Memory Terms and Techniques
Memory: Terms and Technologies.
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
Also, it can't keep up with processors whose buses run faster
than 66 MHz.
A data transmission mode in which data is sent faster than
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
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.
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
Now, most PCs come with a minimum of 8 megabytes of main
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
ROM is nonvolatile, whereas
RAM is volatile.
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
instead of the slower and cheaper dynamic
RAM (DRAM) used for
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
the computer avoids accessing the slower
Some memory caches are built into the architecture of
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
Like L1 caches, L2 caches are composed of
SRAM but they are much
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
be thousands of times faster than accessing a byte on a hard
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
The strategies for determining which information should be
kept in the cache constitute some of the more interesting
problems in computer science.
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
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
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
Video RAM Required for Different Resolutions.
||16 colors (4 bit)
||256 colors (8 bit)
||65 k colors (16 bit)
||16.7 mio colors (24 bit)
| 640 x 480
| 800 x 600
|1024 x 768
|1152 x 1024
|1280 x 1024
|1600 x 1200
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.
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,
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
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
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).
both types of memory
and RAM) allow random access.
precise, therefore, RAM should be referred
to as read/write
ROM as read-only RAM.
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
Short for programmable read-only memory.
is a memory chip on which data can be written only once.
Once a program has been written onto a
PROM, it remains there
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
PROM is manufactured as blank memory,
whereas a ROM is
programmed during the manufacturing process.
To write data
chip, you need a special device called a PROM
programmer or PROM burner.
The process of programming a
is sometimes called burning the PROM.
An EPROM (erasable programmable read-only memory) is a
special type of
that can be erased by exposing it to
Once it is erased, it can be reprogrammed.
is similar to a
PROM, but requires only electricity
to be erased.
Short for electrically erasable programmable read-only
is a special type of
that can be erased by exposing it to an electrical charge.
Like other types of
retains its contents even when the
power is turned off.
Also like other types of
is not as fast as
is similar to flash memory (sometimes called flash EEPROM).
The principal difference is that
data to be written or erased one byte at a time whereas flash
memory allows data to be written or erased in blocks.
makes flash memory faster.
Short for Non-Volatile Random Access Memory.
Any type of memory that retains its contents when power is
Also a type of
that is made non-volatile by connecting it
to a constant power source such as a battery.
Another type of NVRAM uses
chips to save its contents
when power is turned off.
In this case, NVRAM is composed of a combination of
Short for Multibank DRAM.
A relatively new memory technology
developed by MoSys Inc.
MDRAM utilizes small banks of
(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
Another advantage of MDRAM is that memory can be configured
in smaller increments, which can reduce the cost of some
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.
memory architectures, it's necessary to jump all the way to 4
MB. Currently, MDRAM is used in some video adapters and
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
Short for Extended Data Output Dynamic Random Access Memory.
A type of DRAM that is faster than
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
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
Nearly all PCs that support EDO DRAM are equipped with such a
controller, but some Macintosh models are not.
Short for dynamic random access memory.
A type of memory used in most personal computers.
See also SRAM.
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.
Short for Rambus DRAM.
A type of memory (DRAM) developed by
the fastest current memory technologies used by PCs
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
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.
Short for Synchronous DRAM.
A new type of
DRAM that can run at much higher clock speeds than
SDRAM actually synchronizes itself with the CPU's bus and is
capable of running at 100 MHz, about three times faster than
FPM RAM, and about twice as fast
EDO DRAM and
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.
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
SLDRAM, are being developed.
Short for Synchronous Graphic Random Access Memory.
A type of
used increasingly on video adapters and graphics accelerators.
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
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.
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
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.
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
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
because it does not need to pause between accesses.
Unfortunately, it is also much more expensive to produce than
Due to its high speed, SRAM is often used only as a memory cache.
Short for video RAM.
VRAM is special-purpose memory used by video adapters.
RAM, VRAM can be accessed by two different
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.
Short for Window RAM.
A type of
developed by Samsung Electronics that supports
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
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
Pipeline burst caches use two techniques:
- burst mode that pre-fetches memory contents before they
- pipelining so that one memory value can be accessed in the
cache at the same time that another memory value is accessed
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.
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.
A similar technique used in
DRAM, in which the memory loads the
requested memory contents into a small cache composed of
and then immediately begins fetching the next memory contents.
This creates a two-stage pipeline, where data is read from or
SRAM in one stage, and data is read from or
written to memory in the other stage.
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
Last update: 25.03.98