RAM
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Due to cost considerations, all
but the very high-end (and very expensive) computers have utilized DRAM for main
memory. Originally, these were asynchronous, single-bank designs because the
processors were relatively slow. Most recently, synchronous interfaces have been
produced with many advanced features. Though these high-performance DRAMs have
been available for only a few years, it is apparent that they will soon be
replaced by at least one of the protocol-based designs, such as SyncLink or the
DRDRAM design from Rambus, Inc. and Intel.
Basic DRAM operation
A DRAM memory array can be
thought of as a table of cells. These cells are comprised of capacitors, and
contain one or more �bits� of data, depending upon the chip configuration.
This table is addressed via row and column decoders, which in turn receive their
signals from the RAS and CAS clock generators. In order to minimize the package
size, the row and column addresses are multiplexed into row and column address
buffers. For example, if there are 11 address lines, there will be 11 row and 11
column address buffers. Access transistors called �sense amps� are connected
to the each column and provide the read and restore operations of the chip.
Since the cells are capacitors that discharge for each read operation, the sense
amp must restore the data before the end of the access cycle.
The capacitors used for data
cells tend to bleed off their charge, and therefore require a periodic refresh
cycle or data will be lost. A refresh controller determines the time between
refresh cycles, and a refresh counter ensures that the entire array (all rows)
are refreshed. Of course, this means that some cycles are used for refresh
operations, and has some impact on performance.
A typical memory access would
occur as follows. First, the row address bits are placed onto the address pins.
After a period of time the RAS\ signal falls, which activates the sense amps and
causes the row address to be latched into the row address buffer. When the RAS\
signal stabilizes, the selected row is transferred onto the sense amps. Next,
the column address bits are set up, and then latched into the column address
buffer when CAS\ falls, at which time the output buffer is also turned on. When
CAS\ stabilizes, the selected sense amp feeds its data onto the output buffer.
Page Mode Access
By implementing special access
modes, designers were able to eliminate some of the internal operations for
certain types of access. The first significant implementation was called Page
Mode access.
Using this method, the RAS\
signal is held active so that an entire �page� of data is held on the sense
amps. New column addresses can then be repeatedly clocked in only by cycling CAS\.
This provides much faster random access reads, since the row address setup and
hold times are eliminated.
While some applications benefit
greatly from this type of access, there are others that do not benefit at all.
The original Page Mode was improved upon and replaced very quickly so you will
likely never see any memory of this type. Even if you do, it wouldn�t be worth
even getting it for free, considering the advantages of later access modes.
Fast Page Mode
Fast Page mode improved upon
the original page mode by eliminating the column address setup time during the
page cycle. This was accomplished by activating the column address buffers on
the falling edge of RAS\ (rather than CAS\). Since RAS\ remains low for the
entire page cycle, this acts as a transparent latch when CAS\ is high, and
allows address setup to occur as soon as the column address is valid, rather
than waiting for CAS\ to fall.
Fast Page mode became the most
widely used access method for DRAMs, and is still used on many systems. The
benefit of FPM memory is reduced power consumption, mainly because sense and
restore current is not necessary during page mode access. Though FPM was a major
innovation, there are still some drawbacks. The most significant is that the
output buffers turn off when CAS\ goes high. The minimum cycle time is 5ns
before the output buffers turn off, which essentially adds at least 5ns to the
cycle time.
Today, FPM memory is the least
desirable of all available DRAM memory. You should only consider using this if
it is either free, or your system does not support any of the later memory types
(such as a 486 based system). Typical timings are 6-3-3-3 (initial latency of 3
clocks, with a 3-clock page access). Due to the limited demand, FPM is actually
more expensive now than most of the faster memories now available.
HyperPage Mode (EDO)
The last major improvement to
asynchronous DRAMs came with the Hyperpage mode, or Extended DataOut. This
innovation was simply to no longer turn off the output buffers upon the rising
edge of /CAS. In essence, this eliminates the column precharge time while
latching the data out. This allows the minimum time for /CAS to be low to be
reduced, and the rising edge can come earlier.
