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78-473: Compared to single data rate ( SDR ) SDRAM, the DDR SDRAM interface makes higher transfer rates possible through more strict control of the timing of the electrical data and clock signals. Implementations often have to use schemes such as phase-locked loops and self-calibration to reach the required timing accuracy. The interface uses double pumping (transferring data on both the rising and falling edges of

156-473: A cache will generally access memory in units of cache lines . To transfer a 64-byte cache line requires eight consecutive accesses to a 64-bit DIMM, which can all be triggered by a single read or write command by configuring the SDRAM chips, using the mode register, to perform eight-word bursts . A cache line fetch is typically triggered by a read from a particular address, and SDRAM allows the "critical word" of

234-616: A dual-edge clocking RAM and presented their results at the International Solid-State Circuits Convention in 1990. Samsung released the first commercial DDR SDRAM chip (64   Mbit ) in June 1998, followed soon after by Hyundai Electronics (now SK Hynix ) the same year. DDR SDRAM specification was finalized by JEDEC in June 2000 (JESD79). JEDEC has set standards for the data rates of DDR SDRAM, divided into two parts. The first specification

312-467: A read command is issued, the SDRAM will produce the corresponding output data on the DQ lines in time for the rising edge of the clock a few clock cycles later, depending on the configured CAS latency. Subsequent words of the burst will be produced in time for subsequent rising clock edges. A write command is accompanied by the data to be written driven on to the DQ lines during the same rising clock edge. It

390-417: A 1 GB memory module are usually organized as 2 8-bit words, commonly expressed as 64M×8. Memory manufactured in this way is low-density RAM and is usually compatible with any motherboard specifying PC3200 DDR-400 memory. DDR (DDR1) was superseded by DDR2 SDRAM , which had modifications for a higher clock frequency and again doubled throughput, but operates on the same principle as DDR. Competing with DDR2

468-442: A SDR SDRAM running at the same clock frequency, due to this double pumping. With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate (in bytes/s) of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus, with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600  MB/s . In the late 1980s IBM invented DDR SDRAM, they built

546-399: A bus that is 64 data bits wide, and since a byte comprises 8 bits, this equates to 8 bytes of data per transfer. In addition to bandwidth and capacity variants, modules can: Note: DDR2 DIMMs are not backward compatible with DDR DIMMs. The notch on DDR2 DIMMs is in a different position from DDR DIMMs, and the pin density is higher than DDR DIMMs in desktops. DDR2 is a 240-pin module, DDR

624-556: A command is directed toward. Many commands also use an address presented on the address input pins. Some commands, which either do not use an address, or present a column address, also use A10 to select variants. The SDR SDRAM commands are defined as follows: All SDRAM generations (SDR and DDRx) use essentially the same commands, with the changes being: As an example, a 512 MB SDRAM DIMM (which contains 512 MB), might be made of eight or nine SDRAM chips, each containing 512 Mbit of storage, and each one contributing 8 bits to

702-585: A common identifier for 100 MHz SDRAM modules, and modules are now commonly designated with "PC"-prefixed numbers (PC66, PC100 or PC133 - although the actual meaning of the numbers has changed). All commands are timed relative to the rising edge of a clock signal. In addition to the clock, there are six control signals, mostly active low , which are sampled on the rising edge of the clock: SDRAM devices are internally divided into either two, four or eight independent internal data banks. One to three bank address inputs (BA0, BA1 and BA2) are used to select which bank

780-611: A correct match. Most DDR SDRAM operates at a voltage of 2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce power consumption. Chips and modules with the DDR-400/PC-3200 standard have a nominal voltage of 2.6 V. JEDEC Standard No. 21–C defines three possible operating voltages for 184 pin DDR, as identified by the key notch position relative to its centreline. Page 4.5.10-7 defines 2.5V (left), 1.8V (centre), TBD (right), while page 4.20.5–40 nominates 3.3V for

