31:20

0h

RO

Reserved

19:18

10b

RW/O

Optimized Buffer Flush/Fill Supported (OBFFS)

00b: OBFF is not supported.
01b: OBFF is supported using Message signaling only.
10b: OBFF is supported using WAKE# signaling only.
11b: OBFF is supported using WAKE# and Message signaling.
BIOS should program this field to 00b or 10b during system intialization to advertise the level of hardware OBFF support to software. BIOS should never program this field to 01b or 11b since OBFF messaging is not supported.
Note: OBFF is not supported. BIOS should program this field to 00b.

17

0b

RW/O

10-Bit Tag Requester Supported (PX10BTRS)

If this bit is Set, the Function supports 10-Bit Tag Requester capability. Otherwise, the Function does not. This bit must not be Set if the 10-Bit Tag Completer Supported bit is Clear.

16

0b

RW/O

10-Bit Tag Completer Supported (PX10BTCS)

If this bit is Set, the Function supports 10-Bit Tag Completer capability. Otherwise, the Function does not.

15:12

0h

RO

Reserved

11

1b

RW/O

LTR Mechanism Supported (LTRMS)

A value of 1b indicates support for the optional Latency Tolerance Reporting (LTR) mechanism capability.

BIOS must write to this register with either a '1' or a '0' to enable/disable the root port from declaring support for the LTR capability.

10

0h

RO

Reserved

9

0b

RW/O

CAS Completer 128-bit Supported (AC128BS)

Applicable to Functions with Memory Space BARs as well as all Root Ports - must be 0b otherwise. This bit must be set to 1b if the Function supports this optional capability.

8

0b

RW/O

AtomicOp Completer 64-bit Supported (AC64BS)

Applicable to Functions with Memory Space BARs as well as all Root Ports - must be 0b otherwise. Includes FetchAdd, Swap, and CAS AtomicOps. This bit must be set to 1b if the Function supports this optional capability

7

0b

RW/O

AtomicOp Completer 32-bit Supported (AC32BS)

Applicable to Functions with Memory Space BARs as well as all Root Ports - must be 0b otherwise. Includes FetchAdd, Swap, and CAS AtomicOps. This bit must be set to 1b if the Function supports this optional capability

6

0b

RW/O

Atomic Routing Supported (ARS)

This bit must be set to 1b if the Port supports this optional capability

5

1b

RO

ARI Forwarding Supported (AFS)

Applicable only to Switch Downstream Ports and Root Ports - must be 0b for other Function types. This bit must be set to 1b if a Switch Downstream Port or Root Port supports this optional capability.
Note: This bit is not made RWO to simplify implementation, since there is a requirement that the ARI Forwarding Enable bit must be hardwired to 0b if ARI Forwarding Supported bit is 0b. It is low risk to keep this bit 1b.

4

1b

RO

Completion Timeout Disable Supported (CTDS)

A value of 1b indicates support for the Completion Timeout Disable mechanism.

3:0

0111b

RO

Completion Timeout Ranges Supported (CTRS)

This field indicates device support for the optional Completion Timeout programmability mechanism. This mechanism allows system software to modify the Completion Timeout value.

This field is applicable only to Root Ports, Endpoints that issue requests on their own behalf, and PCI Express to PCI/PCI-X Bridges that take ownership of requests issued on PCI Express.

For all other devices this field is reserved and must be hardwired to 0000b.

Four time value ranges are defined:

Range A: 50us to 10ms

Range B: 10ms to 250ms

Range C: 250ms to 4s

Range D: 4s to 64s

Bits are set according to the table below to show timeout value ranges supported.

0000b Completion Timeout programming not supported.

0001b Range A

0010b Range B

0011b Ranges A & B

0110b Ranges B & C

0111b Ranges A, B & C <-- This is what Root Port supports

1110b Ranges B, C & D

1111b Ranges A, B, C & D

All other values are reserved.