In addition to a 40% or greater
improvement in access times, EDO uses the same amount of silicon and the same
package size. EDO has been shown to work well with memory bus speeds up to 83MHz
with little or no performance penalty. If the chips are sufficiently fast (55ns
or faster), EDO can be used even with a 100MHz memory bus. One of the best
reasons to use EDO is that all of the current motherboard chipsets support it
with no compatibility problems, unlike much of the synchronous memory now being
used.
Even with all the stated
advantages, EDO is no longer considered mainstream. Most manufacturers no longer
produce it, or have limited production. It is only a matter of time before the
prices begin to rise, and the equivalent size SDRAM module will be less
expensive.
If you already own EDO memory,
there is no real reason to jump to SDRAM unless you require bus speeds above
83MHz. With a typical EDO timing of 5-2-2-2 at 66MHz, there is almost no
noticeable improvement with SDRAM over EDO, and at 83MHz it is still negligible.
If you require 100MHz bus operation, EDO will lag far behind current SDRAM in
performance even if it does operate at that speed due to the need for 6-3-3-3
timings. On the other hand, with EDO being phased out, you will likely find
SDRAM to be equal to or even lower in price.
Burst EDO (BEDO)
Burst EDO, while a good idea,
was dead before it ever was born. The addition of a burst mode, along with a
dual bank architecture would have provided the 4-1-1-1 access times at 66MHz
that many expected with SDRAM. Burst mode is an advancement over page mode, in
that after the first address input, the next 3 addresses are generated
internally, thereby eliminating the time necessary to input a new column
address. Unfortunately, Intel decided that EDO was no longer viable, and SDRAM
was their preferred memory architecture so they did not implement support of
BEDO into their chipsets. In fact, several large memory manufacturers had put
considerable time and money into the development of SDRAM over the past decade,
and were not very happy with the BEDO design.
Except for support of bus
speeds of 100MHz and faster, BEDO would probably have been a much faster and
more stable memory than SDRAM. Essentially, BEDO lost support as much for
political and economic reasons as for technical ones, it seems.
Synchronous Operation
Once it became apparent that
bus speeds would need to run faster than 66MHz, DRAM designers needed to find a
way to overcome the significant latency issues that still existed. By
implementing a synchronous interface, they were able to do this and gain some
additional advantages as well.
With an asynchronous interface,
the processor must wait idly for the DRAM to complete its internal operations,
which typically takes about 60ns. With synchronous control, the DRAM latches
information from the processor under control of the system clock. These latches
store the addresses, data and control signals, which allows the processor to
handle other tasks. After a specific number of clock cycles the data becomes
available and the processor can read it from the output lines.
Another advantage of a
synchronous interface is that the system clock is the only timing edge that
needs to be provided to the DRAM. This eliminates the need for multiple timing
strobes to be propagated. The inputs are simplified as well, since the control
signals, addresses and data can all be latched in without the processor
monitoring setup and hold timings. Similar benefits are realized for output
operations as well.
JEDEC SDRAM
All DRAMs that have a
synchronous interface are known generically as SDRAM. This includes CDRAM (Cache
DRAM), RDRAM (Rambus DRAM), ESDRAM (Enhanced SDRAM) and others, however the type
that most often is called SDRAM is the JEDEC standard synchronous DRAM.
JEDEC SDRAM not only has a
synchronous interface controlled by the system clock, it also includes a
dual-bank architecture and burst mode (1-bit, 2-bit, 4-bit, 8-bit and full
page). A �mode register� that can be set at power-on and changed during
operation controls the burst mode, burst type (sequential or interleave), burst
length and CAS latency (1, 2 or 3).
CAS Latency is one of several
performance related timings for SDRAM. This measurement is the time it takes to
strobe in the Row Address, and to activate the bank. When a burst read cycle is
initiated, the addresses are set up and RAS\ and CS\ (chip select) are held low
on the next clock cycle (rising edge of CLK), thereby activating the sense
amplifiers on the bank. A period of time equal to tRCD (RAS\ to CAS\ delay) must
pass after which CAS\ and CS\ are held low (again, at the next clock cycle).
