Hardware Reference
In-Depth Information
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.
When we talk about a computer's memory, we usually mean the RAM or physical
memory in the system, which is mainly the memory chips or modules the processor uses
to store primary active programs and data. This often is confused with the term
storage
,
which should be used when referring to things such as disk and tape drives (although they
can be used as a form of RAM called virtual memory).
RAM can refer to both the physical chips that make up the memory in the system and the
logical mapping and layout of that memory.
Logical mapping
and
layout
refer to how the
memory addresses are mapped to actual chips and what address locations contain which
types of system information.
People new to computers often confuse main memory (RAM) with disk storage because
both have capacities that are expressed in similar megabyte or gigabyte terms. The best
analogy I've found to explain the relationship between memory and disk storage is to
think of an office with a desk and a file cabinet.
In this popular analogy, the file cabinet represents the system's hard disk, where both pro-
grams and data are stored for long-term safekeeping. The desk represents the system's
main memory, which allows the person working at the desk (acting as the processor)
direct access to any files placed on it. Files represent the programs and documents you
can “load” into the memory. To work on a particular file, you first must retrieve it from
the cabinet and place it on the desk. If the desk is large enough, you might be able to have
several files open on it at one time; likewise, if your system has more memory, you can
run more or larger programs and work on more or larger documents.
Adding hard disk space to a system is similar to putting a bigger file cabinet in the of-
fice—more files can be permanently stored. And adding more memory to a system is like
getting a bigger desk—you can work on more programs and data at the same time.
One difference between this analogy and the way things really work in a computer is that
when a file is loaded into memory, it is a copy of the file that is actually loaded; the ori-
ginal still resides on the hard disk. Because of the temporary nature of memory, any files
that have been changed after being loaded into memory must then be saved back to the
hard disk before the system is powered off (which erases the memory). If the changed file
