KarbosGuide.com. Module 2e1
What is RAM?
This page should be read together with modules 2a, 2b, 2c, and 2d, which deal with system board, system bus, I/O bus and chip sets. When we talk about motherboard and chip sets, we cannot ignore RAM. Warning: RAM and RAM chips is a very complicated, technical subject area. I can in no way give a complete, comprehensive description of this subject.
RAM is our working memory storage. All the data, which the PC uses and works with during operation, are stored here. Data are stored on drives, typically the hard drive. However, for the CPU to work with those data, they must be read into the working memory storage, which is made up of RAM chips. To examine RAM, we need to look at the following:
RAM types (FPM, EDO, ECC, and SD RAM)
RAM modules (SIMM and DIMM) in different versions
RAM and the system bus
First, let us look back in time. Not too many years ago, Bill Gates said, that with 1 MB RAM, we had a memory capacity, which would never be fully utilized. That turned out to be untrue.
Back in the 80s, PCs were equipped with RAM in quantities of 64 KB, 256 KB, 512 KB and finally 1 MB. Think of a home computer like Commodore 64. It had 64 KB RAM, and it worked fine.
Around 1990, advanced operating systems, like Windows , appeared on the market, That started the RAM race. The PC needed more and more RAM. That worked fine with the 386 processor, which could address larger amount of RAM. The first Windows operated PCs could address 2 MB RAM, but 4 MB soon became the standard. The race has continued through the 90s, as RAM prices have dropped dramatically.
Today. it would be foolish to consider less than 32 MB RAM in a PC. Many have much more. 128 MB is in no way too much for a "power user" with Windows 95/98, it is important with plenty of RAM. Click here to read about the swap file and RAM considerations. Windows 98 is a little better at handling memory, but still a lot af RAM is a good thing.
The traditional RAM type is DRAM (dynamic RAM). The other type is SRAM (static RAM). SRAM continues to remember its content, while DRAM must be refreshed every few milli seconds. DRAM consists of micro capacitors, while SRAM consists of off/on switches. Therefore, SRAM can respond much faster than DRAM. SRAM can be made with a rise time as short as 4 ns. It is used in different versions in L2 cache RAM (for example pipe line BURST Cache SRAM).
DRAM is by far the cheapest to build. Newer and faster DRAM types are developed continuously. Currently, there are at least four types:
FPM (Fast Page Mode)
ECC (Error Correcting Code)
EDO (Extended Data Output)
SDRAM (Synchronous Dynamic RAM)
A brief explanation of DRAM types
FPM was the traditional RAM for PCs, before the EDO was introduced. It is mounted in SIMM modules of 2, 4, 8, 16, or 32 MB. Typically, it is found in 60 ns or 70 ns versions. 60 ns is the fastest and the one to use. You cannot mix different speeds on the same Pentium motherboard.
EDO (Extended Data Out) RAM is an improvement of FPM RAM. Data are read faster. EDO extends the time that output data is valid, which betters timing issues between the CPU and RAM and this way improves the performance.
By switching from FPM to EDO, one could expect a performance improvement of 2 to 5 percent. EDO RAM was usually sold in 60 ns versions. A 50 ns version was available at higher cost.
EDO has now been replaced by the even faster SDRAM.
ECC RAM is a special error correcting RAM type. It is especially used in servers.
SDRAM (synchronous DRAM)): The replacement for DRAM, FPM, and EDO RAM types. SDRAM "locks" (synchronizes) the memory access to the CPU clock. This way we get faster data transfer. While one portion of data is tranported to the CPU another can be being prepared for transfer.
SDRAM comes only in 64 bit modules (long 168 pin DIMMs). SDRAM has a access time of only 6-12 ns. The performance improvement over EDO RAM was a mere 5 percent running at 66 MHz. At 100 and 133 MHz it proves better.
DDR RAM is clock doubled version of SDRAM, which is replacing SDRAM during 2001-2002.
RAMBUS (RDRAM) is a more futuristic RAM type. Intel and others had great expectations from this type, but it flopped in 2000-2001.
8 or 9 bits per byte?
