Hardware Reference
In-Depth Information
Chapter 6. Memory
Memory Basics
This chapter discusses memory from both a physical and a logical point of view. 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 memory and their
uses from the system's point of view. Because the logical layout and uses are within the “mind” of the
processor, memory mapping and logical layout remain perhaps the most difficult subjects to grasp in
the PC universe. This chapter contains useful information 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 programs and
data being operated on by the processor must reside. Memory storage is considered temporary
because the data and programs remain there only as long as the computer has electrical power or is
not reset. Before the computer is shut down or reset, any data that 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.
Main memory is normally called RAM (random access memory), because you can randomly (as
opposed to sequentially) access any location. This designation is somewhat misleading and often
misinterpreted. Read-only memory (ROM), for example, 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 information 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 or so; faster RAM requires refreshing more often than slower RAM. A type
of RAM called 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.
Note
Both DRAM and SRAM memory maintain their contents only as long as power is present.
However, a different type of memory known as
flash memory
can retain its contents without
power, and it is most commonly used today in solid-state drives (SSDs), digital camera and
player media, and USB flash drives. As far as the PC is concerned, a flash memory device
emulates a disk drive (not RAM) and is accessed by a drive letter, just as with any other disk
or optical drive.
