System Address Map

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4.1.1VGA and MDA Memory Space

Video cards use these legacy address ranges to map a frame buffer or a character-based video buffer. The address ranges in this memory space are:

VGAA

0_000A_0000 to 0_000A_FFFF

MDA

0_000B_0000 to 0_000B_7FFF

VGAB

0_000B_8000 to 0_000B_FFFF

By default, accesses to these ranges are forwarded to the hub interface. However, if the VGA_EN1 bit is set in the BCTRL1 configuration register, transactions within the VGA and MDA spaces are sent to AGP. If the MCHCFG.MDAP configuration bit is set, accesses that fall within the MDA range are sent to the hub interface independent of the setting of the VGA_EN1 bit.

If the MCHCFG.MDAP configuration bit is set, accesses in the MDA range are sent to the hub interface, independent of the setting of the VGA_EN1 bit. Legacy support requires the ability to have a second graphics controller (monochrome) in the system. In an 845 chipset system, accesses in the standard VGA range are forwarded to AGP. Since the monochrome adapter may be on the hub interface or (or ISA) bus, the MCH must decode cycles in the MDA range and forward them to the hub interface. This capability is controlled by a configuration bit (MCHCFG.MDAP). In addition to the memory range B0000h to B7FFFh, the MCH decodes I/O cycles at 3B4h, 3B5h, 3B8h, 3B9h, 3BAh, and 3BFh and forwards them to the hub interface.

An optimization allows the system to reclaim the memory displaced by these regions. If SMM memory space is enabled by SMRAM.G_SMRARE and either the SMRAM.D_OPEN bit is set or the system bus receives an SMM-encoded request for code (not data), then the transaction is steered to system memory rather than the hub interface. Under these conditions, the VGA_EN1 bit and the MDAP bit are ignored.

Intel® 82845 MCH for SDR Datasheet

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Intel 845 manual VGA and MDA Memory Space, Vgab