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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.
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.
Chip Packaging:
DIP (DUAL IN-LINE PACKAGE)
When it was common for memory to be installed
directly on the computer's system board, the DIP-style DRAM package was extremely
popular. DIPs are through-hole components, which means they install in holes
extending into the surface of the PCB. They can be soldered in place or installed
in sockets.
SOJ (SMALL OUTLINE J-LEAD)
SOJ packages got their name because the pins
coming out of the chip are shaped like the letter "J". SOJs are
surface-mount components - that is, they mount directly onto the surface of
the PCB.
TSOP (THIN SMALL OUTLINE PACKAGE)
TSOP packaging, another surface-mount design, got its
name because the package was much thinner than the SOJ design. TSOPs were
first used to make thin credit card modules for notebook computers.
CSP (CHIP SCALE PACKAGE)
Unlike DIP,
SOJ, and TSOP packaging, CSP packaging doesn't use pins to connect the chip
to the board. Instead, electrical connections to the board are through a BGA
(Ball Grid Array) on the underside of the package. RDRAM (Rambus DRAM) chips
utilize this type of packaging.
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