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The invent of silicon
has given birth to new modern technology. The development of
Computers is also one of them. It has changed our life to a great
extent. The one of the most important part of a computer is its
primary memory – The RAM. With the advancement in the technology,
the more robust, fast and high capacity RAM are in the market. This
article gives you an introduction about the RAM. Let us divide this
introduction in to following headings.
• What is RAM?
• About RAM types
• About SIMM
• DIMM
• PC100 RAM
• Rambus
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, 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.
Historical review
Back in the 80's, PC's were equipped with RAM in quantities of 64
KB, 256 KB, 512 KB and finally 1 MB. 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 PC's could address 2 MB RAM, but 4 MB soon became
the standard. The race has continued through the 90's, as RAM prices
have dropped dramatically. Today it would be foolish to consider
less than 64 MB RAM in a PC. The OS like windows Xp requires better
than this.
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 milliseconds. DRAM consists of
micro capacitors, while SRAM consists of off/on switches
(flip-flops). Therefore, SRAM can respond much faster than DRAM.
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 PC's, 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
system board.
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 PC's. It comes only in 64 bit
modules (long 168 pin DIMM). SDRAM has a rise 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.
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 system board requires 36 bit modules, you must respect that.
Fortunately, most system boards accepts 32 bit modules, so this
creates no problems.
RAM and system board
You can not freely install your desired RAM type. RAM is controlled
by the chip set on the system board, so you must install a type,
which matches your system board. 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 DRAM's 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 system board. Since came
the SIMM modules. They are mounted on a card, which has an edge
connector. They fit in sockets on the system board, 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.
SIMM's
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 system board
with a 30 pin edge connector. The modules were 8 bit wide. This
meant that 16 bit processors (286 and 386SX) needed 2 SIMM's 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 SIMM's
in a bank, since their banks are 32 bit wide. So, on a typical 1st
generation 486 system board, 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 system board
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 system board 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.
Number of chips per module
Some SIMM's 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. SIMM's with 2 MB, 8 MB and 32 MB are double sided. There
are chips on both sides of the module. All these chips 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 system board with SIMM's
On the Pentium system board, the system bus is 64 bit wide.
Therefore, the SIMM's are installed in pairs. Since the standard
system board 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 system board. All modules must have
the same speed.
DIMM's
The latest RAM type, SDRAM are made in 64 bit wide modules called
DIMM's. They have a 168 pin edge connector. They fit only in the
newer system boards. The 82430 VX and TX chip sets can control SDRAM,
as well as the LX and BX chip sets do. 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 8, 10, and 12 ns speed. There
are usually three DIMM sockets on a system board.
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. Some system boards have both
SIMM and DIMM sockets. The idea is that you can choose between reuse
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.
PC100 RAM
The newest DIMM-modules include a EPROM-chip holding information
about the module. This 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. With BX chip set 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 SD-RAM modules that are constructed according to
these standards are guaranteed to work at 100 MHZ. In some articles
this new RAM is described at 125 MHZ SD-RAM.
Rambus RDRAM
It is a advanced technology from an American company, who sells the
technology to other chip manufactories for just 2% in license... And
since Intel supports the RDRAM, they are going to be rich. Data is
read in packets at a very high clock speed. 600 MHZ works fine, and
GigaHertz will follow. 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.
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