ID 683501
Date 12/19/2022
Public

## 4.3.3.3.1. Implementing MSI-X Interrupts

Section 6.8.2 of the PCI Local Bus Specification describes the MSI-X capability and table structures. The MSI-X capability structure points to the MSI-X Table structure and MSI-X Pending Bit Array (PBA) registers. The BIOS sets up the starting address offsets and BAR associated with the pointer to the starting address of the MSI-X Table and PBA registers.
MSI-X Interrupt Components
1. Host software sets up the MSI-X interrupts in the Application Layer by completing the following steps:
1. Host software reads the Message Control register at 0x050 register to determine the MSI-X Table size. The number of table entries is the <value read> + 1.
The maximum table size is 2048 entries. Each 16-byte entry is divided in 4 fields as shown in the figure below. The MSI-X table can be accessed on any BAR configured. The base address of the MSI-X table must be aligned to a 4 KB boundary.
2. The host sets up the MSI-X table. It programs MSI-X address, data, and masks bits for each entry as shown in the figure below.
Figure 33. Format of MSI-X Table
3. The host calculates the address of the <n th > entry using the following formula:

nth_address = base address[BAR] + 16<n>
2. When Application Layer has an interrupt, it drives an interrupt request to the IRQ Source module.
3. The IRQ Source sets appropriate bit in the MSI-X PBA table.
The PBA can use qword or dword accesses. For qword accesses, the IRQ Source calculates the address of the <m th > bit using the following formulas:
qword address = <PBA base addr> + 8(floor(<m>/64))
qword bit = <m> mod 64
Figure 34. MSI-X PBA Table
4. The IRQ Processor reads the entry in the MSI-X table.
1. If the interrupt is masked by the Vector_Control field of the MSI-X table, the interrupt remains in the pending state.
2. If the interrupt is not masked, IRQ Processor sends Memory Write Request to the TX slave interface. It uses the address and data from the MSI-X table. If Message Upper Address = 0, the IRQ Processor creates a three-dword header. If the Message Upper Address > 0, it creates a 4-dword header.
5. The host interrupt service routine detects the TLP as an interrupt and services it.