858-622: A drop in operating voltage (1.8 V compared to DDR's 2.5 V). The lower memory clock frequency may also enable power reductions in applications that do not require the highest available data rates. According to JEDEC the maximum recommended voltage is 1.9 volts and should be considered the absolute maximum when memory stability is an issue (such as in servers or other mission critical devices). In addition, JEDEC states that memory modules must withstand up to 2.3 volts before incurring permanent damage (although they may not actually function correctly at that level). For use in computers, DDR2 SDRAM

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936-414: A higher-than-standard data rate whilst others simply round up for the name. Note: DDR2-xxx denotes data transfer rate, and describes raw DDR chips, whereas PC2-xxxx denotes theoretical bandwidth (with the last two digits truncated), and is used to describe assembled DIMMs. Bandwidth is calculated by taking transfers per second and multiplying by eight. This is because DDR2 memory modules transfer data on

1014-469: A read command is issued on cycle 0, another read command is issued on cycle 2, and the CAS latency is 3, then the first read command will begin bursting data out during cycles 3 and 4, then the results from the second read command will appear beginning with cycle 5. If the command issued on cycle 2 were burst terminate, or a precharge of the active bank, then no output would be generated during cycle 5. Although

1092-486: A read of that row into the bank's array of all 16,384 column sense amplifiers. This is also known as "opening" the row. This operation has the side effect of refreshing the dynamic (capacitive) memory storage cells of that row. Once the row has been activated or "opened", read and write commands are possible to that row. Activation requires a minimum amount of time, called the row-to-column delay, or t RCD before reads or writes to it may occur. This time, rounded up to

1170-425: A rising edge of its clock input. In SDRAM families standardized by JEDEC , the clock signal controls the stepping of an internal finite-state machine that responds to incoming commands. These commands can be pipelined to improve performance, with previously started operations completing while new commands are received. The memory is divided into several equally sized but independent sections called banks , allowing

1248-405: A row is an automatic side effect of activating it, there is a minimum time for this to happen, which requires a minimum row access time t RAS delay between an active command opening a row, and the corresponding precharge command closing it. This limit is usually dwarfed by desired read and write commands to the row, so its value has little effect on typical performance. The no operation command

1326-406: A sufficient number of auto refresh commands (one per row, 8192 in the example we have been using) every refresh interval (t REF = 64 ms is a common value). All banks must be idle (closed, precharged) when this command is issued. As mentioned, the clock enable (CKE) input can be used to effectively stop the clock to an SDRAM. The CKE input is sampled each rising edge of the clock, and if it is low,

1404-428: Is DDR SDRAM designed to operate at 200 MHz using DDR-400 chips with a bandwidth of 3,200 MB/s. Because PC3200 memory transfers data on both the rising and falling clock edges, its effective clock rate is 400 MHz. 1 GB PC3200 non-ECC modules are usually made with 16 512 Mbit chips, 8 on each side (512 Mbits × 16 chips) / (8 bits (per byte)) = 1,024 MB. The individual chips making up

1482-505: Is a double data rate (DDR) synchronous dynamic random-access memory (SDRAM) interface . It is a JEDEC standard (JESD79-2); first published in September 2003. DDR2 succeeded the original DDR SDRAM specification, and was itself succeeded by DDR3 SDRAM in 2007. DDR2 DIMMs are neither forward compatible with DDR3 nor backward compatible with DDR. In addition to double pumping the data bus as in DDR SDRAM (transferring data on

1560-573: Is a 184-pin module. Notebooks have 200-pin SO-DIMMs for DDR and DDR2; however, the notch on DDR2 modules is in a slightly different position than on DDR modules. Higher-speed DDR2 DIMMs can be mixed with lower-speed DDR2 DIMMs, although the memory controller will operate all DIMMs at same speed as the lowest-speed DIMM present. GDDR2, a form of GDDR SDRAM , was developed by Samsung and introduced in July 2002. The first commercial product to claim using

