Fast Guide to RAM

RAM Defined | Briefly How RAM Works | Kinds of RAM
Main RAM | SRAM | BSRAM | DRAM | FPM DRAM | EDO RAM | EDO DRAM | BEDO DRAM | EDRAM | NVRAM | SDRAM | JEDEC SDRAM | PC100 SDRAM | DDR SDRAM | ESDRAM | DRDRAM | SLDRAM | FRAM
Video RAM | RAMDAC | VRAM | SGRAM | WRAM | MDRAM | RDRAM
The RAM Table | How Much Do You Need?

RAM Defined

RAM (random access memory) is the place in a computer where the operating system, application programs, and data in current use are kept so that they can be quickly reached by the computer's processor. RAM is much faster to read from and write to than the other kinds of storage in a computer, the hard disk, floppy disk, and CD-ROM. However, the data in RAM stays there only as long as your computer is running. When you turn the computer off, RAM loses its data. When you turn your computer on again, your operating system and other files are once again loaded into RAM, usually from your hard disk.

RAM can be compared to a person's short-term memory and the hard disk to the long-term memory. The short-term memory focuses on work at hand, but can only keep so many facts in view at one time. If short-term memory fills up, your brain sometimes is able to refresh it from facts stored in long-term memory. A computer also works this way. If RAM fills up, the processor needs to continually go to the hard disk to overlay old data in RAM with new, slowing down the computer's operation. Unlike the hard disk which can become completely full of data so that it won't accept any more, RAM never runs out of memory. It keeps operating, but much more slowly than you may want it to.

How Big is RAM?

RAM is small, both in physical size (it's stored in microchip modules) and in the amount of data it can hold. It's much smaller than your hard disk. A typical computer may come with 32 million bytes of RAM and a hard disk that can hold 4 billion bytes. RAM comes in the form of "discrete" (meaning separate) microchip modules that plug into holes in the computer's motherboard. These holes connect through a bus or set of electrical paths to the processor. The hard drive, on the other hand, stores data on a magnetized surface that looks like a phonograph record.

Today's personal computers come with 16 or more megabytes of RAM, usually increasing in multiples of 8 megabytes. Users of graphic applications usually need 32, 64, or 128 or even more megabytes of memory. Most personal computers are designed to allow you to add additional RAM modules up to a certain limit (for example, up to 64 or 128 megabytes). Having more RAM in your computer reduces the number of times that the computer processor has to read data in from your hard disk, an operation that takes much longer than reading data from RAM. (RAM access time is expressed in nanoseconds; hard disk access time is expressed in milliseconds.)

Why Random Access

RAM is called "random access" because any storage location can be accessed directly. Originally, the term distinguished regular core memory from offline memory, usually on magnetic tape in which an item of data could only be accessed by starting from the beginning of the tape and finding an address sequentially. Perhaps it should have been called "nonsequential memory" because RAM access is hardly random. RAM is organized and controlled in a way that enables data to be stored and retrieved directly to specific locations. A term IBM has preferred is direct access storage or memory. Note that other forms of storage such as the hard disk and CD-ROM are also accessed directly (or "randomly") but the term random access is not applied to these forms of storage.

In addition to disk, floppy disk, and CD-ROM storage, another important form of storage is read-only memory (ROM), a more expensive kind of memory that retains data even when the computer is turned off. Every computer comes with a small amount of ROM that holds just enough programming so that the operating system can be loaded into RAM each time the computer is turned on.

Briefly How RAM Works

What RAM Looks Like

In general, RAM is much like an arrangement of post-office boxes in which each box can hold a 0 or a 1. Each box has a unique address that can be found by counting across columns and then counting down by row. In RAM, this set of post-office boxes is known as an array and each box is a cell. To find the contents of a box (cell), the RAM controller sends the column/row address down a very thin electrical line etched into the chip. There is an address line for each row and each column in the set of boxes. If data is being read, the bits that are read flow back on a separate data line. In describing a RAM chip or module, a notation such as 256Kx16 means 256 thousand columns of cells standing 16 rows deep.

In the most common form of RAM, dynamic RAM, each cell has a charge or lack of charge held in something similar to an electrical capacitor. A transistor acts as a gate in determining whether the value in the capacitor can be read or written. In static RAM, instead of a capacitor-held charge, the transistor itself is a positional flip/flop switch, with one position meaning 1 and the other position meaning 0.

Externally, RAM is a chip that comes embedded in a personal computer motherboard with a variable amount of additional modules plugged into motherboard sockets. To add memory to your computer, you simply add more RAM modules in a prescribed configuration. These are single in-line memory modules (SIMMs) or dual in-line memory modules (DIMMs). Since DIMMs have a 64-bit pin connection, they can replace two 36-bit (32-bits plus 4 parity bits) SIMMs when synchronous DRAM is used. Laptop and notebook computers contain smaller 32-bit DIMMs known as small outline DIMMs (SO DIMMs).

How Data Is Accessed

When the processor or CPU gets the next instruction it is to perform, the instruction may contain the address of some memory or RAM location from which data is to be read (brought to the processor for further processing). This address is sent to the RAM controller. The RAM controller organizes the request and sends it down the appropriate address lines so that transistors along the lines open up the cells so that each capacitor value can be read. A capacitor with a charge over a certain voltage level represents the binary value of 1 and a capacitor with less than that charge represents a 0. For dynamic RAM, before a capacitor is read, it must be power-refreshed to ensure that the value read is valid. Depending on the type of RAM, the entire line of data may be read that the specific address happens to be located at or, in some RAM types, a unit of data called a page is read. The data that is read is transmitted along the data lines to the processor's nearby data buffer known as level-1 cache and another copy may be held in level-2 cache.

For video RAM, the process is similar to DRAM except that, in some forms of video RAM, while data is being written to video RAM by the processor, data can simultaneously be read from RAM by the video controller (for example, for refreshing the display image).

How RAM Effectiveness is Measured

The amount of time that RAM takes to write data or to read it once the request has been received from the processor is called the access time. Typical access times vary from 9 nanoseconds to 70 nanoseconds, depending on the kind of RAM. Although fewer nanoseconds is better, user-perceived performance is based on coordinating access times with the computer's clock cycles. Access time consists of latency and transfer time. Latency is the time to coordinate signal timing and refresh data after reading it.

Kinds of RAM

RAM can be divided into (1) main RAM, which stores every kind of data and makes it quickly accessible to a microprocessor and (2) video RAM, which stores data intended for your display screen, enabling images to get to your display faster.

Main RAM

Main RAM can be divided into static RAM (SRAM) and dynamic RAM (DRAM).

Static RAM (SRAM)

Static RAM is more expensive, requires four times the amount of space for a given amount of data than dynamic RAM, but, unlike dynamic RAM, does not need to be power-refreshed and is therefore faster to access. One source gives a typical access time as 25 nanoseconds in contrast to a typical access time of 60 nanoseconds for dynamic RAM. (More recent advances in dynamic RAM have improved access time.) Static RAM is used mainly for the level-1 and level-2 caches that the microprocessor looks in first before looking in dynamic RAM.

Burst (or SynchBurst) Static RAM (BSRAM)

Burst SRAM (also known as SynchBurst SRAM) is synchronized with the system clock or, in some cases, the cache bus clock. This allows it be more easily synchronized with any device that accesses it and reduces access waiting time. It is used as the external level-2 cache memory for the Pentium II microprocessor chipset.

Dynamic RAM (DRAM)

Dynamic RAM uses a kind of capacitor that needs frequent power refreshing to retain its charge. Because reading a DRAM discharges its contents, a power refresh is required after each read. Apart from reading, just to maintain the charge that holds its content in place, DRAM must be refreshed about every 15 microseconds. DRAM is the least expensive kind of RAM.

Fast Page Mode DRAM (FPM DRAM)

Prior to newer forms of DRAM, Fast Page Mode DRAM (FPM DRAM) was the most common kind of DRAM in personal computers. Page mode DRAM essentially accesses a row of RAM without having to continually respecify the row. A row access strobe (RAS) signal is held active while the column access strobe (CAS) signal changes to read a sequence of contiguous cells. This reduces access time and lowers power requirements. Clock timings for FPM DRAM are typically 6-3-3-3 (meaning 3 clock cycles for access setup, and 3 clock cycles for the first and each of three successive accesses based on the initial setup).

Enhanced DRAM

Enhanced DRAM (EDRAM) is the combination of SRAM and DRAM in a single package that is usually used for a level-2 cache. Typically, 256 bytes of static RAM is included along with the dynamic RAM. Data is read first from the faster (typically 15 nanoseconds) SRAM and if it is not found there, it is read from the DRAM, typically at 35 nanoseconds.

Extended Data Output RAM or DRAM (EDO RAM or EDO DRAM)

Extended Data Output RAM (EDO RAM) or Extended Data Output Dynamic RAM (EDO DRAM) is up to 25% faster than standard DRAM and reduces the need for level-2 cache memory.

