RAM is an abbreviation for Random Access Memory. RAM is the computer's main memory. The computer
uses RAM constantly to temporarily store information while it is working with it. The photo
above shows what a 128MB SDRAM 100MHz DIMM memory module looks like. SDRAM stands for Synchronous
Dynamic Random Access Memory. SDRAM runs synchronously, (or at the same pace), with the
processor's front side bus at 66MHz, 100MHz, or 133MHz, depending on what type of SDRAM the
computer has. A bus is simply a connection between items on the motherboard. The speed of the
the memory, or its data transfer rate, is how fast the data can travel between the RAM
and the processor. The speed is measured in MHz, (or megahertz). One megahertz is one million
frequency cycles per second. Data travels at a pace of 100 million cycles per second with
100MHz memory. SDRAM that runs at 100MHz is called PC100 memory. SDRAM that runs at 133MHz is
called PC133 memory. Memory modules are available in 16MB, 32MB, 64MB, 128MB, 256MB, 512MB, 1GB,
and 2GB capacities. Each is able to hold different amounts of temporary data. The 16MB module can
hold 16MBs, (or 16,000,000 bytes), of data. The 256MB module can hold 256MBs, (or 256,000,000 bytes),
of data. That's 16 times more than the 16MB module! The memory module shown above is a DIMM module.
DIMM stands for Dual In-line Memory Module. The term DIMM has nothing to do with the speed or
capacity of a memory module. It simply refers to the way the module is designed. DIMM modules
consist of several DRAM chips. DIMM modules have separate contact points on both sides of the module.
The contact points on the older SIMM, (or Single Inline Memory Module), modules are connected together
on both sides of the module. Even though SIMM modules have two sides, with contact points on each,
the connections are actually the same on either side. The memory capacity of a single DRAM memory
chip on a memory module can be 1MB, 2MB, 4MB, 8MB, 16MB, 32MB, 64MB, or 128MB. Below is a picture
of a single DRAM memory chip.
Some memory is capable of checking for errors. This memory is called ECC, (or Error Correction Code),
memory. If a computer has a lot of memory, it can store a lot of temporary data and operate faster.
People with good memories also retain more information and do things faster, because they don't waste
a lot of time trying to remember things. Computers commonly have 512MB to 1GB of memory. Here's
a question for you. If you have 4 4MB DRAM memory chips on both sides of a SDRAM DIMM module, or 4 8MB
DRAM memory chips on one side of a SDRAM DIMM module, how much memory would you have? If you guessed
32MB you're right, because 4 x 4MB x 2 sides = 32MB or 4 x 8MB x 1 side = 32MB.
The DDR SDRAM memory module replaced the SDRAM memory module. DDR stands for Double Data Rate.
SDRAM runs at the same pace the system clock runs. DDR SDRAM runs at double the pace the system clock
runs. After DDR SDRAM came DDR2 SDRAM. DDR2 SDRAM runs at four times the pace the system clock runs.
DDR SDRAM is available in speeds from 266MHz up to 600MHz. DDR2 SDRAM is currently available in speeds
from 400MHz up to 800MHz. Below is a picture of a 2GB DDR2 SDRAM 400MHz DIMM memory module with ECC.
Starting with DDR SDRAM, memory companies often use names like: PC2100, PC2700, PC3200, and etc. to
refer to the speed of a memory module. Before DDR SDRAM, companies used names like PC100 to refer to
memory with a 100MHz bus speed or PC133 for memory with a 133MHz bus speed. PC2100 refers to memory
with a 266MHz bus speed. PC2700 refers to memory with a 333MHz bus speed. At first, this might not
make a lot of sense, but after you see how they arrive at these names, maybe it will begin to make
a little more sense. What they did was take the bus speed and multiplied it by the bus width, rounding
the result to the nearest hundred. The current bus width for SDRAM, DDR SDRAM, and DDR2 SDRAM DIMM modules
is 8 bytes wide. If you take 266MHz and multiply it by 8 you get 2128MHz, which rounded to the nearest
hundred is 2100MHz for PC2100 memory. 333MHz x 8 = 2664MHz, which rounds to 2700MHz for PC2700 memory.
DDR2 SDRAM modules are written with PC2 in front of them. Since 800MHz x 8 = 6400, an 800MHz DDR2 SDRAM
module with an 8 byte wide bus would be written PC2 6400 or PC2-6400, using a space or dash to keep from
confusing the type of memory with the speed. Since data is able to flow at the same time on the different
paths of the RAM bus, this is actually a truer representation of the highest possible data transfer rate
for the complete memory module.