COA-Input/Output versus Memory

In addition to communicate with I/O, the processor must also communicate with memory unit.

And there are three ways that computer buses can be used to communicate with memory as well as I/O.These are:

1. Use two separate buses one for Memory and other for Input/output(This method is named as IOP method).
2. Use one common bus for both Memory and I/O but have separate control lines for each(This method is named as Isolated I/O method).
3. Use one common bus for Memory and I/O with common control lines(This method is named as Memory-Mapped Input/Output method).

Now we, will discuss about each in brief one by one.

      1. IOP(Input-Output Processor) method:

           In this method the computer has independent sets of data, address and control buses, one for                  accessing memory(Known as Memory bus and other for I/O known as I/O bus)

           Means, to handle communication, in this method, IOP method is used.It provides separate Input-             Output Processor(IOP) in addition to Central Processing Unit, and relieves CPU from the task of              I/O transfer.That is, CPU communicate to memory via memory bus, and IOP communicates to                Input Output devices through separate bus known as I/O bus.And memory communicates with                both CPU and IOP through memory bus.Thus, this method provide independent pathways for                  transfer of information between external devices(I/O devices) and internal memory.

      2. Isolated I/O Method:

           In this method, to make processor to communicate with CPU and I/O, common bus is used for                both memory and I/O and distinction between a memory transfer and I/O transfer is made                        through separate read and write lines.Means, "I/O Read" and "I/O Write" control lines are enabled            during and I/O transfer.And "Memory Read" and "Memory Write" control lines are enabled during            a memory transfer.This configuration isolates all I/O interface addresses from the addresses                    assigned to memory and that’s why this method is known as isolated I/O method for assigning                address in a common bus.

      3. Memory-Mapped I/O:

           In this method, one common bus is used for Memory and I/O with no distinction, and then this                  configuration is said to be as Memory-Mapped I/O.Because, there are only memory instruction,                there is no specific input or output instruction.

           And to access or manipulate I/O data, it use some memory instruction that are used to                              manipulate memory words.That is it allows computers to use the same instructions for either                    Input/output transfer or for memory transfer.The advantage of it is that the load and store                          instructions are used for reading and writing from memory can also be used to input and output                data.