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

Host Bridge/DRAM Controller Registers (D0:F0)
4.1.26 TOLUD—Top of Low Usable DRAM (D0:F0)
PCI Device: 0
Address Offset: 9Ch
Default Value: 08h
Access: R/W
Size: 8 bits
This register defines the Top of Low usable DRAM. TSEG and graphics stolen memory are
within the DRAM space defined. From the top of the DRAM space, the (G)MCH optionally
claims 1 to 8 MBs of DRAM for internal graphics if enabled, and 1, 2, or 8 MB of DRAM for
TSEG if enabled.
Bit Access &
Default
7:3 R/W
01h
Description
Top of Low Usable DRAM (TOLUD): This register contains bits 31:27 of an
address one byte above the maximum DRAM memory that is usable by the
operating system. Address bits 31:27 programmed to 01h implies a minimum
memory size of 128 MBs.
Configuration software must set this value to the smaller of the following 2 choices:
� Maximum amount of memory in the system plus one byte or
� Minimum address allocated for PCI memory
Address bits 26:0 are assumed to be 000_0000h for the purposes of address
comparison. The Host interface positively decodes an address intended for DRAM
if the incoming address is less than the value programmed in this register.
If this register is set to 0000 0b, it implies 128 MBs of system memory.
2:0
NOTE: The Top of Low Usable memory is the lowest address above both graphics
stolen memory and TSEG. The host interface determines the base of
graphics stolen memory by subtracting the Graphics Stolen Memory Size
from TOLUD and further decrements by 1 MB to determine the base of
TSEG.
Reserved
Programming Example (82945G GMCH only):
� C1DRB7 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 1GB 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, TOLUD is originally calculated to: 4 GB = 1_0000_0000h
The system memory requirements are:
� 4 GB (max addressable space) – 1 GB (PCI space) – 20 MB (lost memory) =
3 GB – 128 MB (minimum granularity) = B800_0000h
� Since B800_0000h (PCI and other system requirements) is less than 1_0000_0000h; TOLUD
should be programmed to B8h.
84 Intel ® 82945G/82945GZ/82945GC GMCH and 82945P/82945PL MCH Datasheet