(^138) PC Hardware: A Beginner’s Guide
DRAM
The most commonly referenced form of RAM isdynamic RAMorDRAM. Compared to
the other RAM technologies, DRAM is inexpensive and stores the largest number of bits
in the smallest amount of physical space.
A DRAM cell, which stores one bit, is made u pof a single ca pacitor. Acapacitorstores
either a positive or negative voltage value that is used to represent 1 or 0 binary values.
DRAM must be refreshed every two milliseconds. This is done when the contents of
every single DRAM cell (capacitor) is read and then rewritten by a refresh logic circuit.
This constant refreshing contributes to the fact that DRAM is the slowest type of RAM. It
averages transfer speeds of 50ns or higher.DIP Packaging
On PCs with a 386 or earlier processor, DRAM chips were mounted on the motherboard
as individual memory chips in sockets arranged in a group, called amemory bank.On
newersystems(386DXandlater),DRAMchipsareinstalledasapartofintegratedmemory
modules that mount in a special slot on the motherboard (see the following section).
Single DRAM chips are packaged in a DIP (dual inline package), a sample of which is
shown in Figure 7-4. DRAM chips in a DIP packaging were mounted into individual
sockets directly on the motherboard in banks of four or more chips. DIP memories are
rare, except on older systems.Single Inline Memory Modules
With the 386DX, DRAM began to be packaged in modules that mounted to the mother-
board in a single long slot. This single-edge connector package incorporates several DIP
memories into an integrated memory module.Figure 7-4. A DIP chip