If that were the case, then in the time the typist walks a foot, the computer travels
18,750 miles, or about three quarters of the way around the earth!
In the early days of computing, the CPU would wait for each character to be typed. A machine instruction
would ready the keyboard interface to accept a character from the keyboard, and the next instruction would test
to see if the character had been received. If the character had not yet been received, the program would simply
loop, testing (???polling???) to see if the character had been received. This is called ???programmed I/O with polling,???
or ???busy waiting.??? It??™s a simple but prohibitively costly approach.
Today computers use an ???interrupt system??? to avoid busy waiting, and the operating system supervises
all I/O. Each I/O device is connected to the computer via an ???I/O controller.??? An I/O controller is a small,
40 COMPUTER ORGANIZATION [CHAP. 3
special-purpose computer within the computer. It has a few well-defined functions, and a small amount of
memory of its own with which to ???buffer??? (store temporarily) the information being sent or received.
When a program requires output, for example, the operating system moves the data to the buffer memory of
the I/O controller for the device, and commands the I/O controller to start the operation.
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