MBit package

Double data rate SDRAM (DDR SDRAM or 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 adouble data rateinterface 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

Main article:RDRAM

Direct RAMBUS DRAM(DRDRAM) was developed by Rambus. First supported onmotherboardsin 1999, it was intended to become an industry standard, but was outcompeted byDDR SDRAM,making it technically obsolete by 2003.

Reduced Latency DRAM

Main article:RLDRAM

Reduced Latency DRAM (RLDRAM) 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 RAM

Graphics RAMs are asynchronous and synchronous DRAMs designed for graphics-related tasks such astexture memoryandframebuffers,found onvideo cards.

Video DRAM

Main article:VRAM

Video DRAM (VRAM) is adual-portedvariant of DRAM that was once commonly used to store the frame-buffer in somegraphics adaptors.

Window DRAM

Window DRAM (WRAM) is a variant of VRAM that was once used in graphics adaptors such as theMatroxMillennium andATI 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.

Multibank DRAM

MoSysMDRAM MD908

Multibank DRAM (MDRAM) is a type of specialized DRAM developed byMoSys.It is constructed from smallmemory banksof256 kB,which are operated in aninterleavedfashion, providing bandwidths suitable for graphics cards at a lower cost to memories such asSRAM.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 theTseng LabsET6x00 chipsets. Boards based upon this chipset often had the unusual capacity of2.25 MBbecause of MDRAM's ability to be implemented more easily with such capacities. A graphics card with2.25 MBof 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

Synchronous graphics RAM (SGRAM) is a specialized form of SDRAM for graphics adaptors. It adds functions such asbit 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

Main article:GDDR

A 512-MBitQimondaGDDR3 SDRAM package

Inside a Samsung GDDR3 256-MBit package

Graphics double data rate SDRAM is a type of specializedDDRSDRAMdesigned to be used as the main memory ofgraphics 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 2020, there are seven, successive generations of GDDR:GDDR2,GDDR3,GDDR4,GDDR5,GDDR5X,GDDR6andGDDR6X.

Pseudostatic RAM

1 Mbit high speedCMOSpseudostatic RAM, made byToshiba

Pseudostatic RAM (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 is used in the Apple iPhone and other embedded systems such as XFlar Platform.

Some DRAM components have aself-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 than to allow operation without a separate DRAM controller as is in the case of mentioned PSRAMs.

Anembeddedvariant of PSRAM was sold by MoSys under the name1T-SRAM.It is a set of small DRAM banks with an SRAM cache in front to make it behave much like a true SRAM. It is used inNintendoGameCubeandWiivideo game consoles.

Cypress Semiconductor's HyperRAMis a type of PSRAM supporting aJEDEC-compliant 8-pin HyperBusor Octal xSPI interface.

See also

• DRAM price fi xing scandal
• Flash memory
• List of interface bit rates
• Memory bank
• Memory geometry

References

• Keeth, Brent; Baker, R. Jacob; Johnson, Brian; Lin, Feng (2007).DRAM Circuit Design: Fundamental and High-Speed Topics.Wiley.ISBN978-0470184752.

Further reading

• Jacob, Bruce; Wang, David; Ng, Spencer (2010) [2008].Memory Systems: Cache, DRAM, Disk.Morgan Kaufmann.ISBN978-0-08-055384-9.

External links

• Culler, David (2005). "Memory Capacity (Single Chip DRAM)".EECS 252 Graduate Computer Architecture: Lecture 1.Electrical Engineering and Computer Sciences,University of California, Berkeley. p. 15.Logarithmic graph 1980–2003 showing size and cycle time.
• Benefits of Chipkill-Correct ECC for PC Server Main Memory— A 1997 discussion of SDRAM reliability—some interesting information on soft errors fromcosmic rays,especially with respect toerror-correcting codeschemes
• Tezzaron Semiconductor Soft Error White Paper1994 literature review of memory error rate measurements.
• Johnston, A. (October 2000)."Scaling and Technology Issues for Soft Error Rates"(PDF).4th Annual Research Conference on Reliability Stanford University.Archived fromthe original(PDF)on 2004-11-03.
• Mandelman, J. A.; Dennard, R. H.; Bronner, G. B.; Debrosse, J. K.; Divakaruni, R.; Li, Y.; Radens, C. J. (2002)."Challenges and future directions for the scaling of dynamic random-access memory (DRAM)".IBM Journal of Research and Development.46(2.3): 187–212.doi:10.1147/rd.462.0187.Archived fromthe originalon 2005-03-22.
• Ars Technica: RAM Guide
• Wang, David Tawei (2005).Modern DRAM Memory Systems: Performance Analysis and a High Performance, Power-Constrained DRAM-Scheduling Algorithm(PDF)(PhD). University of Maryland, College Park.hdl:1903/2432.Retrieved2007-03-10.A detailed description of current DRAM technology.
• Multi-port Cache DRAM —MP-RAM
• Drepper, Ulrich (2007)."What every programmer should know about memory".