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4. The contents of MDR (old value of the PC) are stored in memory.
The following table shows the sequence of events, where t 1 , t 2 , t 3 .
Step
Micro-operation
t 1
MDR (PC)
t 2
MAR address1 (where to save old PC);
PC address2 (interrupt handling routine)
t 3
Mem[MAR] (MDR)
5.5. CONTROL UNIT
The control unit is the main component that directs the system operations by sending
control signals to the datapath. These signals control the flow of data within the CPU
and between the CPU and external units such as memory and I
O. Control buses
generally carry signals between the control unit and other computer components
in a clock-driven manner. The system clock produces a continuous sequence of
pulses in a specified duration and frequency. A sequence of steps t 0 , t 1 , t 2 ,
/
,
...
Figure 5.7 Timing of control signals
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