1638-808: Is a common belief that number of module ranks equals number of sides. As above data shows, this is not true. One can also find 2-side/1-rank modules. One can even think of a 1-side/2-rank memory module having 16(18) chips on single side ×8 each, but it is unlikely such a module was ever produced. From Ballot JCB-99-70, and modified by numerous other Board Ballots, formulated under the cognizance of Committee JC-42.3 on DRAM Parametrics. Standard No. 79 Revision Log: "This comprehensive standard defines all required aspects of 64Mb through 1Gb DDR SDRAMs with X4/X8/X16 data interfaces, including features, functionality, ac and dc parametrics, packages and pin assignments. This scope will subsequently be expanded to formally apply to x32 devices, and higher density devices as well." PC3200

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1716-539: Is a product of bits per chip and number of chips. It also equals number of ranks (rows) multiplied by DDR memory bus width. Consequently, a module with a greater number of chips or using ×8 chips instead of ×4 will have more ranks. This example compares different real-world server memory modules with a common size of 1 GB. One should definitely be careful buying 1 GB memory modules, because all these variations can be sold under one price position without stating whether they are ×4 or ×8, single- or dual-ranked. There

1794-405: Is a product of one chip's capacity and the number of chips. ECC modules multiply it by 8 ⁄ 9 because they use 1 bit per byte (8 bits) for error correction. A module of any particular size can therefore be assembled either from 32 small chips (36 for ECC memory), or 16(18) or 8(9) bigger ones. DDR memory bus width per channel is 64 bits (72 for ECC memory). Total module bit width

1872-683: Is also available in registered varieties, for systems that require greater scalability such as servers and workstations . Today, the world's largest manufacturers of SDRAM include Samsung Electronics , SK Hynix , Micron Technology , and Nanya Technology . There are several limits on DRAM performance. Most noted is the read cycle time, the time between successive read operations to an open row. This time decreased from 10 ns for 100 MHz SDRAM (1 MHz = 10 6 {\displaystyle 10^{6}}  Hz) to 5 ns for DDR-400, but remained relatively unchanged through DDR2-800 and DDR3-1600 generations. However, by operating

1950-463: Is always permitted, while the load mode register command requires that all banks be idle, and a delay afterward for the changes to take effect. The auto refresh command also requires that all banks be idle, and takes a refresh cycle time t RFC to return the chip to the idle state. (This time is usually equal to t RCD +t RP .) The only other command that is permitted on an idle bank is the active command. This takes, as mentioned above, t RCD before

2028-464: Is any DRAM where the operation of its external pin interface is coordinated by an externally supplied clock signal . DRAM integrated circuits (ICs) produced from the early 1970s to the early 1990s used an asynchronous interface, in which input control signals have a direct effect on internal functions delayed only by the trip across its semiconductor pathways. SDRAM has a synchronous interface, whereby changes on control inputs are recognised after

2106-564: Is available at effective transfer rates of 400 MHz and higher. DDR3 advances extended the ability to preserve internal clock rates while providing higher effective transfer rates by again doubling the prefetch depth. The DDR4 SDRAM is a high-speed dynamic random-access memory internally configured as 16 banks, 4 bank groups with 4 banks for each bank group for ×4/×8 and 8 banks, 2 bank groups with 4 banks for each bank group for ×16 DRAM. The DDR4 SDRAM uses an 8 n prefetch architecture to achieve high-speed operation. The 8 n prefetch architecture

2184-671: Is combined with an interface designed to transfer two data words per clock cycle at the I/O pins. A single read or write operation for the DDR4 SDRAM consists of a single 8 n -bit-wide 4-clock data transfer at the internal DRAM core and 8 corresponding n -bit-wide half-clock-cycle data transfers at the I/O pins. RDRAM was a particularly expensive alternative to DDR SDRAM, and most manufacturers dropped its support from their chipsets. DDR1 memory's prices substantially increased from Q2 2008, while DDR2 prices declined. In January 2009, 1 GB DDR1