Normally you figure 8 bits to one byte. For many years, a ninth bit has been added as parity bit in the RAM blocks to verify correct transmission. That way you have to transmit 9 bits, to store 8 bits in the old 30 pin RAM chips. And it takes 36 bits to store 32 bits in the larger 72 pin chips, which increases the cost of the RAM chip by about 12%.
If your motherboard requires 36 bit modules, you must respect that. Fortunately, most system boards accepts 32 bit modules, so this creates no problems.
RAM and motherboard
You cannot freely install your desired RAM type. RAM is controlled by the chip set on the motherboard, so you must install a type, which matches your motherboard. Furthermore, RAM chips come in different sizes, which must match the system board.
On modern system boards, RAM is installed on SIMM or DIMM modules. Before, small individual DRAMs were used. There was usually room for 36 small chips on the system board. That made it cumbersome to install new RAM. Then, someone figured out to install RAM chips on cards, which are easily installed. First came the SIPP modules. They had multiple pins, which fit in the motherboard. Since then came the SIMM modules. They are mounted on a card, which has an edge connector. They fit in sockets on the motherboard, and anyone can install them.
RAM speed is measured in ns (nano seconds). The fewer ns, the faster is the RAM. Years ago, RAM came in 120, 100 and 80 ns. Today, we are talking about 60 ns and faster.
It becomes complicated to describe the relationship between RAM speed and the ability of the system bus to utilize fast RAM. I will gloss over that. But here is a table which illustrates RAM speed, relative to clock speed:
Clock speed Time per clock tick
20 MHz 50 ns
25 MHz 40 ns
33 MHz 30 ns
50 MHz 20 ns
66 MHz 15 ns
100 MHz 10 ns
133 MHz 6 ns
Here you see the maximal peak bandwidth of the three well known RAM types. The figures illustrates the absolutely maximal transfer from RAM to the L2-cache - in peaks, not as continuously transferred.
RAM type Max. peak bandwidth
FPM 176 MB/sec
EDO 264 MB/sec
SD 528 MB/sec
About SIMM RAM
SIMM (Single Inline Memory Modules) were first made in 8 bit editions. They were small cards with 1, 2 or 4 MB RAM. They were connected to the motherboard with a 30 pin edge connector. The modules were 8 bit wide. This meant that 16 bit processors (286 and 386SX) needed 2 SIMMs in a pair. Thus, there was room for two modules in what is called a bank.
32 bit processors (386DX and 486) need 4 of the small 8 bit SIMMs in a bank, since their banks are 32 bit wide. So, on a typical 1st generation 486 motherboard, you could install 4 X 1 MB, 4 X 2 MB, or 4 X 4 MB in each bank. If you only had one bank (with room for 4 modules), it was expensive to increase the RAM, because you had to discard the old modules.
32 bit modules
With the advent of the 486 processor, demand increased for more RAM. Then the larger 32 bit modules came into use. A 486 motherboard could still have 4 SIMM sockets, but when the modules were 32 bit wide, they could be installed one at a time. This was quite ingenious.
You could add different types of modules and still use the old ones. Also, since the 486 motherboard ran at only 33 MHz on the system bus, the RAM module quality was not so critical. You could mix 60 ns and 70 ns modules of different brands without problems.
Here you see a couple of SIMM modules. On top is a 64 bit module (168 pins - don't try to count them). Next is a 32 bit module with a 72 pin connector. Below is an 8 bit module with a 30 pin connector:
64 bit SDRAM:
32 bit DRAM: and bit DRAM:
Number of chips per module
Some SIMMs have more chips on the module than others. Looking at just the 32 bit modules, we find modules with 2, 4, 8 or chips on each side. SIMMs with 2 MB, 8 MB and 32 MB are double sided. There are chips on both sides of the module, and all these chips are 16 Mbit ones.
The newest DIMM-modules holds 64 Mbit RAM chips. This way a 32 MB module is made of only 4 chips since 4 X 64 / 8 = 32.
Pentium motherboard with SIMMs
On the Pentium motherboard, the system bus is 64 bit wide. Therefore, the 32 bit SIMMs are installed in pairs. Since the standard motherboard only has two banks with a total of four SIMM sockets, RAM expansion possibilities are limited. NOTE: never use different speed RAM modules on the Pentium motherboard. All modules must have the same speed. Here you see a few configurations on an old Pentium motherboard with four SIMM sockets:
16 MB + 16 MB
16 MB + 16 MB
32 MB + 32 MB
32 MB + 32 MB
32 Mb + 32 MB
Certain motherboards (like TYAN) have 6 or 8 SIMM sockets. That provides more RAM expansion flexibility.
