Datasheet

Identifying Motherboards
25
Physical Memory
Physically, memory or RAM is a collection of integrated circuits that store data and pro-
gram information as patterns of 1s and 0s (on and off states) in the chip. Most memory
chips require constant power (also called a constant refresh) to maintain those patterns of
1s and 0s. If power is lost, all those tiny switches revert back to the off position, effectively
erasing the data from memory. Some memory types, however, dont require a refresh.
There are many types of RAM. Lets examine each type in detail.
SRAM
Static RAM (SRAM) stores whatever is placed in it until it’s changed and it is used as cache
memory (discussed later). Unlike dynamic RAM (DRAM), it doesn’t require constant elec-
trical refreshing. Another name for it is nonvolatile RAM (NVRAM). It’s expensive, so it
isn’t typically used for the main memory in a system.
DRAM
Dynamic RAM (DRAM) is an improvement over SRAM. DRAM uses a different approach
to storing the 1s and 0s. Instead of using transistors, DRAM stores information as charges
in very small capacitors. If a charge exists in a capacitor, it’s interpreted as a 1. The absence
of a charge is interpreted as a 0.
Because DRAM uses capacitors instead of switches, it needs to use a constant refresh
signal to keep the information in memory. DRAM requires more power than SRAM for
refresh signals and, therefore, is mostly found in desktop computers.
DRAM technology allows several memory units, called cells, to be packed to a high
density. Therefore, these chips can hold very large amounts of information. Most PCs today
use DRAM of one type or another.
Lets take a brief look at some of the different types of DRAM:
Fast Page Mode (FPM) An older type of RAM (almost always 72-pin SIMM packaging)
that isn’t synchronized in speed with the motherboard. It’s rated in nanoseconds of delay,
with lower numbers being better (for example, 60ns). FPM is now obsolete.
Extended Data Out (EDO) Like FPM, an older type of RAM, usually in 72-pin SIMM
form. It performs a bit better than normal FPM RAM because it needs to be refreshed less
frequently. Like FPM, it’s now obsolete.
Synchronous DRAM (SDRAM) Synchronized to the speed of the motherboard’s system
bus. Synchronizing the speed of the systems prevents the address bus from having to wait for
the memory because of different clock speeds. A 100MHz clock signal produces 800Mbps,
and such memory modules are referred to as PC100. PC133, which replaced PC100, used a
133MHz clock to produce 1067Mbps of throughput.
The relationship between clock speed and throughput is always roughly 1:8 and thus
PC2700 modules are designed specically for a motherboard with a speed of 333MHz,
and PC3200 modules are designed for a motherboard with a speed of 400MHz.
SDRAM typically comes in the form of 168-pin DIMMs or 184-pin RIMMs.
86504c01.indd 25 7/25/09 6:04:12 PM