2262-623: Is encoded on the bank address pins during the load mode register command. For example, DDR2 SDRAM has a 13-bit mode register, a 13-bit extended mode register No. 1 (EMR1), and a 5-bit extended mode register No. 2 (EMR2). It is possible to refresh a RAM chip by opening and closing (activating and precharging) each row in each bank. However, to simplify the memory controller, SDRAM chips support an "auto refresh" command, which performs these operations to one row in each bank simultaneously. The SDRAM also maintains an internal counter, which iterates over all possible rows. The memory controller must simply issue

2340-628: Is for memory chips, and the second is for memory modules. The first retail PC motherboard using DDR SDRAM was released in August 2000. To increase memory capacity and bandwidth, chips are combined on a module. For instance, the 64-bit data bus for DIMM requires eight 8-bit chips, addressed in parallel. Multiple chips with common address lines are called a memory rank . The term was introduced to avoid confusion with chip internal rows and banks . A memory module may bear more than one rank. The term sides would also be confusing because it incorrectly suggests

2418-424: Is greatly increased as a trade-off. The DDR2 prefetch buffer is four bits deep, whereas it is two bits deep for DDR. While DDR SDRAM has typical read latencies of between two and three bus cycles, DDR2 may have read latencies between three and nine cycles, although the typical range is between four and six. Thus, DDR2 memory must be operated at twice the data rate to achieve the same latency. Another cost of

DDR SDRAM - Misplaced Pages Continue

2496-424: Is like power down, but the SDRAM uses an on-chip timer to generate internal refresh cycles as necessary. The clock may be stopped during this time. While self-refresh mode consumes slightly more power than power-down mode, it allows the memory controller to be disabled entirely, which commonly more than makes up the difference. SDRAM designed for battery-powered devices offers some additional power-saving options. One

2574-522: Is manufactured are also standardized by JEDEC. There is no architectural difference between DDR SDRAM modules. Modules are instead designed to run at different clock frequencies: for example, a PC-1600 module is designed to run at 100 MHz , and a PC-2100 is designed to run at 133 MHz . A module's clock speed designates the data rate at which it is guaranteed to perform, hence it is guaranteed to run at lower ( underclocking ) and can possibly run at higher ( overclocking ) clock rates than those for which it

2652-721: Is not inherently lower (faster access times) than asynchronous DRAM. Indeed, early SDRAM was somewhat slower than contemporaneous burst EDO DRAM due to the additional logic. The benefits of SDRAM's internal buffering come from its ability to interleave operations to multiple banks of memory, thereby increasing effective bandwidth . Today, virtually all SDRAM is manufactured in compliance with standards established by JEDEC , an electronics industry association that adopts open standards to facilitate interoperability of electronic components. JEDEC formally adopted its first SDRAM standard in 1993 and subsequently adopted other SDRAM standards, including those for DDR , DDR2 and DDR3 SDRAM . SDRAM

2730-472: Is preferred by Intel for its microprocessors. If the requested column address is at the start of a block, both burst modes (sequential and interleaved) return data in the same sequential sequence 0-1-2-3-4-5-6-7. The difference only matters if fetching a cache line from memory in critical-word-first order. Single data rate SDRAM has a single 10-bit programmable mode register. Later double-data-rate SDRAM standards add additional mode registers, addressed using

2808-504: Is sparse and more common 2GB per DIMM are used. DDR2 SDRAM was first produced by Samsung in 2001. In 2003, the JEDEC standards organization presented Samsung with its Technical Recognition Award for the company's efforts in developing and standardizing DDR2. DDR2 was officially introduced in the second quarter of 2003 at two initial clock rates: 200 MHz (referred to as PC2-3200) and 266 MHz (PC2-4200). Both performed worse than

2886-511: Is supplied in DIMMs with 240 pins and a single locating notch. Laptop DDR2 SO-DIMMs have 200 pins and often come identified by an additional S in their designation. DIMMs are identified by their peak transfer capacity (often called bandwidth). * Some manufacturers label their DDR2 modules as PC2-4300, PC2-5400 or PC2-8600 instead of the respective names suggested by JEDEC. At least one manufacturer has reported this reflects successful testing at

