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74
Disk Buffers
Microcomputer systems that process large and complicated
files often spend considerable time reading from, and writing, mass-
storage devices. This operation creates long processing pauses that
annoy end users. By simply enlarging the PC-DOS/MS-DOS disk-
buffer setting from its normal default value of 2, you can dramatically
reduce disk I/O delays.
A disk buffer is a block of main memory in which the DOS
holds data that is being read from or written to a disk. Each time DOS
is requested to read or write a record, it first looks to see whether the
sector containing that record is already in a buffer. By increasing the
size of the disk buffer, the more likely it is that sought-after data will be
in main memory. If it is, then DOS simply transfers the record to the
application without the need to read the data from the disk which, of
course, saves time.
The logical solution would seem to be to move all the data
used, by an application into the disk-buffer area. Unfortunately,
microcomputer systems are not blessed with huge blocks of main
memory and a compromise must be made between the amount of
memory used for disk buffering and the main memory required for
other system operations.
Many factors interact to cause disk delays (including facets of
the specific hardware configuration and operating system), they
cannot be derived theoretically. Instead, they must be measured
empirically using the actual system configuration, operating system
and "typical" disk transactions made in actual applications.
Measuring something as complicated as the timing of Disk I/O
in a microcomputer system requires a system model.
TEXT 11
Pre-reading task
1. What is the term memory is usually reserved for?
2. What types of a microcomputer memory can you name?
3. Read the text and draw a classification scheme of different
memory types.
4. State the most important characteristics for memories.
Static Memory Devices: Organization and
Characteristics
One can divide memories into two major types: static
39
e) What is processing hardware? What are the basic types
of memory used in a PC?
f) Can a PC-user change the ROM? Who records the
information in ROM?
g) What is storage hardware? What is CD-ROM used for?
Can a user record his or her data on a CD? What kind of
storage hardware can contain more information: CD-
ROM, RAM or ROM?
h) What is modem used for? Can PC-user communicate with
other people without a modem?
Hardware
What is hardware? Webster's dictionary gives us the following
definition of the hardware — the mechanical, magnetic, electronic, and
electrical devices composing a computer system.
Computer hardware can be divided into four categories:
1) input hardware
2) processing hardware
1
3) storage hardware
4) output hardware
Input hardware
The purpose of the input hardware
2
is to collect data and
convert
3
it into a form suitable for computer processing. The most
common input device is a keyboard
4
. It looks very much like a
typewriter. The mouse
5
is a hand held device connected
6
to the
computer by small cable. As the mouse is rolled
7
across the mouse
pad, the cursor moves across the screen. When the cursor reaches
8
the desired location, the user usually pushes a button on the mouse
once or twice to signal a menu selection or a command to the
computer.
The light pen uses a light sensitive
9
photoelectric cell to
signal screen position to the computer. Another type of input hardware
is optic-electronic scanner
10
that is used to input graphics as well as
typeset characters. Microphone and video camera can be also used to
input data into the computer. Electronic cameras are becoming very
popular among the consumers for their relatively low price and
convenience.
Processing hardware
The purpose of processing hardware is retrieve
11
, interpret
and direct
12
the execution
13
of software instructions provided to the
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