Burst Extended Data Output DRAM (BEDO DRAM)

BEDO DRAM improves page mode DRAM by "building in" three successive column address shifts after the first column address is specified so that four bits are read as a burst. Together with a dual-bank architecture, BEDO DRAM promised to offer 4-1-1-1 access times. However, because Intel and other manufacturers preferred SDRAM, BEDO DRAM was not widely used.

Nonvolatile RAM (NVRAM)

Nonvolatile RAM (NVRAM) is a special kind of RAM that retains data when the computer is turned off or there is a power failure. Like the computer's read-only memory (ROM), it is powered by a battery within the computer. It can also work by writing its contents to and restoring them from an EEPROM.

Synchronous DRAM (SDRAM)

Synchronous DRAM (SDRAM) is a generic name for various kinds of DRAM that are synchronized with the clock speed that the microprocessor is optimized for. This tends to increase the number of instructions that the processor can perform in a given time. The speed of SDRAM is rated in MHz rather than in nanoseconds (ns). This makes it easier to compare the bus speed and the RAM chip speed. You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.

JEDEC SDRAM

JEDEC (Joint Electron Device Engineering Council) SDRAM is an industry standard synchronous DRAM. It has a dual-bank architecture and several burst mode accesses that can be preset. JEDEC SDRAM chips operate at either 83 MHz or 100 MHz. JEDEC SDRAM is also known as PC66 SDRAM because it was originally rated for 66 MHz bus operation and to distinguish it from Intel's PC100 architecture.

PC100 SDRAM

PC100 SDRAM is SDRAM that states that it meets the PC100 specification from Intel. Intel created the specification to enable RAM manufacturers to make chips that would work with Intel's i440BX processor chipset. The i440BX was designed to achieve a 100 MHz system bus speed. Ideally, PC100 SDRAM would work at the 100 MHz speed, using a 4-1-1-1 access cycle. It's reported that PC100 SDRAM will improve performance by 10-15% in an Intel Socket 7 system (but not in a Pentium II because its L2 cache speed runs at only half of processor speed).

Double Data Rate SDRAM (DDR SDRAM)

Double Data Rate SDRAM can theoretically improve RAM speed to at least 200 MHz. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling output. It's expected that a number of Socket 7 chipset makers will support this form of SDRAM.

Enhanced SDRAM (ESDRAM)

Enhanced SDRAM (ESDRAM), made by Enhanced Memory Systems, includes a small static RAM (SRAM) in the SDRAM chip. This means that many accesses will be from the faster SRAM. In case the SRAM doesn't have the data, there is a wide bus between the SRAM and the SDRAM because they are on the same chip. ESDRAM is apparently competing with DDR SDRAM as a faster SDRAM chip for Socket 7 processors.

Direct Rambus DRAM (DRDRAM)

Direct Rambus DRAM (DRDRAM) is a proprietary technology proposed by Rambus, Inc. in partnership with Intel. Like SLDRAM, it promises RAM speed up to 800 MHz. It has a smaller bus width (16 bits compared to 64 bits) than current SDRAM designs.

SyncLink DRAM (SLDRAM)

SyncLink DRAM is, along with Direct Rambus DRAM (DRDRAM), a protocol-based approach. In this approach, all signals to RAM are on the same line (rather than having separate CAS, RAS, address, and data lines). Since access time does not depend on synchronizing operations on multiple lines, SLDRAM promises RAM speed of up to 800 MHz. Like Double Data Rate SDRAM, SLDRAM can operate at twice the system clock rate. SyncLink is an open industry standard that is expected to compete and perhaps prevail over Direct Rambus DRAM.

Ferroelectric RAM (FRAM)

FRAM (ferroelectric RAM) is random access memory that combines the fast read and write access of dynamicRAM (DRAM) - the most common kind of personal computer memory - with the ability to retain data whenpower is turned off (as do other non-volatile memory devices such as ROM and flash memory). Because FRAMis not as dense (can not store as much data in the same space) as DRAM and SRAM, it is not likely replacethese technologies. However, because it is fast memory with a very low power requirement, it is expected tohave many applications in small consumer devices such as personal digital assistants (PDAs), handheld phones,power meters, and smart cards, and in security systems. FRAM is faster than flash memory. It is also expectedto replace EEPROM and SRAM for some applications and to become a key component in future wireless products.

