Opcode fetch machine cycle of 8085 :

  • Each instruction of the processor has one byte opcode.
  • The opcodes are stored in memory. So, the processor executes the opcode fetch machine cycle to fetch the opcode from memory.
  • Hence, every instruction starts with opcode fetch machine cycle.
  • The time taken by the processor to execute the opcode fetch cycle is 4T.
  • In this time, the first, 3 T-states are used for fetching the opcode from memory and the remaining T-states are used for internal operations by the processor.

Memory Read Machine Cycle of 8085:

  • The memory read machine cycle is executed by the processor to read a data byte from memory.
  • The processor takes 3T states to execute this cycle.
  • The instructions which have more than one byte word size will use the machine cycle after the opcode fetch machine cycle.
Fig - Timing Diagram for Opcode Fetch Machine Cycle
Fig - Timing Diagram for Memory Read Machine Cycle
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