This section presents the most common memory technologies used for main
:memory: This road map offers an overview of the evolution of memory.
:
:YEAR INTRODUCED TECHNOLOGY SPEED LIMIT
:1987 FPM 50ns
:1995 EDO 50ns
:1997 PC66 SDRAM 66MHz
:1998 PC100 SDRAM 100MHz
:1999 RDRAM 800MHz
:1999/2000 PC133 SRAM 133MHz (VCM option)
:2000 DDR SDRAM 266MHz

MAJOR CHIP TECHNOLOGIES

It's usually pretty easy to tell memory module form factors apart because of
physical differences. Most module form factors can support various memory
technologies so, it's possible for two modules to appear to be the same
when, in fact, they're not. For example, a 168-pin DIMM can be used for EDO,
Synchronous DRAM, or some other type of memory. The only way to tell
precisely what kind of memory a module contains is to interpret the marking
on the chips. Each DRAM chip manufacturer has different markings and part
numbers to identify the chip technology.

FAST PAGE MODE (FPM)

At one time, FPM was the most common form of DRAM found in computers. In
fact, it was so common that people simply called it "DRAM," leaving off the
"FPM". FPM offered an advantage over earlier memory technologies because it
enabled faster access to data located within the same row.

EXTENDED DATA OUT (EDO)

In 1995, EDO became the next memory innovation. It was similar to FPM, but
with a slight modification that allowed consecutive memory accesses to occur
much faster. This meant the memory controller could save time by cutting out
a few steps in the addressing process. EDO enabled the CPU to access memory
10 to 15% faster than with FPM.

SYNCHRONOUS DRAM (SDRAM)

In late 1996, SDRAM began to appear in systems. Unlike previous
technologies, SDRAM is designed to synchronize itself with the timing of the
CPU. This enables the memory controller to know the exact clock cycle when
the requested data will be ready, so the CPU no longer has to wait between
memory accesses. SDRAM chips also take advantage of interleaving and burst
mode functions, which make memory retrieval even faster. SDRAM modules come
in several different speeds so as to synchronize to the clock speeds of the
systems they'll be used in. For example, PC66 SDRAM runs at 66MHz, PC100
SDRAM runs at 100MHz, PC133 SDRAM runs at 133MHz, and so on. Faster SDRAM
speeds such as 200MHz and 266MHz are currently in development.

DOUBLE DATA RATE SYNCHRONOUS DRAM (DDR SDRAM)

DDR SDRAM, is a next-generation SDRAM technology. It allows the memory chip
to perform transactions on both the rising and falling edges of the clock
cycle. For example, with DDR SDRAM, a 100 or 133MHz memory bus clock rate
yields an effective data rate of 200MHz or 266MHz. Systems using DDR SDRAM
are expected to ship at the end of the year 2000.

DIRECT RAMBUS

Direct Rambus is a new DRAM architecture and interface standard that
challenges traditional main memory designs. Direct Rambus technology is
extraordinarily fast compared to older memory technologies. It transfers
data at speeds up to 800MHz over a narrow 16-bit bus called a Direct Rambus
Channel. This high-speed clock rate is possible due to a feature called
"double clocked," which allows operations to occur on both the rising and
falling edges of the clock cycle. Also, each memory device on an RDRAM
module provides up to 1.6 gigabytes per second of bandwidth - twice the
bandwidth available with current 100MHz SDRAM.

In addition to chip technologies designed for use in main memory, there are
also specialty memory technologies that have been developed for video
applications.

MEMORY TECHNOLOGIES FOR VIDEO OR GRAPHICS PROCESSING

VIDEO RAM (VRAM)

VRAM is a video version of FPM technology. VRAM typically has two ports
instead of one, which allows the memory to allocate one channel to
refreshing the screen while the other is focused on changing the images on
the screen. This works much more efficiently than regular DRAM when it comes
to video applications. However, since video memory chips are used in much
lower quantities than main memory chips, they tend to be more expensive. So,
a system designer may choose to use regular DRAM in a video subsystem,
depending on whether cost or performance is the design objective.

WINDOW RAM (WRAM)

WRAM is another type of dual-ported memory also used in graphics-intensive
systems. It differs slightly from VRAM in that its dedicated display port is
smaller and it supports EDO features.

SYNCHRONOUS GRAPHICS RAM (SGRAM)

SGRAM is a video-specific extension of SDRAM that includes graphics-specific
read/write features. SGRAM also allows data to be retrieved and modified in
blocks, instead of individually. This reduces the number of reads and writes
that memory must perform and increases the performance of the graphics
controller by making the process more efficient.

BASE RAMBUS AND CONCURRENT RAMBUS

Before it even became a contender for main memory, Rambus technology was
actually used in video memory. The current Rambus main memory technology is
called Direct Rambus. Two earlier forms of Rambus are Base Rambus and
Concurrent Rambus. These forms of Rambus have been used in specialty video
applications in some workstations and video game systems like Nintendo 64
for several years.