In spite of its name, ferroelectric RAM does not contain iron. Today's FRAM uses lead zirconate titanate (PZT); other materials arebeing considered. The main developer of FRAM is Ramtron International.

Video RAM

Video RAM as "video RAM" means in general all forms of RAM used to store image data for the video display monitor. Somewhat confusingly, the most common type of video RAM is called Video RAM (VRAM). All types of video RAM are special arrangements of dynamic RAM (DRAM). Video RAM is really a buffer between the processor and the display monitor and is often called the frame buffer. When images are to be sent to the display, they are first read by the processor as data from some form of main storage RAM and then written to video RAM. From video RAM (the frame buffer), the data is converted by a RAM digital-to-analog converter (RAMDAC) into analog signals that are sent to the display presentation mechanism such as a cathode ray tube (CRT). Usually, video RAM comes in a 1 or 2 megabyte package and is located on the video or graphics card in the computer. Most forms of video RAM are dual-ported. While the processor is writing a new image to video RAM, the display is reading from video to refresh its current display content. The dual-port design is the main difference between main storage RAM and video RAM.

RAMDAC

RAMDAC (random access memory digital-to-analog converter) is a microchip that converts digital image data into the analog data needed by a computer display. A RAMDAC microchip is built into the video adapter in a computer. It combines a small static RAM (SRAM) containing a color table with three digital-to-analog converters (DACs) that change digital image data into analog signals that are sent to the display's color generators, one for each primary color - red, green, and blue. In a cathode ray tube (CRT) display, an analog signal is sent to each of three electron guns. With displays using other technologies, the signals are sent to a corresponding mechanism.

The SRAM part of the RAMDAC contains a color palette table. A logical color number in the digital data input to SRAM is used to generate three separate values obtained from the table - one for each of red, green, and blue - that are output to one of three digital-to-analog converters. The analog signal output from the converter is input directly to the display electron guns or other image projecting mechanisms. For displays with true color, the digital color data is fed directly to the DACs, bypassing the SRAM table, which is not needed.

Video RAM

Video RAM (VRAM) is the vanilla flavor of video RAM. It is dual-ported, allowing the processor to write to it at the same time that it is refreshing the image on the display monitor.

Synchronous Graphics RAM

Synchronous Graphics RAM (SGRAM) is clock-synchronized RAM that is used for video memory. It is relatively low-cost video memory. It uses masked write, which enables selected data to be modified in a single operation rather as a sequence of read, update, and write operations. It also uses block write, which allows data for background or foreground image fills to be handled more efficiently. SGRAM is single-ported. Its special features are what make it a moderately fast form of video memory. The Matrox Mystique is an example of a video card that uses SGRAM.

Window RAM

Window RAM (WRAM), unrelated to Microsoft Windows, is very high-performance video RAM that is dual-ported and has about 25% more bandwidth than VRAM but costs less. It has features that make it more efficient to read data for use in block fills and text drawing. It can be used for very high resolution (such as 1600 by 1200 pixels) projection using true color). It's used in the Matrox Millenium video card.

Multibank Dynamic RAM

Multibank Dynamic RAM (MDRAM) is a high-performance RAM, developed by MoSys, that divides memory into multiple 32 KB parts or "banks" that can be accessed individually. Traditional video RAM is monolithic; the entire frame buffer is accessed at one time. Having individual memory banks allows accesses to be interleaved concurrently, increasing overall performance. It's also cheaper since, unlike other forms of video RAM, cards can be manufactured with just the right amount of RAM for a given resolution capability instead of requiring it to be in multiples of megabytes.

Rambus Dynamic RAM

Rambus Dynamic RAM (RDRAM) is a video RAM designed by Rambus that includes a proprietary bus that speeds up the data flow between video RAM and the frame buffer. It's optimized for video streaming.

The RAM Table

Read more about it at: > The PC Technology Guide has a section on "System Memory." > Tom's Hardware Guide includes "A RAM Guide." > Micron has a product selection guide for its DRAM, SRAM, and flash memory products. > Enhanced Memory Systems, a subsidiary of Ramtron, makes EDRAM and ESDRAM. > Matrox describes its Mystique video card with SGRAM and its Millenium card with WRAM. > Rambus describes how the Rambus DRAM works.

This word suggested by: Aista, Mark Burrett, Con Giakamozis, Chaminda Jayasekara, Harry Kachline, Carlos Eugenio Machado, Jeff Martin, Dayne Miller, Mike Peters, Simon Smith, and Shaun Trivedi

Last updated on: Apr 29, 2008

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