Address Spaces, Transaction Type

Memory Transactions

Memory Transactions include the following types:

• Read Request/Completion

• Write Request

• AtomicOp Request/Completion

Memory Transactions use two different address formats:

• Short Address Format: 32-bit address

• Long Address Format: 64-bit address

Certain Memory Transactions can optionally have a PASID TLP Prefix containing the Process Address Space ID (PASID).

I/O Transactions

PCI Express supports I/O Space for compatibility with legacy devices that require their use. Future revisions of this

specification may deprecate the use of I/O Space. I/O Transactions include the following types:

• Read Request/Completion

• Write Request/Completion

I/O Transactions use a single address format:

• Short Address Format: 32-bit address

Configuration Transactions

Configuration Transactions are used to access configuration registers of Functions within devices.

Configuration Transactions include the following types:

• Read Request/Completion

• Write Request/Completion

Message Transactions

The Message Transactions, or simply Messages, are used to support in-band communication of events between devices.

In addition to specific Messages defined in this document, PCI Express provides support for vendor-defined Messages

using specified Message codes. Except for Vendor-Defined Messages that use the PCI-SIG® Vendor ID (0001h), the

definition of specific vendor-defined Messages is outside the scope of this document.

This specification establishes a standard framework within which vendors can specify their own Vendor-Defined

Messages tailored to fit the specific requirements of their platforms (see Section 2.2.8.6 ).

Note that these vendor-defined Messages are not guaranteed to be interoperable with components from different

vendors.