2964-544: Is temperature-dependent refresh; an on-chip temperature sensor reduces the refresh rate at lower temperatures, rather than always running it at the worst-case rate. Another is selective refresh, which limits self-refresh to a portion of the DRAM array. The fraction which is refreshed is configured using an extended mode register. The third, implemented in Mobile DDR (LPDDR) and LPDDR2 is "deep power down" mode, which invalidates

3042-493: Is the duty of the memory controller to ensure that the SDRAM is not driving read data on to the DQ lines at the same time that it needs to drive write data on to those lines. This can be done by waiting until a read burst has finished, by terminating a read burst, or by using the DQM control line. When the memory controller needs to access a different row, it must first return that bank's sense amplifiers to an idle state, ready to sense

3120-402: Is the following word if an even address was specified, and the previous word if an odd address was specified. For the sequential burst mode , later words are accessed in increasing address order, wrapping back to the start of the block when the end is reached. So, for example, for a burst length of four, and a requested column address of five, the words would be accessed in the order 5-6-7-4. If

3198-442: Is the heart of a read operation, as it involves the careful sensing of the tiny signals in DRAM memory cells; it is the slowest phase of memory operation. However, once a row is read, subsequent column accesses to that same row can be very quick, as the sense amplifiers also act as latches. For reference, a row of a 1 Gbit DDR3 device is 2,048 bits wide, so internally 2,048 bits are read into 2,048 separate sense amplifiers during

DDR SDRAM - Misplaced Pages Continue

3276-497: Is the increase in prefetch length. In DDR SDRAM, the prefetch length was two bits for every bit in a word; whereas it is four bits in DDR2 SDRAM. During an access, four bits were read or written to or from a four-bit-deep prefetch queue. This queue received or transmitted its data over the data bus in two data bus clock cycles (each clock cycle transferred two bits of data). Increasing the prefetch length allowed DDR2 SDRAM to double

3354-405: The clock signal ) to double data bus bandwidth without a corresponding increase in clock frequency. One advantage of keeping the clock frequency low is that it reduces the signal integrity requirements on the circuit board connecting the memory to the controller. The name "double data rate" refers to the fact that a DDR SDRAM with a certain clock frequency achieves nearly twice the bandwidth of

3432-508: The "DDR2" technology was the Nvidia GeForce FX 5800 graphics card. However, this GDDR2 memory used on graphics cards is not DDR2 per se, but rather an early midpoint between DDR and DDR2 technologies. Using "DDR2" to refer to GDDR2 is a colloquial misnomer . In particular, the performance-enhancing doubling of the I/O clock rate is missing. It had severe overheating issues due to the nominal DDR voltages. ATI has since designed

3510-477: The DIMM's 64- or 72-bit width. A typical 512 Mbit SDRAM chip internally contains four independent 16 MB memory banks. Each bank is an array of 8,192 rows of 16,384 bits each. (2048 8-bit columns). A bank is either idle, active, or changing from one to the other. The active command activates an idle bank. It presents a two-bit bank address (BA0–BA1) and a 13-bit row address (A0–A12), and causes

3588-462: The DQ lines to the SDRAM in time for the write operation. Because the effects of DQM on read data are delayed by two cycles, but the effects of DQM on write data are immediate, DQM must be raised (to mask the read data) beginning at least two cycles before write command but must be lowered for the cycle of the write command (assuming the write command is intended to have an effect). Doing this in only two clock cycles requires careful coordination between

3666-569: The GDDR technology further into GDDR3 , which is based on DDR2 SDRAM, though with several additions suited for graphics cards. GDDR3 was commonly used in graphics cards and some tablet PCs. However, further confusion has been added to the mix with the appearance of budget and mid-range graphics cards which claim to use "GDDR2". These cards actually use standard DDR2 chips designed for use as main system memory although operating with higher latencies to achieve higher clock rates. These chips cannot achieve

