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 :
- Separate set of address, control and data bus to I/O and memory.
- Have common bus (data and address) for I/O and memory but separate control lines.
- 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/O | Memory Mapped I/O |
|---|---|
| Memory and I/O have seperate address space | Both have same address space |
| All address can be used by the memory | Due to addition of I/O addressable memory become less for memory |
| Separate instruction control read and write operation in I/O and Memory | Same 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 seperate buses | Lesser efficient |
| Larger in size due to more buses | Smaller 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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