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THE FETCH-EXECUTE CYCLE

THE FETCH-EXECUTE CYCLE. REGISTERS. ACC. Check for interrupts. Fetch instruction from memory. MDR. MAR. 1. CIR. Execute instruction. Update Program counter. PC. 1. MEMORY Location Contents. THE PROGRAM. The address of the first instruction is copied from the PC to the MAR.

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THE FETCH-EXECUTE CYCLE

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  1. THE FETCH-EXECUTE CYCLE

  2. REGISTERS ACC Check for interrupts Fetch instruction from memory MDR MAR 1 CIR Execute instruction Update Program counter PC 1 MEMORY Location Contents THE PROGRAM The address of the first instruction is copied from the PC to the MAR

  3. REGISTERS ACC Check for interrupts Fetch instruction from memory MDR LDA 23 MAR 1 CIR Execute instruction Update Program counter PC 1 MEMORY Location Contents THE PROGRAM The first instruction is read into the MDR

  4. REGISTERS ACC Check for interrupts Fetch instruction from memory MDR LDA 23 MAR 1 CIR LDA 23 Execute instruction Update Program counter PC 1 MEMORY Location Contents THE PROGRAM The first instruction is copied into the CIR

  5. REGISTERS ACC Check for interrupts Fetch instruction from memory MDR LDA 23 MAR 1 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents THE PROGRAM The program counter is updated to point to the next instruction

  6. REGISTERS ACC Check for interrupts Fetch instruction from memory MDR LDA 23 MAR 23 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents THE PROGRAM The address part of the current instruction is placed in the MAR

  7. REGISTERS ACC Check for interrupts Fetch instruction from memory MDR 14 MAR 23 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents The contents of the location whose address is in the MAR are loaded into the MDR THE PROGRAM

  8. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR 14 MAR 23 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents THE PROGRAM The contents of the MDR are copied into the ACC

  9. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR 14 MAR 23 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents THE PROGRAM The Status Register is checked for interrupts

  10. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR 14 MAR 2 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents THE PROGRAM The address of the second instruction is copied from the PC to the MAR

  11. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR ADD 72 MAR 2 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents THE PROGRAM The second instruction is read into the MDR

  12. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR ADD 72 MAR 2 CIR ADD 72 Execute instruction Update Program counter PC 2 MEMORY Location Contents THE PROGRAM The second instruction is copied into the CIR

  13. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR ADD 72 MAR 2 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The program counter is updated to point to the next instruction

  14. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR ADD 72 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The address part of the current instruction is placed in the MAR

  15. REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR 35 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents The contents of the location whose address is in the MAR are loaded into the MDR THE PROGRAM

  16. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 35 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The contents of the MDR are added to the ACC

  17. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 35 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The Status Register is checked for interrupts

  18. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 35 MAR 3 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The address of the third instruction is copied from the PC to the MAR

  19. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 3 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The third instruction is read into the MDR

  20. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 3 CIR STA 53 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The third instruction is copied into the CIR

  21. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 3 CIR STA 53 Execute instruction Update Program counter PC 4 MEMORY Location Contents THE PROGRAM The program counter is updated to point to the next instruction

  22. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 MEMORY Location Contents THE PROGRAM The address part of the current instruction is placed in the MAR

  23. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 49 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 MEMORY Location Contents THE PROGRAM The contents of the ACC are copied into the MDR

  24. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 49 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 MEMORY Location Contents The contents of the MDR are copied into the memory location whose address is in the MAR THE PROGRAM

  25. REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 49 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 MEMORY Location Contents THE PROGRAM The Status Register is checked for interrupts

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