3744-422: The SDRAM automatically enters power-down mode, consuming minimal power until CKE is raised again. This must not last longer than the maximum refresh interval t REF , or memory contents may be lost. It is legal to stop the clock entirely during this time for additional power savings. Finally, if CKE is lowered at the same time as an auto-refresh command is sent to the SDRAM, the SDRAM enters self-refresh mode. This

3822-525: The SDRAM's mode register and expected by the DRAM controller. Any value may be programmed, but the SDRAM will not operate correctly if it is too low. At higher clock rates, the useful CAS latency in clock cycles naturally increases. 10–15 ns is 2–3 cycles (CL2–3) of the 200 MHz clock of DDR-400 SDRAM, CL4-6 for DDR2-800, and CL8-12 for DDR3-1600. Slower clock cycles will naturally allow lower numbers of CAS latency cycles. SDRAM modules have their own timing specifications, which may be slower than those of

3900-423: The bank address pins. For SDR SDRAM, the bank address pins and address lines A10 and above are ignored, but should be zero during a mode register write. The bits are M9 through M0, presented on address lines A9 through A0 during a load mode register cycle. Later (double data rate) SDRAM standards use more mode register bits, and provide additional mode registers called "extended mode registers". The register number

3978-521: The burst length were eight, the access order would be 5-6-7-0-1-2-3-4. This is done by adding a counter to the column address, and ignoring carries past the burst length. The interleaved burst mode computes the address using an exclusive or operation between the counter and the address. Using the same starting address of five, a four-word burst would return words in the order 5-4-7-6. An eight-word burst would be 5-4-7-6-1-0-3-2. Although more confusing to humans, this can be easier to implement in hardware, and

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4056-486: The cache line to be transferred first. ("Word" here refers to the width of the SDRAM chip or DIMM, which is 64 bits for a typical DIMM.) SDRAM chips support two possible conventions for the ordering of the remaining words in the cache line. Bursts always access an aligned block of BL consecutive words beginning on a multiple of BL. So, for example, a four-word burst access to any column address from four to seven will return words four to seven. The ordering, however, depends on

4134-460: The chips on the module. When 100 MHz SDRAM chips first appeared, some manufacturers sold "100 MHz" modules that could not reliably operate at that clock rate. In response, Intel published the PC100 standard, which outlines requirements and guidelines for producing a memory module that can operate reliably at 100 MHz. This standard was widely influential, and the term "PC100" quickly became

4212-487: The data to be written into the memory array. For a pipelined read, the requested data appears a fixed number of clock cycles (latency) after the read command, during which additional commands can be sent. The earliest DRAMs were often synchronized with the CPU clock (clocked) and were used with early microprocessors. In the mid-1970s, DRAMs moved to the asynchronous design, but in the 1990s returned to synchronous operation. In

4290-457: The device to operate on a memory access command in each bank simultaneously and speed up access in an interleaved fashion. This allows SDRAMs to achieve greater concurrency and higher data transfer rates than asynchronous DRAMs could. Pipelining means that the chip can accept a new command before it has finished processing the previous one. For a pipelined write, the write command can be immediately followed by another command without waiting for

4368-601: The effective clock rates of DDR2 are higher than DDR, the overall performance was not greater in the early implementations, primarily due to the high latencies of the first DDR2 modules. DDR2 started to be effective by the end of 2004, as modules with lower latencies became available. Memory manufacturers stated that it was impractical to mass produce DDR1 memory with effective transfer rates in excess of 400 MHz (i.e. 400 MT/s and 200 MHz external clock) due to internal speed limitations. DDR2 picks up where DDR1 leaves off, utilizing internal clock rates similar to DDR1, but

4446-401: The following rising edge of the clock is ignored for all purposes other than checking CKE. As long as CKE is low, it is permissible to change the clock rate, or even stop the clock entirely. If CKE is lowered while the SDRAM is performing operations, it simply "freezes" in place until CKE is raised again. If the SDRAM is idle (all banks precharged, no commands in progress) when CKE is lowered,

