Short for Burst EDO DRAM.
A new type of EDO DRAM that can process four memory addresses in one burst.
Unlike SDRAM, however, BEDO DRAM can only stay synchronized with the CPU clock for short periods (bursts).
Also, it can't keep up with processors whose buses run faster than 66 MHz.

Burst Mode

A data transmission mode in which data is sent faster than normal.
There are a number of techniques for implementing burst modes.
In a data bus, for example, a burst mode is usually implemented by allowing a device to seize control of the bus and not permitting other devices to interrupt.
In RAM, burst modes are implemented by automatically fetching the next memory contents before they are requested.
This is essentially the same technique used by disk caches.

The one characteristic that all burst modes have in common is that they are temporary and unsustainable.
They allow faster data transfer rates than normal, but only for a limited period of time and only under special conditions.

Main Memory

Refers to physical memory that is internal to the computer. The word main is used to distinguish it from external mass storage devices such as disk drives.
Another term for main memory is RAM.

The computer can manipulate only data that is in main memory. Therefore, every program you execute and every file you access must be copied from a storage device into main memory. The amount of main memory on a computer is crucial because it determines how many programs can be executed at one time and how much data can be readily available to a program.

Because computers often have too little main memory to hold all the data they need, computer engineers invented a technique called swapping, in which portions of data are copied into main memory as they are needed. Swapping occurs when there is no room in memory for needed data. When one portion of data is copied into memory, an equal-sized portion is copied (swapped) out to make room. Another technique, called virtual memory, enables a computer to access larger amounts of data than main memory can hold at one time, but it is a relatively slow process. Therefore, the more memory a computer has and the more it can avoid swapping, the faster it will be able to execute large programs.

Now, most PCs come with a minimum of 8 megabytes of main memory. This is adequate for some applications, but you may need more memory to run sophisticated applications, particularly those that include graphics. You can usually increase the amount of memory by inserting extra memory in the form of chips or memory expansion boards.

Non Volatile Memory

Types of memory that retain their contents when power is turned off.
ROM is nonvolatile, whereas RAM is volatile.

Cache Memory

A special high-speed storage mechanism.
It can be either a reserved section of main memory or an independent high-speed storage device.
Two types of caching are commonly used in personal computers:

- memory caching
- disk caching

A memory cache, sometimes called a cache store or RAM cache, is a portion of memory made of high-speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main memory.
Memory caching is effective because most programs access the same data or instructions over and over.
By keeping as much of this information as possible in SRAM, the computer avoids accessing the slower DRAM.

Some memory caches are built into the architecture of microprocessors.
The Intel 80486 microprocessor, for example, contains an 8K memory cache, and the Pentium has a 16K cache.
Such internal caches are often called Level 1 (L1) caches.
Most modern PCs also come with external cache memory, called Level 2 (L2) caches.
These caches sit between the CPU and the DRAM.
Like L1 caches, L2 caches are composed of SRAM but they are much larger.

Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory.
The most recently accessed data from the disk (as well as adjacent sectors) is stored in a memory buffer.
When a program needs to access data from the disk, it first checks the disk cache to see if the data is there.
Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.

When data is found in the cache, it is called a cache hit, and the effectiveness of a cache is judged by its hit rate.
Many cache systems use a technique known as smart caching, in which the system can recognize certain types of frequently used data.
The strategies for determining which information should be kept in the cache constitute some of the more interesting problems in computer science.

Video Memory

RAM installed on a video adapter.
Before an image can be sent to a display monitor it is first represented as a bit map in an area of video memory called the frame buffer.
The amount of video memory, therefore, dictates the maximum resolution and color depth available (see table below).

With a conventional video adapter, the bit map to be displayed is first generated by the computer's microprocessor and then sent to the frame buffer. Most modern video adapters, however, are actually graphics accelerators.
This means that they have they're own microprocessor that is capable of manipulating bit maps and graphics objects.
A small amount of memory is reserved for these operations as well.