After the time period for tCAC (column access time) has passed the first bit of
data is on the output line and can be retrieved (at the next clock cycle). The
basic rule is that CAS latency times the clock speed (tCLK) must be equal or
greater than tCAC (or CL x tCLK >= tCAC). This means that the column access
time is the limiting factor for CAS Latency.
SDRAM was initially introduced
as the answer to all performance problems, however it quickly became apparent
that there was little performance benefit and a lot of compatibility problems.
The first SDRAM modules contained only two clock lines, but it was soon
determined that this was insufficient. This created two different module designs
(2-clock and 4-clock), and you needed to know which your motherboard required.
Though the timings were theoretically supposed to be 5-1-1-1 @ 66MHz, many of
the original SDRAM would only run at 6-2-2-2 when run in pairs, mostly because
the chipsets (i430VX, SiS5571) had trouble with the speed and coordinating the
accesses between modules. The i430TX chipset and later non-Intel chipsets
improved upon this, and the SPD chip (serial presence detect) was added to the
standard so chipsets could read the timings from the module. Unfortunately, for
quite some time the SPD EEPROM was either not included on many modules, or not
read by the motherboards.
SDRAM chips are officially
rated in MHz, rather than nanoseconds (ns) so that there is a common denominator
between the bus speed and the chip speed. This speed is determined by dividing 1
second (1 billion ns) by the output speed of the chip. For example a 67MHz SDRAM
chip is rated as 15ns. Note that this nanosecond rating is not measuring
the same timing as an asynchronous DRAM chip. Remember, internally all DRAM
operates in a very similar manner, and most performance gains are achieved by
�hiding� the internal operations in various ways.
The original SDRAM modules
either used 83MHz chips (12ns) or 100MHz chips (10ns), however these were only
rated for 66MHz bus operation. Due to some of the delays introduced when having
to deal with the various synchronization of signals, the 100MHz chips will
produce a module that operates reliably at about 83MHz, in many cases. These
SDRAM modules are now called PC66, to differentiate them from those conforming
to Intel�s PC100 specification.
PC100 SDRAM
When Intel decided to
officially implement a 100MHz system bus speed, they understood that most of the
SDRAM modules available at that time would not operate properly above 83MHz. In
order to bring some semblance of order to the marketplace, Intel introduced the
PC100 specification as a guideline to manufacturers for building modules that
would function properly on their upcoming i440BX. With the PC100 specification,
Intel laid out a number of guidelines for trace lengths, trace widths and
spacing, number of PCB layers, EEPROM programming specs, etc.
There is still quite a bit of
confusion regarding what a �true� PC100 module actually consists of.
Unfortunately, there are quite a few modules being sold today as PC100, yet do
not operate reliably at 100MHz. While the chip speed rating is used most often
to determine the overall performance of the chip, a number of other timings are
very important. tRCD (RAS to CAS Delay), tRP (RAS precharge time) and CAS
Latency all play a role in determining the fastest bus speed the module will
operate on to still achieve a 4-1-1-1 timing.
PC100 SDRAM on a 100MHz (or
faster) system bus will provide a performance boost for Socket 7 systems of
between 10% and 15%, since the L2 cache is running at system bus speed. Pentium
II systems will not see as big a boost, because the L2 cache is running at �
processor speed anyway, with the exception of the cacheless Celeron chips of
course.
DDR SDRAM
One limitation of JEDEC SDRAM
is that the theoretical limitation of the design is 125MHz, though technology
advances may allow up to 133MHz operation. It is obvious that bus speeds will
need to increase well beyond that in order for memory bandwidth to keep up with
future processors. There are several competing new standards on the horizon that
are very promising, however most of them require special pinouts, smaller bus
widths, or other design considerations. In the short term, Double Data Rate
SDRAM looks very appealing. Essentially, this design allows the activation of
output operations on the chip to occur on both the rising and falling edge of
the clock. Currently, only the rising edge signals an event to occur, so the DDR
SDRAM design can effectively double the speed of operation up to at least
200MHz.
