Appeal No. 1998-2767
Application 08/391,541
card systems. Typically, a processor simply writes a pattern
of data to a particular memory location and then reads back
the data from that location to verify proper operation. This
process is then repeated for different patterns and different
memory locations. The invention provides a diagnostic state
machine distinct from the central processor unit. To isolate
a fault between a computer system and an external device
connected to the computer via an external bus, the computer
system simulates a direct memory access (DMA) by the external
device without actually employing the external device. To
accomplish this, the computer system stores self-generated
data and address signals from a back connector that would
otherwise connect to the external device. The method of the
invention is performed by writing a first value to a first
address space defined by addresses on a bus, latching the
first value in a first register with a diagnostic state
machine, writing the first value from the first register to a
second address within the second address space defined by
addresses on the bus, reading a second value from the memory
over the bus, at the second address, and comparing the second
2
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