Because of the demands of video systems, video memory needs to be faster than main memory. There are many different types of video memory, including VRAM, WRAM, RDRAM, and SGRAM.

Video RAM Required for Different Resolutions.

Resolution	16 colors (4 bit)	256 colors (8 bit)	65 k colors (16 bit)	16.7 mio colors (24 bit)
640 x 480	512K	512K	1 MB	2 MB
800 x 600	512K	512K	1 MB	2 MB
1024 x 768	1 MB	1 MB	2 MB	4 MB
1152 x 1024	1 MB	2 MB	2 MB	4 MB
1280 x 1024	1 MB	2 MB	4 MB	4 MB
1600 x 1200	2 MB	2 MB	4 MB	8 MB

ROM

Short for read-only memory.
Computer memory on which data has been prerecorded.
Once data has been written onto a ROM chip, it cannot be removed and can only be read.

Unlike main memory (RAM), ROM retains its contents even when the computer is turned off. ROM is referred to as being nonvolatile, whereas RAM is volatile.

Most personal computers contain a small amount of ROM that stores critical programs such as the program that boots the computer. In addition, ROMs are used extensively in calculators and peripheral devices such as laser printers, whose fonts are often stored in ROMs.

A variation of a ROM is a PROM (programmable read-only memory). PROMs are manufactured as blank chips on which data can be written with a special device called a PROM programmer.

RAM

Short for random access memory.
A type of computer memory that can be accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes.
RAM is the most common type of memory found in computers and other devices, such as printers.

There are two basic types of RAM:

- Dynamic RAM (DRAM)
- Static RAM (SRAM)

The two types differ in the technology they use to hold data, dynamic RAM being the more common type.
Dynamic RAM needs to be refreshed thousands of times per second.
Static RAM needs to be refreshed less often, which makes it faster; but it is also more expensive than dynamic RAM.
Both types of RAM are volatile, meaning that they lose their contents when the power is turned off.

In common usage, the term RAM is synonymous with main memory, the memory available to programs. For example, a computer with 8M RAM has approximately 8 million bytes of memory that programs can use.
In contrast, ROM (read-only memory) refers to special memory used to store programs that boot the computer and perform diagnostics.
Most personal computers have a small amount of ROM (a few thousand bytes).
In fact, both types of memory (ROM and RAM) allow random access.
To be precise, therefore, RAM should be referred to as read/write RAM and ROM as read-only RAM.

RAMDAC

Short for Random Access Memory Digital-to-Analog Converter.
A single chip on video adapter cards.
The RAMDAC's role is to convert digitally encoded images into analog signals that can be displayed by a monitor.
A RAMDAC actually consists of four different components

- SRAM to store the color map
- digital-to-analog converters (DAC) for monitor's red electron gun
- digital-to-analog converters (DAC) for monitor's green electron gun
- digital-to-analog converters (DAC) for monitor's blue electron gun

PROM

Short for programmable read-only memory.
A PROM is a memory chip on which data can be written only once. Once a program has been written onto a PROM, it remains there forever.
Unlike main memory, PROMs retain their contents when the computer is turned off.

The difference between a PROM and a ROM (read-only memory) is that a PROM is manufactured as blank memory, whereas a ROM is programmed during the manufacturing process.
To write data onto a PROM chip, you need a special device called a PROM programmer or PROM burner.
The process of programming a PROM is sometimes called burning the PROM.

An EPROM (erasable programmable read-only memory) is a special type of PROM that can be erased by exposing it to ultraviolet light.
Once it is erased, it can be reprogrammed. An EEPROM is similar to a PROM, but requires only electricity to be erased.

EEPROM

Short for electrically erasable programmable read-only memory.
An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.
Like other types of PROM, EEPROM retains its contents even when the power is turned off.
Also like other types of ROM, EEPROM is not as fast as RAM.

