PCI Bus Interface
Initiator Transfers
Copyright 1997-2000, V3 Semiconductor Inc.
EPC User
’
s Manual Revision 1.05
61
7.2.4
PCI Exceptions During EPC Initiated Cycles
There are two categories of exceptions that can be generated on the PCI bus:
Fatal Exceptions:
These are conditions from which recovery is not possible such as Master
Abort (attempted access to non-existent device) and Target Abort (unrecoverable error
within a target).
Recoverable Exceptions:
These are exceptions from which recovery is guaranteed such
as Disconnect and Retry. A target disconnect, for example, is used by a target to discontinue
a burst that is longer than it can accommodate in a single transfer. This exception is
recovered by simply starting another PCI cycle to complete the transaction.
Fatal exceptions are reported to the local processor by a processor interrupt. Recoverable
exceptions are handled transparently by the EPC.
7.2.4.1
Fatal Exception: Master Abort (Reads)
A Master Abort occurs when the EPC does not receive a DEVSEL in response to a read or
write attempt. This typically results from an attempt to access a non-existent device.
The EPC responds to Master Abort conditions during a read as follows:
READY is returned to the local processor to
“
unlock
”
the local bus. The data
returned is FFFFFFFFH. Optionally, a PCI read exception interrupt can be
generated.
The M_ABORT bit in the PCI_STAT register is set.
7.2.4.2
Fatal Exception: Master Abort (Writes)
Since PCI writes are posted by the EPC, it is conceivable that a Master Abort exception can
occur long after the local bus write completes. In this case it makes no sense to generate a
bus error on the local bus.
The EPC responds to Master Abort conditions during a write as follows:
A PCI write exception interrupt is generated (maskable).
The M_ABORT bit in the PCI_STAT register is set.
7.2.4.3
Fatal Exception: Target Abort (Reads)
A Target Abort occurs when the EPC receives a target abort indication from the currently
addressed target. Target Abort is only used in the most extreme cases since it implies that
the target is in the system (i.e. a DEVSEL is received), however it is permanently incapable
of responding to the request.
The EPC responds to Target Abort conditions during a read as follows: