HP D315 manual AGP BUS Overview, BUS Transactions

Technical Reference Guide

4.3AGP BUS OVERVIEW

NOTE: For a detailed description of AGP bus operations refer to the AGP Interface Specification Rev. 2.0 available at the following AGP forum web site: http://www.agpforum.org/index.htm

The Accelerated Graphics Port (AGP) bus is specifically designed as an economical yet high- performance interface for graphics adapters, especially those designed for 3D operations. The AGP interface is designed to give graphics adapters dedicated pipelined access to system memory for the purpose of off-loading texturing, z-buffering, and alpha blending used in 3D graphics operations. By off-loading a large portion of 3D data to system memory the AGP graphics adapter only requires enough memory for frame buffer (display image) refreshing.

4.3.1 BUS TRANSACTIONS

The operation of the AGP bus is based on the 66-MHz PCI specification but includes additional mechanisms to increase bandwidth. During the configuration phase the AGP bus acts in accordance with PCI protocol. Once graphics data handling operation is initiated, AGP-defined protocols take effect. The AGP graphics adapter acts generally as the AGP master, but can also behave as a “PCI” target during fast writes from the PCI bus controller.

Key differences between the AGP interface and the PCI interface are as follows:

♦Address phase and associated data transfer phase are disconnected transactions. Addressing and data transferring occur as contiguous actions on the PCI bus. On the AGP bus a request for data and the transfer of data may be separated by other operations.

♦Commands on the AGP bus specify system memory accesses only. Unlike the PCI bus, commands involving I/O and configuration are not required or allowed. The system memory address space used in AGP operations is the same linear space used by PCI memory space commands, but is further specified by the graphics address re-mapping table (GART) of the north bridge component.

♦Data transactions on the AGP bus involve eight bytes or multiples of eight bytes. The AGP memory addressing protocol uses 8-byte boundaries as opposed to PCI’s 4-byte boundaries. If a transfer of less than eight bytes is needed, the remaining bytes are filled with arbitrary data that is discarded by the target.

♦Pipelined requests are defined by length or size on the AGP bus. The PCI bus defines transfer lengths with the FRAME- signal.

There are two basic types of transactions on the AGP bus: data requests (addressing) and data transfers. These actions are separate from each other.

Compaq D315 and hp d325 Personal Computers 4-9

Featuring the AMD Athlon XP Processor

Second Edition - April 2003