EEPROM is similar to flash memory (sometimes called flash EEPROM).
The principal difference is that EEPROM requires data to be written or erased one byte at a time whereas flash memory allows data to be written or erased in blocks.
This makes flash memory faster.

NVRAM

Short for Non-Volatile Random Access Memory.
Any type of memory that retains its contents when power is turned off.

Also a type of SRAM that is made non-volatile by connecting it to a constant power source such as a battery.
Another type of NVRAM uses EEPROM chips to save its contents when power is turned off.
In this case, NVRAM is composed of a combination of SRAM and EEPROM chips.

MDRAM

Short for Multibank DRAM.
A relatively new memory technology developed by MoSys Inc.
MDRAM utilizes small banks of DRAM (32 KB each) in an array, where each bank has its own I/O port that feeds into a common internal bus.
Because of this design, data can be read or written to multiple banks simultaneously, which makes it much faster than conventional DRAM.

Another advantage of MDRAM is that memory can be configured in smaller increments, which can reduce the cost of some components.
For example, it's possible to produce MDRAM chips with 2.5 MB, which is what is required by video adapters for 24-bit color at a resolution of 1,024x768.
With conventional memory architectures, it's necessary to jump all the way to 4 MB. Currently, MDRAM is used in some video adapters and graphics accelerators.

FPM RAM

Short for Fast Page Mode RAM.
A type of Dynamic RAM ( DRAM) that allows faster access to data in the same row or page.
Page-mode memory works by eliminating the need for a row address if data is located in the row previously accessed.
It is sometimes called page mode memory.

FPM RAM is being replaced by newer types of memory, such as Extended Data Out (EDO) DRAM, BEDO DRAM, and SDRAM.

EDO DRAM

Short for Extended Data Output Dynamic Random Access Memory.
A type of DRAM that is faster than conventional DRAM.
Unlike conventional DRAM which only allows one byte to be read at a time.
EDO DRAM can copy an entire block of memory to its internal cache.
While the processor is accessing this cache, the memory can collect a new block to send.

Note that EDO DRAM is faster than conventional DRAM only if the cache controller supports a transfer mode known as pipeline burst.
Nearly all PCs that support EDO DRAM are equipped with such a controller, but some Macintosh models are not.

DRAM

Short for dynamic random access memory.
A type of memory used in most personal computers.
See also SRAM.

DDR SDRAM

Short for Double Data Rate-Synchronous DRAM.
A type of SDRAM that supports data transfers on both edges of each clock cycle, effectively doubling the memory chip's data throughput.
DDR-SDRAM is also called SDRAM II.

RDRAM

Short for Rambus DRAM.
A type of memory (DRAM) developed by Rambus, Inc.
Whereas the fastest current memory technologies used by PCs (SDRAM) can deliver data at a maximum speed of about 100 MHz, RDRAM transfers data at up to 600 MHz.

In 1997, Intel announced that it would license the Rambus technology for use on its future motherboards, thus making it the likely de facto standard for memory architectures.
However, a consortium of computer vendors is working on an alternative memory architecture called SyncLink DRAM (SLDRAM).

RDRAM is already being used in place of VRAM in some graphics accelerator boards, but it is not expected to be used for the main memory of PCs until 1998 or 1999.
Intel and Rambus are also working a new version of RDRAM, called nDRAM, that will support data transfer speeds at up to 1,600 MHz.

SDRAM

Short for Synchronous DRAM.
A new type of DRAM that can run at much higher clock speeds than conventional memory.
SDRAM actually synchronizes itself with the CPU's bus and is capable of running at 100 MHz, about three times faster than conventional FPM RAM, and about twice as fast EDO DRAM and BEDO DRAM.
SDRAM is replacing EDO DRAM in many newer computers.

Today's fastest Pentium systems use CPU buses running at 100 MHz, so SDRAM can keep up with them, though barely.
Future PCs, however, are expected to have CPU buses running at 200 MHz or faster.
SDRAM is not expected to support these high speeds which is why new memory technologies, such as RDRAM and SLDRAM, are being developed.