About new fast RAM
The most used modern RAM type, SDRAM is made in 64 bit wide modules called DIMMs (Dual Inline Memory Module).
They have a 168 pin edge connector. Here you see one module:
Since the DIMM modules are 64 bits wide, you can install one module at a time. They are available in 8, 16, 32, 64, 128, 256 MB, and 512MB with 6, 8, 10, and 12 ns speed. There are usually 2 -4 DIMM sockets on a motherboard.
The advantage of SDRAM is increased speed. That allows you to increase system bus speed. With 60 ns EDO-RAM, you can run at a maximum of 75 MHz on the system bus, while SDRAM speed can increase to 133 MHz and above. Also the SDRAM work synchronous with the system bus for a better performance.
Most chip sets are made for SDRAM. Some motherboards had both SIMM and DIMM sockets. The idea was that you could reuse old EDO RAM in the SIMM sockets, or choose to install SDRAM in the DIMM sockets. They were not designed to mix RAM types although it works at some boards.
Above: a 64 MB DIMM-module holding 32 chips each of 16 Mbit (32 X 16 Mbit / 8 bit = 64 MB).
It is better to use DIMMs made of the new 64 Mbit chips. A 64 MB module is this way made of only 8 chips (8 X 64 Mbit / 8 bit = 64 MB).
Intel have managed to speed up the processors power by factor 200 times the last ten years. That is a lot, but it is a problem that RAM memory technology only has improved by factor 20 in the same period.
Today we hope and dream of new fast RAM types, that will help us to get the full potential from our powerful PCs.
The first attempt to improving RAM speed was the PC100 standard. With chip sets like BX the system bus speed has come up to 100 MHz. Hence Intel has made a new standard called PC100. Only 8 ns SD-RAM modules that are constructed according to these standards are guaranteed to work at 100 MHz. In some articles this RAM is described at 125 SD-RAM.
The new DIMM-modules include a EPROM-chip holding information about the module. This little 8-pin chip works as a SPD (Serial Presence Detect) - a unit storing information about the RAM type. The idea is that BIOS can read this information and this way tune the system bus and the timings for a perfect CPU-RAM performance.
You can find a program, that tests the contents of the SPD at this c't homepage. It works with the Intel chip sets holding a 82371 south bridge like BX and GX.
Another program is called DIMM_ID.
The PC133 RAM running at 133 MHz is the latest version of SDRAM. Specifications are made by VIA, Micron, NEC, Samsung, SIS, Acer Labs and other vendors. The first production (from Corsair, June 1999) used 7.5 ns RAM modules from Micron.
VIA supports the PC133 RAM with their Apollo Pro Plus chip set (693A). Later they launched support for PC266 DDR RAM!
Also AMD's K7 Athlon may use PC133 RAM with the VIA KX133Pro chipset.
Virtual Channel 133 is another flavour of the PC133 standard. The modules holds a small cache of superfast SRAM. According to tests, these modules perform very well, but due to unknown reasons, it never became popular.
Intel and PC133
Originally Intel planned to by-pass PC133 RAM in their roadmaps. They intended to migrate from PC100-based chip sets (like BX) to Rambus-based chip sets (like i820).
For a period of 12 months in 1999-2000, Intel experienced several disastrous incidents from their attempt to implement Rambus in chip sets and motherboards. During this period they were forced (by taiwanese motherboard manufactures) to adapt the PC133 standard.
The chip set i815 was the result of this revision of strategies.
Intel's problem is that they have "sold their soul" to Rambus Inc. According to their agreement, until 2003 Intel can only implement other RAM types than RDRAM if the bandwidth is less than 1 GB/sec. This agreement does not include server chipsets, from what we understand.
About Rambus RAM
While the CPUs has become around 200 times faster in ten years, the RAM performance has only gone up 20 times. So we need new RAM types. But which?
