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Address Spaces

Each bus exists in a seperate address space from the CPU - they exist in I/O space. Although the CPU can access I/O space memory, the controllers themselves can only access system memory indirectly with the help of the CPU. From the point of view of the device, say the floppy disk controller, it will see only the address space that its control registers are in (ISA), and not the system address space. Direct Memory Access (DMA) controllers are used to allow controllers to directly access system memory but this access is under strict control and supervision of the system processor.

David A. Rusling