SGRAM

Short for Synchronous Graphic Random Access Memory.
A type of DRAM used increasingly on video adapters and graphics accelerators. Like SDRAM, SGRAM can synchronize itself with the CPU bus clock up to speeds of 100 MHz.
In addition, SGRAM uses several other techniques, such as masked writes and block writes, to increase bandwidth for graphics-intensive functions.

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.

SLDRAM

Short for SyncLink DRAM.
A new type of memory being developed by a consortium of computer manufacturers called the Synclink Consortium.
SLDRAM is competing with Rambus memory (RDRAM) as the future PC memory architecture.
But whereas actual RDRAM chips are already in use on video boards and other devices, SLDRAM is still on paper only.
Moreover, Intel is backing RDRAM, which reduces the odds of SLDRAM becoming an important technology.

SRAM

Short for static random access memory.
SRAM is a type of memory that is faster and more reliable than the more common DRAM (dynamic RAM).
The term static is derived from the fact that it needs to be refreshed less often than dynamic RAM.

While DRAM supports access times of about 60 nanoseconds, SRAM can give access times as low as 10 nanoseconds.
In addition, its cycle time is much shorter than that of DRAM because it does not need to pause between accesses.
Unfortunately, it is also much more expensive to produce than DRAM. Due to its high speed, SRAM is often used only as a memory cache.

VRAM

Short for video RAM.
VRAM is special-purpose memory used by video adapters.
Unlike conventional RAM, VRAM can be accessed by two different devices simultaneously.
This enables a monitor to access the VRAM for screen updates at the same time that a graphics processor provides new data.
VRAM yields better graphics performance but is more expensive than normal RAM.

WRAM

Short for Window RAM.
A type of RAM developed by Samsung Electronics that supports two ports.
This enables a video adapter to fetch the contents of memory for display at the same time that new bytes are being pumped into memory.
This results in much faster display than is possible with conventional single-port RAM.

WRAM is similar to VRAM, but achieves even faster performance at less cost.

Pipeline Burst Cache

A type of memory cache built into many modern DRAM designs. Pipeline burst caches use two techniques:

- burst mode that pre-fetches memory contents before they are requested
- pipelining so that one memory value can be accessed in the cache at the same time that another memory value is accessed in DRAM

The purpose of pipeline burst caches is to minimize wait states so that memory can be accessed as fast a possible by the microprocessor.

An alternative DRAM design that provides even faster DRAM access is Synchronous DRAM (SDRAM).
SDRAM also uses a burst mode, but because memory accesses are synchronized with the microprocessor bus, pipelining is not necessary.

Pipelining

(1)

A technique used in advanced microprocessors where the microprocessor begins executing a second instruction before the first has been completed.
That is, several instructions are in the pipeline simultaneously, each at a different processing stage.

The pipeline is divided into segments and each segment can execute its operation concurrently with the other segments.
When a segment completes an operation, it passes the result to the next segment in the pipeline and fetches the next operation from the preceding segment.
The final results of each instruction emerge at the end of the pipeline in rapid succession.

Although formerly a feature only of high-performance and RISC -based microprocessors, pipelining is now common in microprocessors used in personal computers.
Intel's Pentium chip, for example, uses pipelining to execute as many as four instructions simultaneously.

Pipelining is called pipeline processing.

(2)

A similar technique used in DRAM, in which the memory loads the requested memory contents into a small cache composed of SRAM and then immediately begins fetching the next memory contents.
This creates a two-stage pipeline, where data is read from or written to SRAM in one stage, and data is read from or written to memory in the other stage.

DRAM pipelining is usually combined with another performance technique called burst mode.
The two techniques together are called a pipeline burst cache.

Thanks for joining.

This is the end of the show.

Juergen Theis, IBM Germany
e-mail: jtheis@vnet.ibm.com
Last update: 25.03.98