82845MP Datasheet, PDF (105/157 Pages) Intel Corporation – Intel 845 Family Chipset-Mobile 82845MP/82845MZ Chipset Memory Controller Hub Mobile (MCH-M)

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Intel^® 82845MP/82845MZ Chipset-Mobile (MCH-M)
4. System Address Map
A system based on the Intel 845MP/845MZ Chipset supports 4 GB of addressable memory space and 64
KB+3 of addressable I/O space. The I/O and memory spaces are divided by system configuration
software into regions. The memory ranges are useful either as system memory or as specialized memory,
while the I/O regions are used solely to control the operation of devices in the system.
4.1. Memory Address Ranges
The system memory map is broken into two categories:
Extended Memory Range (1MB to 4GB) - Extended memory, existing between 1 MB and 4 GB. It
contains a 32-bit memory space, which is used for mapping PCI, AGP, APIC, SMRAM, and BIOS
memory spaces.
DOS Compatible Area (below 1 MB) - A DOS legacy space, which is used for BIOS and legacy
devices on the LPC interface.
Figure 5. Addressable Memory Space
PCI Memory Address
Range
4 GB
Hublink_A
AGP
Graphics
Aperture
IO
Aperture
APICs
Main Memory Address
Range
DOS legacy Address
Range
Top of Low Memory
1 MB
Independently
Programmable
Non-overlapping
Windows
These address ranges are always mapped to system memory, regardless of the system configuration.
Memory may be carved out of the MAINMEM segment for use by System Management Mode (SMM)
hardware and software. The Top of Low Memory (TOM) register defines the top of Main Memory.
Note: The address of the highest 16-MB quantity of valid memory in the system is placed into the GBA15
register.
For memory populations less than 3 GB, this value will be the same as the one programmed into the
TOM register. For other memory configurations, the two are unlikely to be the same, since the PCI
configuration portion of the BIOS software will program the TOM register to the maximum value that is
less than the amount of memory in the system and that allows enough room for all populated PCI
devices.
250687-002
Datasheet
105