System
DRAM Technologies
DRAM Technology Types
As discussed
earlier, DRAM was originally created to act as a fast middle-man to
slow storage devices. Once the small section of data actually being
used was offloaded into memory, it could work with the microprocessor
much faster and nearly eliminate delays.
As time
passed, however, hard disks and microprocessors began to hit the market
in faster and faster models, to the point where the DRAM itself could
be considered a major bottleneck. In order to eliminate that bottleneck,
DRAM itself needed to evolve.
Today,
there are 10 different commercially available types of system DRAM
(excluding parity / ECC functions).
Page
Mode DRAM - (DRAM)
The first
brand of DRAM. It provided a fine solution for ancient ISA Video cards
and 286-486 PCs. However, when the performance of CPU's began to increase
more rapidly, it was quickly thrown out to make way for faster and
more efficient DRAM technology.
Fast
Page Mode DRAM - (FPM DRAM)
The first
popular redesign of DRAM used a new feature called Fast Page Mode,
which by default receives performance gains if the next access is
identical or concurrent to the prior access. This form of DRAM was
also manufactured at sub 60 ns ratings for various video cards.
Extended
Data Output DRAM - (EDO DRAM)
EDO DRAM
provides another performance boost by decreasing latency timing and
providing quicker sequential reads. EDO DRAM is also seemingly more
capable of handling 60 ns, which makes it ideal for 66 MHz motherboards.
Burst
Extended Data Output DRAM - (BEDO DRAM)
BEDO
provided a performance increase much like the small evolution that
Fast Page mode does. BEDO improves over EDO by allowing bursts of
data to be read, rather than one set at a time. It is hard to find,
however, as it was only useable with certain 3rd party chipsets, and
quickly overshadowed by SDRAM.
Enhanced
DRAM - (EDRAM)
An older
product from Enhanced Memory Systems, inc. It combines SRAM caches
with FPM DRAM to provide excellent performance for 50 MHz bus speeds.
By hiding DRAM charge time, it also improves page miss latency.
Multibank
(Burst) Extended Data Output Enhanced DRAM - (Multibank EDO EDRAM)
(Multibank BEDO EDRAM)
A specific
product available from Enhanced Memory Systems, Inc. It combines the
multibank SRAM caches of MDRAM with the read/write advancements of
EDO/BEDO DRAM, combined into a SIMM/DIMM package for system memory.
Synchronous
DRAM - (SDRAM)
System
Memory finally took the needed step up into high performance with
SDRAM. Unlike prior DRAMs, SDRAM operates synchronously with the System
Clock Speed! Combined with the incredibly low timing rates, it nearly
matches SRAM for speed.
PC100
Specification Synchronous DRAM - (PC100 SDRAM)
Technically,
there is no difference between this and normal SDRAM - the only change
is the better (lower) nanosecond rates. There are actually several
levels of PC100 specification, dealing with latency and PCB layers,
but anyone who wants to use it with a 100 MHz motherboard should stick
with 8 ns or lower. Most PC100 SDRAMs also include an EEPROM with
Serial Presence Detection, which helps the motherboard automatically
adjust settings.
Enhanced
Synchronous DRAM - (ESDRAM)
Another
company decided to try a new memory technology, called ESDRAM.
It is
supposedly capable of reducing internal latency and cycle times by
2x while offering twice the peak bandwidth of RDRAM. Like CDRAM, it
apparently uses hybrid technology to achieve the gains in performance.
Specific information on this memory technology can be found at the
ESDRAM Website.
Double
Data Rate Synchronous DRAM - (DDR SDRAM)
The first
evolution to SDRAM. It doubles potential bandwidth by sending data
on the rise and the fall of each clock signal. Because it is compatible
with traditional SDRAM architecture it is expected to supplant SDRAM
in the near future.
Direct
Rambus DRAM - (DRDRAM)
In an
attempt to provide the memory market with a high speed RAM capable
of staying in the marketplace for years, RDRAM was born. It uses 16bit
DRAM chips clocked at 400-800 MHz combined with DDR technology to
achieve its unmatched clock speed. Specific information on this memory
technology can be found at the Rambus
Website.
Synchronous
Link DRAM - (SLDRAM)
The third
powerful next generation memory is known as SLDRAM. It utilizes multiplexing
technology to achieve higher bandwidth through each of the pins. It
is also considered to be the strongest competitor against RDRAM. SLDRAM,
Inc. has more information on this emerging technology.
What
are Parity and ECC (Error Checking and Correction)?
Early
on, RAM was not as stable a solution as it is today. Irregularities
could cause the data in memory to corrupt or alter in ways that often
led to a system crash or hard disk data damage. This problem was first
solved with Parity RAM. Through additional or modified chips, it added
an additional bit to each byte of RAM which verified the validity
of each byte. If the data did not check out properly, your computer
would typically halt to avoid further problems.
ECC added
a further process to the cycle. Instead of merely checking the bytes,
it can correct most errors with an extra bit. It is fairly popular
with the CAD crowd, as it helps maintains strict accuracy. For most
consumers, however, it is not necessary due to the low rate of errors
in today's memory, and actually involves a slight performance hit.
What
are RAS and CAS?
They
stand for Row Access Strobe (RAS) and Column Access Strobe (CAS).
Each describes how long it takes to read a row or column of memory
cells, known as the CAS/RAS Latency. Each is described with a rating
number, where lower numbers are better. The rating is also dependant
on the front side bus speed of your motherboard so the rating may
rise on higher speeds.
Direct
RDRAM versus SLDRAM
The speed
at which technology gets faster is increasing exponentially as the
millenium approaches, with the invent of three distinct new memory
technologies: Direct RDRAM and SLDRAM.
They
both offer clock speeds of 400 MHz and beyond (4 times the speed of
today's system bus!) with better latencies and timing rates than today's
SDRAM. On paper, the differences seem to be extreme enough to give
hesitation to people buying memory today.
Whether
these two new types can coexist is seriously in doubt, however. While
Rambus has Intel on its side (Intel is responsible for motherboard
chipsets which control what types of RAM can be used), memory manufacturers
around the industry have complained about the licensing fees, patent
clauses and new form factor involved.
Companies
like Micron are pointing to SLDRAM as an easily comparable technology
that doesn't require fees (SLDRAM, inc. is non profit) or patent clauses,
while still using today's DIMM form factor and interface.
Currently
only Fujitsu is a member of the Rambus consortium - only time will
tell if Intel or the world's memory manufacturers will prevail.
>>
System
DRAM Form Factors <<