4524-492: The increased bandwidth is the requirement that the chips are packaged in a more expensive and difficult to assemble BGA package as compared to the TSSOP package of the previous memory generations such as DDR SDRAM and SDR SDRAM . This packaging change was necessary to maintain signal integrity at higher bus speeds. Power savings are achieved primarily due to an improved manufacturing process through die shrinkage, resulting in

4602-487: The interface circuitry at increasingly higher multiples of the fundamental read rate, the achievable bandwidth has increased rapidly. Another limit is the CAS latency , the time between supplying a column address and receiving the corresponding data. Again, this has remained relatively constant at 10–15 ns through the last few generations of DDR SDRAM. In operation, CAS latency is a specific number of clock cycles programmed into

4680-421: The interrupting read may be to any active bank, a precharge command will only interrupt the read burst if it is to the same bank or all banks; a precharge command to a different bank will not interrupt a read burst. Interrupting a read burst by a write command is possible, but more difficult. It can be done if the DQM signal is used to suppress output from the SDRAM so that the memory controller may drive data over

4758-513: The late 1980s IBM invented DDR SDRAM, they built a dual-edge clocking RAM and presented their results at the International Solid-State Circuits Convention in 1990. In 1998, Samsung released a double data rate SDRAM, known as DDR SDRAM , chip (64   Mbit ) followed soon after by Hyundai Electronics (now SK Hynix ) the same year and mass-produced in 1993. By 2000, SDRAM had replaced virtually all other types of DRAM in modern computers, because of its greater performance. SDRAM latency

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4836-423: The manufacturer. Such chips draw significantly more power than slower-clocked chips, but usually offered little or no improvement in real-world performance, unless bandwidth dependent tasks such as integrated graphics rendering are used. DDR2 started to become competitive against the older DDR standard by the end of 2004, as modules with lower latencies became available. The key difference between DDR2 and DDR SDRAM

4914-500: The memory and requires a full reinitialization to exit from. This is activated by sending a "burst terminate" command while lowering CKE. DDR SDRAM employs prefetch architecture to allow quick and easy access to multiple data words located on a common physical row in the memory. The prefetch architecture takes advantage of the specific characteristics of memory accesses to DRAM. Typical DRAM memory operations involve three phases: bitline precharge, row access, column access. Row access

4992-495: The next multiple of the clock period, specifies the minimum number of wait cycles between an active command, and a read or write command. During these wait cycles, additional commands may be sent to other banks; because each bank operates completely independently. Both read and write commands require a column address. Because each chip accesses eight bits of data at a time, there are 2,048 possible column addresses thus requiring only 11 address lines (A0–A9, A11). When

5070-426: The next row. This is known as a "precharge" operation, or "closing" the row. A precharge may be commanded explicitly, or it may be performed automatically at the conclusion of a read or write operation. Again, there is a minimum time, the row precharge delay, t RP , which must elapse before that row is fully "closed" and so the bank is idle in order to receive another activate command on that bank. Although refreshing

5148-404: The original DDR specification due to higher latency, which made total access times longer. However, the original DDR technology tops out at a clock rate around 200 MHz (400 MT/s). Higher performance DDR chips exist, but JEDEC has stated that they will not be standardized. These chips are mostly standard DDR chips that have been tested and rated to be capable of operation at higher clock rates by

5226-718: The physical placement of chips on the module. All ranks are connected to the same memory bus (address + data). The chip select signal is used to issue commands to specific rank. Adding modules to the single memory bus creates additional electrical load on its drivers. To mitigate the resulting bus signaling rate drop and overcome the memory bottleneck , new chipsets employ the multi-channel architecture. Note: All items listed above are specified by JEDEC as JESD79F. All RAM data rates in-between or above these listed specifications are not standardized by JEDEC – often they are simply manufacturer optimizations using tighter tolerances or overvolted chips. The package sizes in which DDR SDRAM

5304-398: The rate at which data could be transferred over the data bus without a corresponding doubling in the rate at which the DRAM array could be accessed. DDR2 SDRAM was designed with such a scheme to avoid an excessive increase in power consumption. DDR2's bus frequency is boosted by electrical interface improvements, on-die termination , prefetch buffers and off-chip drivers. However, latency

