from memory storage, executes the instructions sequentially, and displays
or stores the results in appropriate locations. Transfer of data from one loca-
tion to another is performed by the CPU using buses, which are essentially
a set of electrical connections.
How fast a program is executed depends on the speed of the computer
operation, which increases with the faster electrical components of the
CPU. The efficiency of the computer operation is further increased by using
parallel transfer of data (where many transfers are performed simultane-
ously) rather than serial transfer (where only one transfer is carried out at
a time).
Computer Memory
The memory of a computer is a section assigned for temporary storage of
data during the operation of a program. The program instructions and
processed data are all stored in the computer memory with the help of the
CPU. When the CPU sends data to the memory, it writes the data into the
memory. If the CPU retrieves data from the memory, it is then said to read
the data from the memory. The memory can be of two types: random access
memory (RAM) and read-only memory (ROM). RAM has the advantage
of both write and read capacity; however, the data stored in it is lost when
the computer is shutoff or the electrical power is lost. With larger RAMs,
computation time becomes shorter. On the other hand, data stored in
ROMs such as CD-ROMs, DVD-ROMs, etc. cannot be erased by electrical
failure or computer shutoff.
External Storage Devices
Floppy disks, hard disk drives, CD-ROMs, DVD-ROMs, magnetic tapes,
and optical (laser) disks are varieties of external storage devices that are
commonly used for storage of programs and data. Each of them has vari-
able storage space. Hard drives are installed virtually in all computers for
internal storage of the programs and data. Floppy disks are commonly used
for storing data externally as backup copies, although in some applications
programs and data can be stored for input into the computer for execution.
While hard drives have the storage capacity of hundreds of gigabytes, floppy
disks can only store up to a few megabytes and are getting out of use. Cur-
rently, CD-ROMs are available with capacities of 650–700 megabytes and
DVD-ROMs with 4.7–9 gigabytes. Magnetic tapes and laser optical disks
have large storage space in compact form and can be utilized primarily for
archiving of patient data that can be retrieved for future reference.
Input/Output Devices
Input/output (I/O) devices are essential for input of the initial data and for
output of the processed data. Input and output of data are carried out by
Basics of a Computer 141