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Cadenti
logic family
medium scale integration
metal oxide semiconductor
ac register
accumulator logic
adder circuit
basic computer design
branch unconditionally
flowchart
input and output communication
input output instruction
input register
interrupt cycle
logic adder circuits
logic gates
output register
register and memory
binary code
binary number
clock pulse
data types
decimal numbers
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Program Interrupt
The process of communication just described is referred to as programmed control transfer. The computer keeps checking the flag bit, and when it finds it set, it initiates an information transfer. The difference of information flow rate between the computer and that of the input-output device makes this type of transfer inefficient. To see why this is inefficient, consider a computer that can go through an instruction cycle in 1 (xs. Assume that the input-output device can transfer information at a maximum rate of 10 characters per second. This is equivalent to one character every 100,000 |xs. Two instructions are executed when the computer checks the flag bit and decides not to transfer the information. This means that at the maximum rate, the computer will check the flag 50,000 times between each transfer. The computer is wasting time while checking the flag instead of doing some other useful processing task.
An alternative to the programmed controlled procedure is to let the external device inform the computer when it is ready for the transfer. In the meantime the computer can be busy with other tasks. This type of transfer uses the interrupt facility. While the computer is running a program, it does not check the flags. However, when a flag is set, the computer is momentarily interrupted from proceeding with the current program and is informed of the fact that a flag has been set. The computer deviates momentarily from what it is doing to take care of the input or output transfer. It then returns to the current program to continue what it was doing before the interrupt.
The interrupt enable flip-flop IEN can be set and cleared with two instructions. When IEN is cleared to 0 (with the IOF instruction), the flags cannot interrupt the computer. When IEN is set to 1 (with the ION instruction), the computer can be interrupted. These two instructions provide the programmer with the capability of making a decision as to whether or not to use the interrupt facility.