5382-421: The requested address, and the configured burst type option: sequential or interleaved. Typically, a memory controller will require one or the other. When the burst length is one or two, the burst type does not matter. For a burst length of one, the requested word is the only word accessed. For a burst length of two, the requested word is accessed first, and the other word in the aligned block is accessed second. This

5460-428: The right notch position. The orientation of the module for determining the key notch position is with 52 contact positions to the left and 40 contact positions to the right. Increasing the operating voltage slightly can increase maximum speed but at the cost of higher power dissipation and heating, and at the risk of malfunctioning or damage. Module and chip characteristics are inherently linked. Total module capacity

5538-500: The rising and falling edges of the bus clock signal ), DDR2 allows higher bus speed and requires lower power by running the internal clock at half the speed of the data bus. The two factors combine to produce a total of four data transfers per internal clock cycle. Since the DDR2 internal clock runs at half the DDR external clock rate, DDR2 memory operating at the same external data bus clock rate as DDR results in DDR2 being able to provide

5616-533: The row access phase. Row accesses might take 50 ns , depending on the speed of the DRAM, whereas column accesses off an open row are less than 10 ns. Traditional DRAM architectures have long supported fast column access to bits on an open row. For an 8-bit-wide memory chip with a 2,048 bit wide row, accesses to any of the 256 datawords (2048/8) on the row can be very quick, provided no intervening accesses to other rows occur. DDR2 SDRAM Double Data Rate 2 Synchronous Dynamic Random-Access Memory ( DDR2 SDRAM )

5694-420: The row is fully open and can accept read and write commands. When a bank is open, there are four commands permitted: read, write, burst terminate, and precharge. Read and write commands begin bursts, which can be interrupted by following commands. A read, burst terminate, or precharge command may be issued at any time after a read command, and will interrupt the read burst after the configured CAS latency. So if

5772-407: The same bandwidth but with better latency . Alternatively, DDR2 memory operating at twice the external data bus clock rate as DDR may provide twice the bandwidth with the same latency. The best-rated DDR2 memory modules are at least twice as fast as the best-rated DDR memory modules. The maximum capacity on commercially available DDR2 DIMMs is 8GB, but chipset support and availability for those DIMMs

5850-401: The time the SDRAM takes to turn off its output on a clock edge and the time the data must be supplied as input to the SDRAM for the write on the following clock edge. If the clock frequency is too high to allow sufficient time, three cycles may be required. If the read command includes auto-precharge, the precharge begins the same cycle as the interrupting command. A modern microprocessor with

5928-425: Was Rambus XDR DRAM . DDR2 dominated due to cost and support factors. DDR2 was in turn superseded by DDR3 SDRAM , which offered higher performance for increased bus speeds and new features. DDR3 has been superseded by DDR4 SDRAM , which was first produced in 2011 and whose standards were still in flux (2012) with significant architectural changes. DDR's prefetch buffer depth is 2 (bits), while DDR2 uses 4. Although

6006-516: Was 2–3 times more expensive than 1 GB DDR2. MDDR is an acronym that some enterprises use for Mobile DDR SDRAM, a type of memory used in some portable electronic devices, like mobile phones , handhelds , and digital audio players . Through techniques including reduced voltage supply and advanced refresh options, Mobile DDR can achieve greater power efficiency. Synchronous dynamic random-access memory#SDR Synchronous dynamic random-access memory ( synchronous dynamic RAM or SDRAM )

6084-488: Was made. DDR SDRAM modules for desktop computers, dual in-line memory modules (DIMMs) , have 184 pins (as opposed to 168 pins on SDRAM, or 240 pins on DDR2 SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two). DDR SDRAM for notebook computers, SO-DIMMs , have 200 pins, which is the same number of pins as DDR2 SO-DIMMs. These two specifications are notched very similarly and care must be taken during insertion if unsure of

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