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Intel
82865G/82865GV GMCH Datasheet
81
Register Description
3.5.30
TOUD—Top of Used DRAM Register (Device 0)
Address Offset:
Default Value:
Access:
Size:
C4–C5h
0400h
RO, R/W
16 bits
Bit
Descriptions
15:3
Top of Usable DRAM (TOUD)—R/W.
This register contains bits 31:19 of the maximum system
memory address that is usable by the operating system. Address bits 31:19 imply a memory
granularity of 512 KB. Configuration software should set this value to either the maximum amount
of usable memory (minus TSEG, graphics stolen memory, and CSA stolen memory) in the system
or to the minimum address allocated for PCI memory or the graphics aperture (minus TSEG,
graphics stolen memory), whichever is smaller. Address bits 18:0 are assumed to be 0000h for the
purposes of address comparison.
This register must be set to at least 0400h for a minimum of 64 MB of system memory. To calculate
the value of TOUD, configuration software should set this value to the
smaller
of the following 2
cases:
The maximum amount of usable memory in the system minus optional TSEG, optional
graphics stolen memory.
The address allocated for PCI memory or the graphics aperture minus optional TSEG,
optional graphics stolen memory.
Programming Example
:
DRB7 is set to 4 GB.
TSEG is enabled and TSEG size is set to 1 MB.
Internal Graphics is enabled and Graphics Mode Select is set to 32 MB.
BIOS knows the OS requires 1 GB of PCI space.
BIOS also knows the range from FEC0_0000h to FFFF_FFFFh is not usable by the
system. This 20-MB range at the very top of addressable memory space is lost to APIC.
According to the above equation, TOUD is originally calculated to:
4 GB (DRB7) – 1 MB (TSEG) – 32 MB (graphics) = FDF0_0000h
The system memory requirements are:
4 GB (max addressable space) – 1 GB (PCI space) – 33 MB (TSEG, graphics)
– 20 MB (lost memory) = BCB0_0000h
Since BCB0_0000h (PCI and other system memory requirements) is less than FDF0_0000h,
TOUD should be programmed to BCB0_0000h.
NOTE:
Even if the OS does not need any PCI space, TOUD should never be programmed above
FEC0_0000h. If TOUD is programmed above this, address ranges that are reserved will
become accessible to applications.
2:0
Reserved.