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Out-of-order program control flow - branching

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• Instructions that update the Program Counter
  • Important fact:

    Some assembler instructions can update the program counter !!!

    When an instruction updates the program counter, the processor will fetch the next instruction at the updated memory location !!!

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    Slideshow:  

     

     

                 

    (Initial state: PC = 44 (bin !), CPU just finished executing the instruction in IR)

                 

    (Instruction Fetch: CPU sends PC on address bus and issues READ command)

                 

    (Instruction Fetch: in response, memory sends content at address (= instruction "mov pc, #4") to CPU)

                 

    (Instruction fetch: CPU stores the data (= instruction "mov pc, #4") in the IR register)

                 

    (CPU increments PC to point to new next instruction - Instruction Fetch step ends)

                 

    ("mov pc, #4" will update PC to 4 !!! -- Next instruction will now be fetched from a new location !!!)

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    In other word:

    When you update the Program Counter, you will break the normal program flow and introduce an out-of-order program flow !!!