About RAM
When we talk about motherboard and chip sets, we can not 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. But, here you can read a little about:
· What is RAM?
· About RAM types
· About SIMMs
· DIMMs
· PC100 RAM
· Rambus or DDR
What is RAM?
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,
· 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.
Historical review
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. Windows 98 is a little better at handling memory, but still a lot af RAM is a
good thing.
RAM types
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 (Synchron Data 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 is an improvement of FPM RAM. Data are read faster. By switching from FPM to EDO,
one can expect a performance improvement of 2 to 5 percent. EDO RAM are usually sold in 60
ns versions. A 50 ns version is available at higher cost.
ECC RAM is a special error correcting RAM type. It is especially used in servers.
SDRAM is the newest RAM type for PCs. It comes only in 64 bit modules (long 168 pin DIMMs).
SDRAM has a access time of only 8-12 ns. The performance improvement over EDO RAM is
only 5 percent running at 66 MHz, but at 100 MHz it will prove a lot better.
RAMBUS (RDRAM) is a future RAM type. Intel and others have great expectations from this
type.
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 can not 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 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 speeds
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
Peak Bandwidth
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 a continuously
transfer.
RAM type Max. peak bandwidth
FPM 176 MB/sec
EDO 264 MB/sec
SD 528 MB/sec
SIMMs
SIMM 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:
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:
Bank 1 Bank 2 Total RAM
16 MB + 16 MB - 32 MB
16 MB + 16 MB 32 MB + 32 MB 96 MB
32 MB + 32 MB 32 Mb + 32 MB 128 MB
Certain motherboards (like TYAN) have 6 or 8 SIMM sockets. That provides more RAM expansion
flexibility.
DIMMs
The next RAM type, SDRAM are made in 64 bit wide modules called DIMMs. They have a 168 pin
edge connector. They fit only in the newer motherboards.
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 and 256 MB, with 6, 8, 10, and 12 ns speed. There are usually three 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 at least 100 MHz. Also the SDRAM work synchronous with the system bus for a better
performance.
All new chip sets can control SDRAM. Some motherboards have both SIMM and DIMM sockets.
The idea is that you can choose between re use EDO RAM in the SIMM sockets, or install SDRAM
in the DIMM sockets. They are 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 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).
PC100 RAM
With new chip sets the system bus speed has come up to 100 MHz. This puts new focus on the quality
of the RAM modules. 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 or 133 MHz SD-RAM.
The newest 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 these 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 centents of the SPD at this c’t homepage. It works with the Intel
chip sets holding a 82371 south bridge like BX and GX.
Rambus RDRAM or DDR RAM
Intel expects soon to use the so-called Rambus RAM (RDRAM or nDRAM). It is a advanced technology
from an American company, who sells the technology to other chip manufactories for just 2%
in license. Intel supports the RDRAM, which should be rather cheap to produce.
RDRAM is developed from the traditional DRAM, but the architecture has been streamed and
optimized to yeld new performance. Data is read in packets at a very high clock speed. 600 MHz
works fine, and GigaHertz will follow. Perhaps we can drop the L2-cache if it works. The RDRAM
chips have to be placed very close to the CPU to reduce radio noise.
Another interesting RAM type is the DDR RAM (not that DDR). DDR stands for Double Data Rate:
a technology that transmits data on both sides of a tact signal. DDR-SDRAM is getting more and
more hot. All major vendors except Intel support DDR-SDRAM. DDR-SDRAM will to be cheaper
than Rambus RAM yet giving almost the same performance.
