Datasheet

Functional Description
R
158 Datasheet
Optionally the FIELD pin can indicate to the overlay which field is currently being displayed at
the display device.
6.5.2.10 Intel 855GME GMCH AGP Interface Overview
The GMCH support 1.5 V AGP 1X/2X/4X devices. The AGP signal buffers are 1.5 V
drive/receive (buffers are not 3.3 V tolerant). The GMCH support 2X/4X source synchronous
clocking transfers for read and write data, and sideband addressing. The GMCH also support 2X
and 4X clocking for Fast Writes initiated from the GMCH (on behalf of the processor).
AGP PIPE# or SBA[7:0] transactions to DRAM do not get snooped and are, therefore, not
coherent with the processor caches. AGP FRAME# transactions to DRAM are snooped. AGP
PIPE# and SBA[7:0] accesses to and from the hub interface are not supported. AGP FRAME#
access from an AGP master to the hub interface is also not supported. Only the AGP FRAME
memory writes from the hub interface are supported.
6.5.2.11 AGP Target Operations
As an initiator, the GMCH does not initiate cycles using AGP enhanced protocols. The GMCH
supports AGP cycles targeting interface to main memory only. The GMCH supports interleaved
AGP PIPE#] and AGP FRAME#, or AGP SBA[7:0] and AGP FRAME# transactions.
Table 39. AGP Commands Supported by the GMCH when Acting as an AGP Target
GMCH Host Bridge
AGP Command
C/BE[3:0]#
Encoding
Cycle Destination Response as PCIx Target
Read 0000 Main Memory Low Priority Read
0000 The Hub interface Complete locally with random data; does not go
to the hub interface
Hi-Priority Read 0001 Main Memory High Priority Read
0000 The Hub interface Complete locally with random data; does not go
to the hub interface
Reserved 0010 N/A No Response
Reserved 0011 N/A No Response
Write 0100 Main Memory Low Priority Write
0100 The Hub interface Cycle goes to DRAM with BE’s inactive; does
not go to the hub interface
Hi-Priority Write 0101 Main Memory High Priority Write
0101 The Hub interface Cycle goes to DRAM with BE’s inactive; does
not go to the hub interface
Reserved 0110 N/A No Response
Reserved 0111 N/A No Response
Long Read 1000 Main Memory Low Priority Read
The Hub interface Complete locally with random data; does not go
to the hub interface