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Computer Organization
Instructors :
Dr. Abdul Raouf Khan
Mr.Marwan El-Haj
Program Interrupts
The data transfer from/ to input/output
devices, which uses checking the input/output
flag is called programmed data transfer. In
this scheme the computer time is wasted by
checking flags instead of doing some useful
work.

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Program Interrupts
The data transfer scheme in which an
external device informs the computer, when
it is ready for the transfer of data, by
interrupting the computer, is called Interrupt
driven data transfer. While the computer is
running a program, it does not check the flag.
However, when a flag is set, the computer is
momentarily interrupted from proceeding with
the current program.
Program Interrupts
In our Basic computer, the interrupt driven
data transfer scheme can be implemented
with the help of IEN flip flop.
If IEN =0, the computer cannot be interrupted
If IEN =1, the computer is interrupted
The following flow chart summarizes the
schemes

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Modified Fetch & decode cycle
The microoperation for fetch and decode phase will
now be specified as
R’T0 : AR PC
R’T1 : IR M[AR], PC PC + 1
R’T2 : D0 D7 Decode IR(12-14),AR IR(0-11), I IR(15)

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Interrupt cycle microoperation
The micro-operation for interrupt phase is
specified as
– RT0 : AR 0, TR PC
– RT1 : M[AR] TR, PC 0
– RT2 : PC PC +1, IEN 0, R 0, SC 0
When R is set to 1?

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Basic Computer Consists of: