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Understanding Processor Timing: Microprocessor Operation and Instruction Execution Cycle

This chapter delves into the fundamental workings of a microprocessor, specifically focusing on the instruction execution cycle comprising fetch, decode, execute, and repeat phases. Using a practical example with initial register values and an assembly instruction, it illustrates how the program counter (IP) is incremented and the instruction is processed step-by-step. Additionally, it covers detailed timing diagrams for memory read and write operations for the 8088 microprocessor, providing insights into the intricacies of CPU operations and memory interactions.

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Understanding Processor Timing: Microprocessor Operation and Instruction Execution Cycle

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  1. Chapter 5 Processor Timing

  2. How does the Microprocessor works? • Fetch • Increment Program Counter by 1 (IP=IP+1) • Decode • Execute • Repeat

  3. Example ;assume that initially ;ds = 2000, bx = 0023, ax = 351C ;cs = 1000, ip = 0005 mov [bx], al ;8807 hlt ;F4

  4. CPU MEMORY FETCH CS:IP

  5. FETCH 1000:0005

  6. FETCH 10005

  7. FETCH 10005 LOW HIGH

  8. FETCH 10005 88 LOW HIGH

  9. FETCH 10005 88 LOW HIGH

  10. INC. PC

  11. INC. PC

  12. DECODE mov [bx], ?

  13. FETCH CS:IP

  14. FETCH 1000:0006

  15. FETCH 10006

  16. FETCH 10006 LOW HIGH

  17. FETCH 10006 07 LOW HIGH

  18. FETCH 10006 07 LOW HIGH

  19. INC. PC

  20. INC. PC

  21. DECODE mov [bx], al

  22. EXECUTE mov [bx], al DS:BX

  23. EXECUTE mov [bx], al 2000:0023

  24. EXECUTE mov [bx], al 20023

  25. EXECUTE mov [bx], al 20023 1C

  26. EXECUTE mov [bx], al 20023 1C HIGH LOW

  27. EXECUTE mov [bx], al 20023 1C HIGH LOW

  28. FETCH CS:IP

  29. FETCH 1000:0007

  30. FETCH 10007

  31. FETCH 10007 LOW HIGH

  32. FETCH 10007 F4 LOW HIGH

  33. FETCH 10007 F4 LOW HIGH

  34. INC. PC 10007 F4 LOW HIGH

  35. INC. PC 10007 F4 LOW HIGH

  36. DECODE 10007 F4 LOW hlt HIGH

  37. EXECUTE hlt

  38. Machine Cycle Timing Diagram Instruction Cycle

  39. Processor Timing Diagram of 8088 for Memory Read T1 T2 T3 T4 CLOCK __ DT/R ALE AD7 - AD0 A7 - A0 D7 - D0 (from memory) A15 - A8 A15 - A8 A19/S6 - A16/S3 A19 - A16 S6 - S3 __ IO/M ____ RD ______ DEN

  40. Processor Timing Diagram of 8088 for Memory Write T1 T2 T3 T4 CLOCK __ DT/R ALE AD7 - AD0 A7 - A0 D7 - D0 (from CPU) A15 - A8 A15 - A8 A19/S6 - A16/S3 A19 - A16 S6 - S3 __ IO/M ____ WR ______ DEN

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