| 摘要 |
An integrated processor is provided that employs an improved address decoding method during bus cycles of an external master. An external PCI master may initiate a cycle (either memory or I/O) on the PCI bus by asserting an address signal on the PCI bus along with the FRAME signal which indicates the start of the PCI cycle. After the address becomes stable, the bus interface unit transfers the address signal to the CPU local bus. The bus interface unit does not assert or drive the address strobe signal ADS at this time, however, and thus a CPU local bus cycle is not initiated. The decode logic within the memory or I/O control unit responsively decodes the address signal to determine whether the address is mapped within the address space of the respective control unit. If the address is not within the mapped space of the respective control unit, the control unit does not assert the hit signal, and thus the bus interface unit does not initiate a corresponding CPU local bus cycle and does not drive the PCI device select signal DEVSEL. The PCI bus cycle may then proceed normally in that either another PCI slave may assert the device select DEVSEL signal to indicate that it is responding to the cycle, or the cycle will be aborted by the PCI master. If the address is determined to be within the mapped space of the respective control unit, the control unit asserts the hit signal to inform the bus interface unit that the address is mapped to a device situated on the CPU local bus. The bus interface unit accordingly asserts the device select signal DEVSEL to claim the current cycle, and initiates a corresponding cycle, memory or I/O cycle, or the CPU local bus.
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