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I/O Mapping in 8085 Microprocessor

  • I/O interfacing

    There are two methods of interfacing the Input / Output devices with the microprocessor. They are,

    1) Memory mapped I/O and
    2) I/O mapped I/O.

    Memory mapped I/O

    In this method the I/O devices are treated like the memory. A part of the memory address space is used for the I/O devices. The memory mapped I/O scheme is shown in figure.
    Figure: Memory mapped I/O scheme


    • In memory mapped I/O scheme, the same address space is used for both memory and I/O devices.
    • The microprocessor uses the sixteen address line A0 – A7 and A8 – A15 for the memory as well as for the I/O devices.
    • The I/O devices share the address space with the memory. All the memory related instructions are used for addressing I/O devices also.
    • No separate IN and OUT instructions are required in memory mapped I/O scheme.
    • IO/𝑀’ pin is not required.



    Steps for memory operations (memory read and memory write) :

    1. When the memory related instructions like LDA and STA are used, the microprocessor places the 16-bit address on the address bus.
    2. 𝑅𝐷’ is activated for read operation and 𝑊𝑅’ is activated for write operation.
    Steps for I/O operations (I/O read and I/O write) :
    The same steps used for memory operations are used for I/O operations also.

    I/O mapped I/O

    In this method, I/O devices are treated as I/O devices and memory is treated as memory. Separate address space is used for memory and I/O. The I/O mapped I/O scheme is shown in figure.

    Figure: I/O mapped I/O scheme
    • In I/O mapped I/O scheme, the microprocessor uses the sixteen address lines A0 – A7 and A8 – A15 for the memory and eight address lines A0 to A7 to identify an input / output device.
    • Here, the full address space 0000 – FFFF is used for the memory and a separate address space 00 – FF is used for the I/O devices.
    • Hence, the microprocessor can address 65536 (216) memory locations 256 (28) input devices and 256 (28) output devices separately.
    • IN and OUT instructions are used to activate the IO/𝑀’ signal.
    • When IO/𝑀’ is low, the memory is selected for reading and writing operations.
    • When IO/𝑀’ is high, the I/O port is selected for reading and writing operations.

    Steps for memory operations (memory read and memory write) :

    1. When the memory related instructions like LDA and STA are used, the microprocessor places the 16-bit address on the address bus.
    2. The microprocessor makes the IO/𝑀’ linelow.
    3. The microprocessor makes the 𝑅𝐷’ low for read operation and 𝑊𝑅’ low for write operation.

    Steps for I/O operations (I/O read and I/O write) :

    1. When the I/O related instructions like IN and OUT are used, the microprocessor places the 8-bit address on the address bus A0 – A7 as well as A8 – A15.
    2. IO/𝑀’ line is made high.
    3. The microprocessor makes the 𝑅𝐷’ low for read operation and 𝑊𝑅’ low for write operation.

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