Bus Cycles of 8086 Microprocessor

The bus cycle is also named as machine cycle. Bus cycle of 8086 is used to access memory, peripheral devices (Input/Output devices), and Interrupt controller. Bus cycle corresponds to a sequence of events that starts with an address being output on system address bus followed by a write or read data transfer. During these operations, a series of control signals are also produced by microprocessor to control direction and timing of bus.

There are at least four clock periods in a bus cycle of 8086 microprocessor. These four clock periods are called T1, T2, T3 and T4 states.

These four clock states gives bus cycle duration T of 200 ns *4 = 800 ns in 5-MHz 8086 system.

  1. Read Cycle :
    When a read cycle is to be performed, during T1 microprocessor puts an address on address bus, and then bus is put in high impedance state during T2 state. Data to be read must be out on bus during T3 and T4. During T3 bus is made “reserved for data in” and finally data is read during T4.



  2. Write Cycle :
    In case of write memory cycle, during T1 state microprocessor puts an address on address bus. Data is put on data bus by CPU during T2 state and maintained during T3 and T4 states, that is written out to memory or I/O devices.

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