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Memory mapped I/O and Isolated I/O

  • Difficulty Level : Basic
  • Last Updated : 21 Nov, 2019
Geek Week

As a CPU needs to communicate with the various memory and input-output devices (I/O) as we know data between the processor and these devices flow with the help of the system bus. There are three ways in which system bus can be allotted to them :

  1. Separate set of address, control and data bus to I/O and memory.
  2. Have common bus (data and address) for I/O and memory but separate control lines.
  3. Have common bus (data, address, and control) for I/O and memory.

In first case it is simple because both have different set of address space and instruction but require more buses.

Isolated I/O –

Then we have Isolated I/O in which we Have common bus(data and address) for I/O and memory but separate read and write control lines for I/O. So when CPU decode instruction then if data is for I/O then it places the address on the address line and set I/O read or write control line on due to which data transfer occurs between CPU and I/O. As the address space of memory and I/O is isolated and the name is so. The address for I/O here is called ports. Here we have different read-write instruction for both I/O and memory.

Memory Mapped I/O –

In this case every bus in common due to which the same set of instructions work for memory and I/O. Hence we manipulate I/O same as memory and both have same address space, due to which addressing capability of memory become less because some part is occupied by the I/O.



Differences between memory mapped I/O and isolated I/O –

Isolated I/OMemory Mapped I/O
Memory and I/O have separate address spaceBoth have same address space
All address can be used by the memoryDue to addition of I/O addressable memory become less for memory
Separate instruction control read and write operation in I/O and MemorySame instructions can control both I/O and Memory
In this I/O address are called ports.Normal memory address are for both
More efficient due to separate busesLesser efficient
Larger in size due to more busesSmaller in size
It is complex due to separate separate logic is used to control both.Simpler logic is used as I/O is also treated as memory only.

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