82845G Datasheet, PDF (42/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

Register Description
Term
Description
Reserved
Bits
Some of the GMCH registers described in this chapter contain reserved bits. These bits are
labeled Reserved (Rsvd). Software must deal correctly with fields that are reserved. On reads,
software must use appropriate masks to extract the defined bits and not rely on reserved bits
being any particular value. On writes, software must ensure that the values of reserved bit
positions are preserved. That is, the values of reserved bit positions must first be read, merged
with the new values for other bit positions, and then written back. Note the software does not
need to perform read, merge, and write operations for the configuration address register.
Reserved
Registers
Default
Value upon
Reset
In addition to reserved bits within a register, the GMCH contains address locations in the
configuration space of the Host-Hub Interface Bridge entity that are marked either “Reserved” or
“Intel Reserved.” The GMCH responds to accesses to “Reserved” address locations by
completing the host cycle. When a “Reserved” register location is read, a zero value is returned.
(“Reserved” registers can be 8-, 16-, or 32-bits in size). Writes to “Reserved” registers have no
effect on the GMCH. Registers that are marked as “Intel Reserved” must not be modified by
system software. Writes to “Intel Reserved” registers may cause system failure. Reads to “Intel
Reserved” registers may return a non-zero value.
Upon Reset, the GMCH sets all of its internal configuration registers to predetermined default
states. Some register values at reset are determined by external strapping options. The default
state represents the minimum functionality feature set required to successfully bring up the
system. Hence, it does not represent the optimal system configuration. It is the responsibility of
the system initialization software (usually BIOS) to properly determine the DRAM configurations,
operating parameters and optional system features that are applicable, and to program the
GMCH registers accordingly.
3.2
Platform Configuration
In some previous chipsets the MCH (or GMCH) component and the I/O Controller Hub component
were physically connected by PCI bus #0. From a configuration standpoint, both components
appeared to be on PCI bus #0 which was also the system’s primary PCI expansion bus. The north
bridge contained two PCI devices while the south bridge was considered one PCI device with
multiple functions.
In the 845G chipset the configuration structure is significantly different. The GMCH and the ICH4
are physically connected by the hub interface; thus, from a configuration standpoint, the hub
interface is logically PCI bus #0. As a result, all devices internal to the GMCH and ICH4 appear to
be on PCI bus #0. The system’s primary PCI expansion bus is physically attached to the ICH4 and,
from a configuration perspective, appears to be a hierarchical PCI bus behind a PCI-to-PCI bridge
and therefore has a programmable PCI Bus number. The AGP appears to system software to be a
real PCI bus behind PCI-to-PCI bridges resident as devices on PCI bus #0.
Note: The primary PCI bus is referred to as PCI_A in this document and is not PCI bus #0 from a
configuration standpoint.
The GMCH contains the following PCI devices within a single physical component. The
configuration registers for the devices are mapped as devices residing on PCI bus #0.
• Device 0: Host-HI Bridge/DRAM controller. Logically this appears as a PCI device residing
on PCI bus #0. Physically Device 0 contains the standard PCI registers, DRAM registers, AGP
capabilities registers, the Graphics Aperture controller, and other GMCH specific registers.
• Device 1: Host-AGP Bridge. Logically this appears as a “virtual” PCI-to-PCI bridge residing
on PCI bus #0. Physically Device 1 contains the standard PCI-to-PCI bridge registers and the
standard AGP/PCI configuration registers (including the AGP I/O and memory address
mapping).
• Device 2: Integrated Graphics controller. Logically this appears as a PCI device residing on
PCI bus #0. Physically Device 2 contains the configuration registers for 3D, 2D and display
functions.
• Device 6: Intel Reserved.
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Intel^® 82845G/82845GL/82845GV GMCH Datasheet