Many vendors decided to go for DDR RAM as described in . Where DDR RAM is a development of the existing SDRAM technology, Intel choose RDRAM, which represents a much more revolutionary change in RAM design.
Intel and RDRAM without succes
Intel is comitted to the Rambus RAM, which also is called RDRAM (Rambus Direct RAM), nDRAM, or RIMM (Rambus Inline Memory Modules).
RDRAM is an advanced technology patented by a company, who sells the technology to other chip manufactures for a 2% in license. In 1997 Intel signed a contract that apparently commits them to support RDRAM only in all new chipset up to 2002.
Originally AMD also expected to support the Rambus RAM for its Athlon processor. But having seen all Intel's problems with the technology, AMD is not so keen on the Rambus anymore. However, RDRAM is already used in Sony PlayStation 2 and in Nintendo64 machines. In the Sony PlayStation 2 you find 32 MB of RDRAM delivering a bandwidth of 3.2 GB/sec.
During 1999 and 2000, Rambus was not very successful. In fact, Intel has suffered a serious set-back due to their commitment to the Rambus design. The chip set i820 "Camino" became a little disaster.
Intel failed to produce a reliable way to interface SDRAM to the 820 chipset. The MTH (Memory Translator Hub - which translated RDRAM bus to SDRAM modules) had some timing or noise issues that caused unreliable operation. Intel replaced CC820 boards with VC820 boards (with 128MB RDRAM included) as the CC820 use the MTH and SDRAM while the VC820 used RDRAM.
But, on the paper, Rambus sounds great:
Intelligent Rambus design
RDRAM is developed from the traditional DRAM, but the architecture is completely new. It has been streamed and optimized to yield new performance.
The RAMBUS-design gives a more intelligent access to the RAM, meaning that units can "prefetch" data and this way free the CPU some work. The idea is that data is read in small packets at a very high clock speed.
The RIMM modules are only 16 bit wide compared to the traditional 64 bit SDRAM DIMMs, but they work at a much higher clock frequency:
The Rambus modules work on 2.5 volts, which internally is reduced down to 0.5 volt when possible. This helps to reduce heat and radio signals.
The RIMMs hold controlling chips that turns off the power to sections not in use. They can also reduce the memory speed if thermal sensors report of overheating.
All RAM slots have to be full; this is new, with RAMBUS we have to fill in blank modules in slots which are not in use. The blank modules are called CRIMMs (with a 'C' for continuity).
The RIMM modules hold 184 pins.
The RDRAM chips have to be placed very close to the CPU to reduce radio noise. This indicates, that RIMM technology is rather sensitive; Intel seems to have made that discovery as well.
High clock rates
As mentioned, the modules are only 16 bit wide, but they work at 600, 700 and 800 MHz. Actually a PC800 RIMM runs on a 400 MHz clock using both rising and falling edges, being clockdoubled just as DDR RAM.
More confusing the PC600 RIMM actually runs on a 266/532 MHz clock, and the PC700 works at 366/712 MHz.
This gives the bandwidth of up to 1.6 GB per second - compared to the 500-800 MB/sec of PC100 SDRAM - of a single Rambus channel. You may find a chart comparing the bandwiths of different RAM types in the next page.
Multichannel memory design
You may bundle four channels to a 64 bit wide RAM bus giving 6.4 GB/sec:
This is not possible using the existing RAMBUS-based chip sets like i820. They only operate with one RAMBUS channel onboard. The high-end chip set i840 operates with dual RDRAM channels, as the up-coming i850 will.
RIMM in the future, says Intel
GigaHertz versions of Rambus RAM will probably follow, so the technology has potential for much higher bandwidths.
In 1999 it seemed that Intel was having big problems with the Rambus technology in the ill-faited i820 chip set (the so-called "Caminogate" tragedy). Hence they were forced to support PC133 RAM as seen in the i815 chipset.
Poor performance so far
Unfortunately it was soon obvious that the i815 chip set with its PC133 RAM was performing slightly better than the i820 chip set with its still very expensive RDRAM. You have to use dual Rambus channels (as in upcomming Intel chip set i850 "Tehema") to benefit from a higher bandwidth. But this doubling is also possible from using DDR RAM.
A test between a i840-based dual Rambus PC and a Micron DDR-based PC gave the same result; all benchmarks were better on the DDR system.