There is already one Socket 7
chipset that has support for DDR SDRAM, and more will certainly follow if
manufacturers decide to make this memory available. In this industry, many times
it is the first to market that gains the support, rather than the best
technology.
Enhanced SDRAM (ESDRAM)
In order to overcome some of
the inherent latency problems with standard DRAM memory modules, several
manufacturers have included a small amount of SRAM directly into the chip,
effectively creating an on-chip cache. One such design that is gaining some
attention is ESDRAM from Ramtron International Corporation.
ESDRAM is essentially SDRAM,
plus a small amount of SRAM cache which allows for lower latency times and burst
operations up to 200MHz. Just as with external cache memory, the goal of a cache
DRAM is to hold the most frequently used data in the SRAM cache to minimize
accesses to the slower DRAM. One advantage to the on-chip SRAM is that a wider
bus can be used between the SRAM and DRAM, effectively increasing the bandwidth
and increasing the speed of the DRAM even when there is a cache miss.
As with DDR SDRAM, there is
currently at least one Socket 7 chipset with support for ESDRAM. The deciding
factor in determining which of these solutions will succeed will likely be the
initial cost of the modules. Current estimates show the cost of ESDRAM at about
4 times that of existing DRAM solutions, which will likely not go over well with
most users.
Protocol Based DRAM
All of the previously discussed
DRAM have separate address, data and control lines which limits the speed at
which the device can operate with current technology. In order to overcome this
limitation, several designs implement all of these signals on the same bus. The
two protocol based designs currently getting the most attention are SyncLink
DRAM (now called SLDRAM due to trademark issues) and Direct Rambus DRAM (DRDRAM)
licensed by Rambus, Inc.
DRDRAM
Intel has placed their money on
the proprietary memory design developed by Rambus, Inc. On the surface, this
looks to be a very fast solution for system memory due to its fast operation (up
to 800MHz). The reality is, however, that the design is only up to twice as fast
as current SDRAM operation due to the smaller bus width (16 bits vs. 64 bits).
Despite the claims from Intel
and Rambus, Inc., there are some potentially serious issues which need to be
addressed with this technology. The higher speeds require short wire lengths and
additional shielding to prevent problems with EMI. In addition, latency times
are actually worse than currently available fast SDRAM. Since most of today�s
applications do not actually utilize the full bandwidth of the memory bus even
today, simply increasing the bandwidth while ignoring latency issues will likely
not provide any real performance improvements. In addition, processors operating
with 800MHz bus speeds will certainly require more than double the current
memory bandwidth.
While these issues are serious
enough, the biggest drawback to the technology is that it is proprietary
technology. Manufacturers wishing to implement a solution with DRDRAM will be
required to pay a royalty to Intel and Rambus, Inc., and will also have no real
control over the technology. This is not an attractive outlook for most memory
manufacturers who have no desire to essentially become chip foundries.
SLDRAM
Many memory manufacturers are
putting their support behind SLDRAM as the long-term solution for system
performance. While SLDRAM is a protocol-based design, just as RDRAM is, it is an
open-industry-standard, which requires no royalty payments. This alone should
allow for lower cost. Another cost advantage for the SLDRAM design is that it
does not require a redesign of the RAM chips.
Due to the use of packets for
address, data and control signals, SLDRAM can operate on a faster bus than
standard SDRAM � up to at least 200MHz. Just as DDR SDRAM operates the output
signal at twice the clock rate, so can SLDRAM. This puts the output operation as
high as 400MHz, with some engineers claiming it can reach 800MHz in the near
future.
Compared to DRDRAM, it seems
that SLDRAM is a much better solution due to the lower actual clock speed
(reducing signal problems), lower latency timings and lower cost due to the
royalty-free design and operation on current bus designs. It appears that even
the bandwidth of SLDRAM is much higher than DRDRAM at 3.2GB/s vs. 1.6GB/s
Though Intel initially intended
to support only DRDRAM in future chipsets, competing chipset manufacturers,
memory manufacturers and pressure from end users may force them to include
support for SLDRAM as well. If the marketplace can successfully influence Intel
to provide this support, we may actually see a situation where the best
technology wins over marketing hype.
