RG82855GMESL72L Datasheet, PDF (127/213 Pages) Intel Corporation – Intel® 855GM/855GME Chipset Graphics and Memory Controller Hub (GMCH)

5.3
Intel® 855GM/GME GMCH System Address Map
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Monochrome Display Adapter (MDA) Range (0B0000h - 0B7FFFh)
Legacy support requires the ability to have a second graphics controller (monochrome) in the
system. Accesses in the standard VGA range are forwarded to IGD and the Hub interface
(depending on configuration bits). Since the monochrome adapter may be mapped to anyone of
these devices, the GMCH must decode cycles in the MDA range and forward them either to IGD
or to Hub interface. This capability is controlled by a VGA steering bits and the legacy
configuration bit (MDAP bit). In addition to the system memory range B0000h to B7FFFh, the
GMCH decodes IO cycles at 3B4h, 3B5h, 3B8h, 3B9h, 3Bah, and 3BFh and forwards them to the
either the IGD or the Hub interface.
Expansion Area (0C0000h-0DFFFFh)
This 128-kB ISA Expansion region is divided into eight, 16-kB segments. Each segment can be
assigned one of four Read/Write states: read-only, write-only, read/write, or disabled. Typically,
these blocks are mapped through GMCH and are subtractively decoded to ISA space. System
memory that is disabled is not remapped.
Extended System BIOS Area (0E0000h-0EFFFFh)
This 64-kB area is divided into four, 16-kB segments. Each segment can be assigned independent
read and write attributes so it can be mapped either to main DDR SDRAM or to Hub interface.
Typically, this area is used for RAM or ROM. System memory segments that are disabled are not
remapped elsewhere.
System BIOS Area (0F0000h-0FFFFFh)
This area is a single 64-kB segment. This segment can be assigned Read and Write attributes. It
is by default (after Reset) Read/Write disabled and cycles are forwarded to Hub interface. By
manipulating the Read/Write attributes, the GMCH can “shadow” BIOS into the main DDR
SDRAM. When disabled, this segment is not remapped.
Extended System Memory Area
This system memory area covers 100000h (1 MB) to FFFFFFFFh (4 GB-1) address range and it
is divided into the following regions:
• Main system memory from 1 MB to the top of system memory.
• PCI Memory space from the top of system memory to 4 GB with two specific ranges.
• APIC Configuration Space from FEC0_0000h (4 GB–20 MB) to FECF_FFFFh (4 GB–19
MB -1) and FEE0_0000h (4 GB–18 MB) to FEEF_FFFFh (4 GB–17 MB-1).
• High BIOS area from 4 GB to 4 GB - 2 MB
Datasheet
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