So far Rambus RAM is of no big interest. It is too expensive, and there is nothing to gain from it. However the Rambus technology stil is quite promising, but the prices has to come down, and it better be soon. DDR RAM is closing in.
Intel claims that DDR is to slow for the new Pentium 4 processor. It would require dual channel DDR RAM to get the required bandwidth. And dual channel DDR RAM meens a 128 bit wide bus, which is no good solution. The north bridge and the motherboard would be loaded with hundreds of signal lines.
In 2001 RDRAM is being used with great success on the GB850 Pentium 4 board and RDRAM prices are tumbling steadily.
Rambus plans to speed up the bandwith a factor two using a Quad Rambus Signaling Level. This should happen without any increase in clock frequency.
About DDR RAM
A very interesting RAM type is the DDR RAM, which is expected to hit the market in 2001.
DDR stands here for Double Data Rate. It is a technology that transmits data on both sides of a tact signal.
This way the performance has been doubled; a 133 MHz SDRAM chip can very easy become altered to a 266 MHz DDR chip:
It should be pretty easy for the market to change for DDR RAM. The modules look like and operate quite similar to existing SDRAMs. We just need new chipsets to start the migration.
However, the modules hold 16 pins more than SDRAM do, so they do not fit into the same sockets.
The Taiwanese company VIA, which produces chip sets and CPUs and who are fighting Intel, is fully supporting the DDR RAM strategy. Soon we shall see 266 MHz moduls (being "overclocked"133 MHz SDRAM modules).The 266 MHz modules reaches a 2.1 GB/sec bandwidth. Hence they are to be sold as PC2100 RAM.
Other terms used are:
DDR200 (200 MHz)
DDR266 (266 MHz)
DDR333 (333 MHz)
VIA expects DDR to be used in all segments of the pc market. Intel, who is behind the Rambus technology, only expects to use DDR in large servers, where you find several Gigabytes of RAM installed, and where RAM price really matters.
Intel is dedicated to the Rambus technology. In the summer 2000 it was revealed that Intel has comitted itself to the RAMBUS technology so they cannot implement DDR! This goes for all future desktop PCs until 2003, according to their agreement with Rambus Inc. Only the 64 bit server Itanium processor and it succesors Foster and McKinley are using DDR RAM.
We hope that Intel will change their strategy. We expect DDR-SDRAM to be cheaper than Rambus RAM for quite some time; yet it should give the same performance. Rambus represents a sophisticated technology, but with prices 5 times higher it is not a low-end product. Intel produces great chipsets for desktop PCs like i815E, and it would be sad if they abandoned this market. We want Intel and PC2100!
Reports in the summer 2000 told that Intel has licensed VIA to develop DDR-enabled chip sets for Pentium 4.
Evolutionary changes of design
Where RDRAM requires completely new production plants, DDR represents an evolutionary progress. The chip manufactures may re-use their SDRAM fabs for the production without many problems.
Hence it seems quite natural and in tune with the previous changes in RAM technology that we use the DDR standard for a couple of years. Before Rambus (or something even better) enters the market.
Below you see the theoretical bandwidts of different RAM types. However, SDRAM does not perform as good as the figures show. This is due to latencies; the CPU and other units cannot read the data at these speeds; they have to wait some clock circles in between each reading before the data transfers start. The same goes for DDR RAM.
Theoretical max. bandwidth
SDRAM 100 MHz
100 MHz X 64 bit= 800 MB/sec
SDRAM 133 MHz
133 MHz X 64 bit= 1064 MB/sec
DDRAM 200 MHz (PC1600)
2 X 100 MHz X 64 bit= 1600 MB/sec
DDRAM 266 MHz (PC2100)
2 X 133 MHz X 64 bit= 2128 MB/sec
DDRAM 366 MHz (PC2600)
2 X 166 MHz X 64 bit= 2656 MB/sec
RDRAM 600 MHz
600 MHz X 16 bit= 1200 MB/sec
RDRAM 700 MHz
700 MHz X 16 bit= 1400 MB/sec
RDRAM 800 MHz
800 MHz X 16 bit= 1600 MB/sec
A new version of DDR RAM is scheduled for 2003. Using another technique, it should be possible to double the performance!