• The 8085 instructions consist of one to five machine cycles.
  • Actually the execution of an instruction is the execution of the machine cycles of that instruction in the predefined order.
  • The timing diagram of an instruction ate obtained by drawing the timing diagrams of the machine cycles of that instruction, one by one in the order of execution.
Timing diagram for IN C0H.

  • Fetching the Opcode DBH from the memory 4125H.
  • Read the port address C0H from 4126H.
  • Read the content of port C0H and send it to the accumulator.
  • Let the content of port is 5EH.
Fig - Timing Diagram for Opcode Fetch Machine Cycle
TIMING DIAGRAM OF 8085 INSTRUCTIONS
Timing diagram for STA 526AH.

  • STA means Store Accumulator -The contents of the accumulator is stored in the specified address(526A).
  • The opcode of the STA instruction is said to be 32H. It is fetched from the memory 41FFH(see fig). - OF machine cycle
  • Then the lower order memory address is read(6A). - Memory Read Machine Cycle
  • Read the higher order memory address (52).- Memory Read Machine Cycle
  • The combination of both the addresses are considered and the content from accumulator is written in 526A. - Memory Write Machine Cycle
  • Assume the memory address for the instruction and let the content of accumulator is C7H. So, C7H from accumulator is now stored in 526A.

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