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Pogram Counter

Posted on 2002-03-26
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why there is bidirectional path between "Program Counter" & "Data Bus" in micro processors of Intel Series.
As Memory address of Instruction is placed from Data Bus to Program Counter in one direction What is moved back from Program Counter to Data Bus in reverse direction.
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Question by:SairaZaidi
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There must be a bi-directional path between the PC (program counter) and the data bus on Intel architecture CPUs to support instructions that:

1) Set the PC from an instruction.  Like an absolute JMP.  The data following the JMP opcode is the new PC and must be able to get from the data bus to the PC.

2) Set memory from the PC.  A good example is the CALL instruction.  CALL pushes the PC onto the STACK.  Since the stack is in memory on Intel CPUs, there must be a path from the PC to data bus.

Of course it's possible to have the PC go somewhere else BEFORE sending it to the data bus and therefore eliminate the direct PC <--> data bus path but the frequency of such instructions as noted above would make this missing path a major bottleneck.

CPU architecture designers spend a lot of time modelling things to figure out where the bottlenecks are and how to best eliminate them.
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No comment has been added lately, so it's time to clean up this TA.
I will leave a recommendation in the Cleanup topic area that this question is:
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Do it.  This question has been hanging for > 1 year!!!
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