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Register Description
R
Intel
®
82845 MCH for SDR Datasheet 73
3.5.33 TOM—Top of Low Memory Register (Device 0)
Address Offset: C4–C5h
Default Value: 0100h
Access: R/W
Size: 16 bits
This register contains the maximum address below 4 GB that should be treated as a memory
access. Note that this register must be set to a value of 0100h (16 MB) or greater. Usually it will
sit below the areas configured for the hub interface, PCI memory, and the graphics aperture.
Bit Description
15:4 Top of Low Memory (TOM). This register contains the address that corresponds to bits 31 to
20 of the maximum system memory address that lies below 4 GB. Configuration software
should set this value to either the maximum amount of memory in the system or to the
minimum address allocated for PCI memory or the graphics aperture, whichever is smaller.
Programming Example: 400h = 1 GB. An access to 4000_0000h or above will be considered
above the TOM and therefore not routed to system memory. It may go to AGP, aperture, or
subtractively decode to the hub interface.
3:0 Reserved.