• It seperate all I/O address from the memory address(that is address space is seperate for I/O & Memory).
  • CPU has distinct I/O instructions so complexity in instructions set increases.
  • If the addresses of the interfacing registors(I/O) is placed on the address lines, the I/O Read & I/O Write control lines are enable.If the memory address is placed on the address line, the Memory Read & Memory Write control lines are enable.
  • This technique can be used only with microprocessor that have seperate IN & OUT instructions & special I/O Write & Read control outputs.


  • This technique can be used with any microprocessor.
  • Same address space is used for both Memory & I/O.
  • It allows the microprocessor to send the same instructions for both I/O transfer & Memory transfer.
  • Some of CPUs addressable space is reservedfor Memory or same for I/O.
  • If total address location = 0 –>K

M = no of address for I/O

N = no of address for M

then K = M + N

  • Only one set of Read/Write control signals are there.


  • In this technique, CPU must regularly check/poll each channel or post in turn.
  • It is time consuming as CPU has pause & poll every port.
  • Post status is examined in case action is required by computer for example: CPU is programmed to read the keyboard, it is periodically checks if a character has been entered.
  • Usually used for small systems.

         The idea of following is to repeatedly input &/ or output by the using a polling loop in the program.


  • It is wasteful of CPU time as it involves CPU through out the I/O process.


  • In interrupt I/O, processor after sending command to I/O module(for execution of any instruction) is free to perform any other task. When I/O module checks the states of device and if it is ready then I/O module will it self interrupt the microprocessor to request service. On receiving interrupt request, CPU finished is the instruction it is executing, save the contents  of IP & Flag register & then executes the interrupt service routine.
  • More complex Hardware & Software.
  • It makes for efficient use of CPUs time & capacities.

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