82845G Datasheet, PDF (113/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

Functional Description
4.3.1.3
4.3.1.4
4.3.1.5
AGP Transaction Ordering
The GMCH observes transaction ordering rules as defined by the Accelerated Graphics Port
Interface Specification, Revision 2.0. The GMCH implements read after write hazard protection for
normal priority commands through the use of a “pseudo FENCE.” When a normal priority read
command is placed in the command queue, it is checked for possible conflicts with any normal
priority write commands that have been received but not yet delivered to SDRAM. If a potential
conflict is detected, the GMCH inserts a FENCE between the read and all previous normal priority
commands in the queue. This ensures that any normal priority write that was received prior to the
read will be pushed to SDRAM before the read is serviced. As a result the read will be guaranteed
to receive the new data when it is serviced. Note that all reads received prior to the read that
potentially conflicts will also be serviced prior to the conflicting read.
High priority reads and writes are not checked for conflicts between themselves or normal priority
reads and writes. AGP commands (delivered via PIPE# or SBA, not FRAME#) snoop the global
SDRAM write buffer.
AGP Electrical Characteristics
The 4X data transfers use 1.5 V signaling levels as described in the Accelerated Graphics Port
Interface Specification, Revision 2.0. The GMCH supports 1X/2X/PCI data transfers using 1.5 V
signaling levels. The following table shows the data rates and signaling levels supported by the
GMCH:
Data Rate
PCI-66
1X AGP
2X AGP
4X AGP
Signaling Level
1.5 V
Yes
Yes
Yes
Yes
3.3 V
No
No
No
No
Support for PCI-66 Devices
The GMCH’s AGP interface can be used as a PCI-66 MHz interface with the following
restrictions:
• Support for 1.5 V operation only.
• Support for only one device. GMCH does not provide arbitration or electrical support for more
than one PCI-66 device.
• The PCI-66 device must meet the Accelerated Graphics Port Interface Specification, Revision
2.0.
• The GMCH does not provide full PCI-to-PCI bridge support between AGP/PCI and hub
interface. Traffic between AGP and hub interface is limited to hub interface-to-AGP memory
writes.
• LOCK# signal is not present. Neither inbound nor outbound locks are supported.
• SERR#/PERR# signals are not present.
• 16 clock Subsequent Data Latency timer (instead of 8).
Intel^® 82845G/82845GL/82845GV GMCH Datasheet
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