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Software Developer’s Manual 83
PCI Local Bus Interface
4.1.3 Message Signaled Interrupts
1
Message Signaled Interrupt (MSI) capability is optional for PCI 2.2 or 2.3, but required for PCI-X.
When Message Signaled Interrupts are enabled, instead of asserting an interrupt pin, the Ethernet
controller generates an interrupt using a memory write command. The address and most of the data
of the command are determined by the system and programmed in configuration registers. This
permits the system to program a different message for each function so it can speed up interrupt
delivery.
To enable Message Signaled Interrupts, the system software writes to the “MSI Enable” bit in the
MSI “Message Control” register. When Message Signaled Interrupts are enabled, the Ethernet
controller no longer asserts its INTA# pin to signal interrupts.
MSI systems allow a function to request up to 32 messages, but does not guarantee that all of them
are allocated. The Ethernet controller supports only a single message. When Message Signaled
Interrupts are enabled, the Ethernet controller generates a message when any of the unmasked bits
in the Interrupt Cause Read register (ICR) are set to 1b. The Ethernet controller does not generate
the message again until the ICR is read and a subsequent interrupt event occurs.
In conventional PCI mode, Message Signaled Interrupts can also be disabled in the EEPROM. If
MSI is disabled, the Message Signaled Interrupt registers is not visible.
4.1.3.1 Message Signaled Interrupt Configuration Registers
Figure 4-3. Message Signaled Interrupt Configuration Registers
4.1.3.1.1 MSI Capability ID
4.1.3.1.2 Next Capability
1. Not applicable to the 82541xx or 82547GI/EI.
Byte Offset Byte 3 Byte 2 Byte 1 Byte 0
F0h Message Control Next Capability
MSI
Capability ID
F4h Message Address
F8h Message Upper Address
FCh Reserved Message Data
Bits
Read/
Write
Initial
Value
Description
7:0 R 05h
Capability ID - Identifies the Message Signaled Interrupt register set in
the capabilities linked list.
Bits
Read/
Write
Initial
Value
Description
7:0 R 00h
Next Capability – points to the next capability in the capabilities
linked list. Its value is 0b since the Message Signaled Interrupt is the
last item in the list.