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Intel® 945G/945GZ/945GC/ 945P/945PL Express Chipset Family ...

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4.1.25 LAC—Legacy Access Control (D0:F0)<br />

PCI Device: 0<br />

Address Offset: 97h<br />

Default Value: 00h<br />

Access: R/W<br />

Size: 8 bits<br />

This register controls a fixed DRAM hole from 15–16 MB.<br />

Bit Access<br />

&<br />

Default<br />

7 R/W<br />

0 b<br />

6:1 Reserved<br />

0 R/W<br />

0 b<br />

Host Bridge/DRAM Controller Registers (D0:F0)<br />

Description<br />

Hole Enable (HEN): This field enables a memory hole in DRAM space. The DRAM<br />

that lies "behind" this space is not remapped.<br />

0 = No memory hole.<br />

1 = Memory hole from 15 MB to 16 MB.<br />

MDA Present (MDAP): This bit works with the VGA Enable bits in the BCTRL<br />

register of Device 1 to control the routing of processor-initiated transactions targeting<br />

MDA compatible I/O and memory address ranges. This bit should not be set if<br />

device 1's VGA Enable bit is not set.<br />

If device 1's VGA enable bit is not set, accesses to I/O address range x3BCh–x3BFh<br />

are forwarded to the DMI.<br />

If the VGA enable bit is set and MDA is not present, accesses to I/O address range<br />

x3BCh–x3BFh are forwarded to PCI <strong>Express</strong>*, if the address is within the<br />

corresponding IOBASE and IOLIMIT; otherwise, the accesses are forwarded to the<br />

DMI.<br />

MDA resources are defined as the following:<br />

Memory: 0B0000h–0B7FFFh<br />

I/O: 3B4h, 3B5h, 3B8h, 3B9h, 3BAh, 3BFh,<br />

(Including ISA address aliases, A [15:10] are not used in decode)<br />

Any I/O reference that includes the I/O locations listed above, or their aliases, will be<br />

forwarded to the DMI even if the reference includes I/O locations not listed above.<br />

The following table shows the behavior for all combinations of MDA and VGA:<br />

VGAEN MDAP Description<br />

0 0 All References to MDA and VGA space are routed to the<br />

DMI<br />

0 1 Invalid combination<br />

1 0 All VGA and MDA references are routed to PCI <strong>Express</strong><br />

graphics attach.<br />

1 1 All VGA references are routed to PCI <strong>Express</strong> graphics<br />

attach. MDA references are routed to the DMI<br />

VGA and MDA memory cycles can only be routed across the PCI <strong>Express</strong> when<br />

MAE (PCICMD1[1]) is set.<br />

VGA and MDA I/O cycles can only be routed across the PCI <strong>Express</strong> if IOAE<br />

(PCICMD1[0]) is set.<br />

Intel ® 82<strong>945G</strong>/82<strong>945G</strong>/82<strong>945G</strong>C GMCH and 82<strong>945P</strong>/82<strong>945P</strong>L MCH Datasheet 83

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