Interrupts
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6.3
Message Signaled Interrupt (MSI) Support
One of the interrupt schemes supported by the PEX 8532 is the MSI mechanism, which is required for
PCI Express devices. The MSI method uses Memory Write transactions to deliver interrupts. MSIs are
edge-triggered interrupts.
Note: MSI and INTx are mutually exclusive. These interrupt mechanisms cannot
be simultaneously enabled.
6.3.1
MSI Operation
At configuration time, system software traverses the function Capability list. If a Capability ID of 05h is
found, the function implements MSI. System software reads the MSI Capability Structure registers to
determine function capabilities.
The PEX 8532 supports only one message for MSI; therefore, the Message Control register Multiple
Message Enable and Multiple Message Capable fields (offset 48h[22:20, 19:17], respectively) are
always cleared to 000b.
The Message Control register MSI 64-Bit Address Capable bit is enabled (offset 48h[23]=1),
by default.
System software initializes the MSI Capability Structure registers with a system-specified message.
If the MSI function is enabled, after an Interrupt event occurs, the Interrupt Generation module
generates a DWord Memory Write to the address specified by the Message Address[31:0] register
(offset 4Ch) contents. Data written is the contents of the Message Data register (offset 54h) lower two
bytes and zeros (0) in the upper two bytes. Because the Message Control register Multiple Message
Enable field (offset 48h[22:20]) field is always cleared to 000b, the Interrupt Generation module is not
allowed to change the low-order bits of Message data.
When the Hot Plug and/or device-specific error events that caused the interrupt are serviced, the device
can generate a new MSI Memory Write as a result of new events.
6.3.2
MSI Capability Registers
MSI Capability registers are described in Section 11.8, “Message Signaled Interrupt Capability
Registers.”
84
ExpressLane PEX 8532AA/BA/BB/BC 8-Port/32-Lane Versatile PCI Express Switch Data Book
Copyright © 2007 by PLX Technology, Inc. All Rights Reserved – Version 1.6
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