What is a 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. 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 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 32 MB RAM in a PC. Many have much more. I
have 128 MB, and that is in no way too much. Especially, if you use Windows 95, it is important with plenty of RAM. Click here to read about the swap file and RAM considerations.

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 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's). 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.
RAMBUS (RDRAM) is a future RAM type. Intel and others have great expectations from this

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.

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.


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.

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: