THE 8085 MACHINE CYCLES AND BUS TIMING
The 8085 micro-processor is designed to execute 74 different instruction types. Each instruction has two parts: operation code, an operand. The op-code is a command such as add and the operand is the object to be operated on, such as a byte or the content of register. Some instruction is 1-bytes instruction and some are multi bytes instruction
To execute instruction, the 8085 needs to perform various operations such as memory read/write and I/O read/write. However there is no direct relationship between the number of the bytes of an instruction and the number of the operation s 8085 signals and to perform.
Basically the micro-processor have three major function
These function are further divided to various operation
OP-CODE FETCH
The first operation in any instruction is op-code fetch. The micro-processor needs to get this machine code from the memory register where it is stored before the micro-processor can begin execute the instruction
This op-code fetch cycle is called M1 cycle and has four t-state . this 8085 uses the first three states T1-T3 to fetch the code and T4 to decode and execute the op-code .in the 8085 the instruction set some instruction have six T-state .
timing diagram of op-code fetch
