Dynamic random-access memory: Difference between revisions

Revision as of 15:57, 16 October 2003

Revision as of 12:00, 3 November 2003

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== Special Types of DRAM ==

*fast=== Fast page mode DRAM ===

*video DRAM or VRAM

*extended data out (EDO) DRAM

*synchronous DRAM (SDRAM or sometimes SD-RAM)

*[[Rambus]] DRAM or [[RDRAM]]

*[[DDR SDRAM|double data rate (DDR) SDRAM]]

*pseudostatic RAM (PSRAM)

=== Fast page mode DRAM (or FPM DRAM or Page mode DRAM or Fast page mode memory or Page mode memory) ===

=== Fast page mode DRAM (oris also called FPM DRAM or Page mode DRAM or Fast page mode memory or Page mode memory) ===.

In many applications, data is often transferred to and from DRAM in bursts to consecutive addresses. Fast page mode DRAM simplifies this type of operation by providing an automatic column counter. The controller selects a row (also called a ''page'') and column, as with plain DRAM, and reads or writes the selected ___location. The DRAM then automatically increments the column address, allowing the controller to access the next ___location without having to supply a new address. This saves time, and increases the performance of the system when reading or writing bursts of data.

=== Video DRAM (VRAM) ===

'''VRAM''' is a dual-ported version of DRAM formerly used in [[graphics card|graphics adaptors]]. It is now amost obsolete, having been superseded by SDRAM and SGRAM. VRAM has two paths (or ports) to its memory array that can be used simultaneously. The first port, the DRAM port, is accessed as with plain DRAM. The second port, the video port, is read-only, and is dedicated to feeding a fast stream of data to the display. To use the video port, the controller first uses the DRAM port to select the row of the memory array that is to be displayed. The VRAM then copies that entire row to an internal shift-register. The controller can then continue to use the DRAM port for drawing objects on the display. Meanwhile, the controller feeds a clock called the ''shift clock (SCLK)'' to the VRAM's video port. Each SCLK pulse causes the VRAM to deliver the next item of data, in strict address order, from the shift-register to the video port. For simplicity, the graphics adaptor is usually designed so that the contents of a row, and therefore the contents of the shift-register, corresponds to a complete horizontal line on the display.

=== Synchronous Dynamic RAM (SDRAM) ===

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SDRAM has now ([[2000s]]) replaced plain DRAM in modern computers, because of its greater speed.

=== SynchronousExtended graphicsdata RAMout (SGRAMEDO) DRAM ===

It is a specialised form of SDRAM for graphics adaptors. It adds functions such as bit masking (writing to a specified bit plane without affecting the others) and block write (filling a block of memory with a single colour).

=== Double data rate (DDR) SDRAM ===

It[[DDR SDRAM|double data rate (DDR) SDRAM]] is a later development of SDRAM. All types of SDRAM use a clock signal that is a square wave. This means that the clock alternates regularly between one voltage (low) and another (high), usually millions of times per second. Plain SDRAM, like most synchronous logic circuits, acts on the low-to-high transition of the clock and ignores the opposite transition. DDR SDRAM acts on both transitions, thereby halving the required clock rate for a given data transfer rate.

=== Rambus DRAM (RDRAM) ===

It[[Rambus]] DRAM ([[RDRAM]]) is internally similar to DDR SDRAM, but uses a special method of signalling developed by the Rambus Company that allows faster clock speeds. RDRAM chips are packaged on modules called RIMMs, which are not compatible with the DIMMs used for plain SDRAM. The Rambus technology has now been taken over by [[Intel]].

=== Video DRAM (VRAM) ===

'''VRAM''' is a dual-ported version of DRAM formerly used in [[graphics card|graphics adaptors]]. It is now amost obsolete, having been superseded by SDRAM and SGRAM. VRAM has two paths (or ports) to its memory array that can be used simultaneously. The first port, the DRAM port, is accessed as with plain DRAM. The second port, the video port, is read-only, and is dedicated to feeding a fast stream of data to the display. To use the video port, the controller first uses the DRAM port to select the row of the memory array that is to be displayed. The VRAM then copies that entire row to an internal shift-register. The controller can then continue to use the DRAM port for drawing objects on the display. Meanwhile, the controller feeds a clock called the ''shift clock (SCLK)'' to the VRAM's video port. Each SCLK pulse causes the VRAM to deliver the next item of data, in strict address order, from the shift-register to the video port. For simplicity, the graphics adaptor is usually designed so that the contents of a row, and therefore the contents of the shift-register, corresponds to a complete horizontal line on the display.

=== Synchronous graphics RAM (SGRAM) ===

It is a specialised form of SDRAM for graphics adaptors. It adds functions such as bit masking (writing to a specified bit plane without affecting the others) and block write (filling a block of memory with a single colour).

=== Pseudostatic RAM (PSRAM) ===