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The main memory can be thought of as a series of
storage locations.Each location has a unique
address,counting from zero upwards.The address is
called memory address.The main memory is
connected to the CPU by buses.When the CPU wants
to access a particular memory location,it sends the
address to the memory on an address bus.The data
held in that location are returned to the CPU on the
data bus.
The size of the main memory is measured in
kilobytes (KB),which is equal to 1024 bytes. For
convenience,we sometimes round off the value of
K to 1000.For example,640 KB is approximately
640 000 bytes (640 x 1000).To measure memory
of large sizes,we use megabytes (MB) and
gigabytes (GB).
1 MB = 1024 KB = 220 bytes ~ 1 million bytes
1 GB = 1024 MB = 230 bytes ~ 1 billion bytes
A semiconductor memory chip is an integrated
circuit (IC) that contains thousands of
transistors.A transistor is an electronic component
that can be either on or off.
A semiconductor chip is very
small.This chip contains
256KB
Semiconductor chips have several advantages over
previous devices:
1. A chip,although small,is very dense.One chip can
therefore store thousands or even million of bytes.
2. As the chip is very small,the components are
stored very close together.The amount of time for
an electrical pulse to pass from one component to
another is therefore very small.Memory made
from semiconductor chips is therefore very
efficient.
3. As the chips are so small,the size of the computer
is reduced as well.
4. Chips are cheaper to manufacture.
5. If the chip has a fault,the whole chip can be
easily replaced.
There are two main types of semiconductor
memory chips in a computer: random access
memory (RAM) and read only memory(ROM).
ROM (top) and RAM
(bottom) on a
motherboard (right)
RAM:
1.Data can be read from any word or written onto
any word in the RAM.
2.When a program is run,the instructions and the
working data are temporarily stored in the RAM.
3.When the computer is switched off,all the data
held in the RAM are lost.RAM is therefore
volatile.
ROM:
1.Data can only be read from the ROM.The
contents cannot be overwritten.
2.ROM is used by the computer manufacturer to
store the systems programs.These programs
control the fundamental operations.
3.ROM is non-volatile.When the power is switched
off,the contents of the ROM remain unaffected.
Backing storage devices exist to save programs and
data for future processing.It is not possible to put
all programs and data in to the main memory for
several reasons.
1.Data in the main memory (except in the ROM)are
lost when the machine is switched off.
2.Main memory is very expensive.
3.Data for an application program may be so large
that it is physically impossible for the main memory
to hold all the data.
Backing storage devices provide a more permanent
from of storage than main memory,Backing storage
devices are used to record data and programs onto
various storage media,Commonly used devices are
disk drives and tape drives.
Backing storage devices can be treated as
input/output devices.They function as input devices
when the stored data are transferred to the
computer for processing.They function as output
devices when processed result are written to them.
In this chapter,we will discuss three storage
media:magnetic disk,magnetic tape,and optical
disk(CD-ROM).
Magnetic disk:Floppy disk
Hard disk
Magnetic tape:Reel-to-reel tape
Cartridge tape
Cassette tape
Optical disk: