Hardware Reference
In-Depth Information
Chapter 6. Memory
Memory Basics
Thischapterdiscussesmemoryfrombothaphysicalandalogicalpointofview.First,we'll
examine what memory is, where it fits into the PC architecture, and how it works. Then
we'll look at the various types of memory, speeds, and packaging of the chips and memory
modules you can buy and install.
This chapter also covers the logical layout of memory, defining the various areas of
memoryandtheirusesfromthesystem'spointofview.Becausethelogicallayoutanduses
arewithinthe“mind”oftheprocessor,memorymappingandlogicallayoutremainperhaps
the most difficult subjects to grasp in the PC universe. This chapter contains useful inform-
ation that removes the mysteries associated with memory and enables you to get the most
out of your system.
Memory is the workspace for the processor. It is a temporary storage area where the pro-
grams and data being operated on by the processor must reside. Memory storage is con-
sideredtemporarybecausethedataandprogramsremainthereonlyaslongasthecomputer
haselectrical powerorisnotreset.Beforethecomputer isshutdownorreset,anydatathat
has been changed in memory should be saved to a more permanent storage device (usually
a hard disk) so it can be reloaded into memory in the future.
Memoryofteniscalled RAM ,for random access memory .MainmemoryiscalledRAMbe-
cause you can randomly (as opposed to sequentially) access any location. This designation
issomewhatmisleadingandoftenmisinterpreted.Read-onlymemory(ROM),forexample,
is also randomly accessible, yet it is usually differentiated from the system RAM because
it maintains data without power and can't normally be written to. Although a hard disk can
be used as virtual random access memory, we don't consider that RAM either.
Over the years, the definition of RAM has changed from a simple acronym to become
something that means the primary memory workspace the processor uses to run programs,
which usually is constructed out of a type of chip called dynamic RAM (DRAM). One of
the characteristics of DRAM chips (and therefore most types of RAM in general) is that
they store data dynamically, which really has two meanings. One meaning is that the in-
formation can be written to RAM repeatedly at any time. The other has to do with the fact
that DRAM requires the data to be refreshed (essentially rewritten) every few milliseconds
orso;fasterRAMrequiresrefreshingmoreoftenthanslowerRAM.AtypeofRAMcalled
static RAM (SRAM) does not require the periodic refreshing. An important characteristic
of RAM in general is that data is stored only as long as the memory has electrical power.
 
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