
System Address Map
R
4.1.1VGA and MDA Memory Space
Video cards use these legacy address ranges to map a frame buffer or a
• | 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
Intel® 82845 MCH for SDR Datasheet | 99 |