Filter
Top Products
System Address Map
R
Intel
®
82845 MCH for SDR Datasheet 97
4 System Address Map
A system based on the 845 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.
When the MCH receives a write request whose address targets an invalid space, the data is
ignored. For reads, the MCH responds by returning all zeros on the requesting interface.
4.1 Memory Address Ranges
The system memory map is broken into two categories:
• Extended Memory Range (1 MB to 4 GB). The second is extended memory, existing
between 1MB and 4GB. 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). The final range is a DOS legacy space, which is used
for BIOS and legacy devices on the LPC interface.
Figure 3. Addressable Memory Space
DOS Legacy Address
Range
System Memory
Address Range
PCI Memory Address
Range
Top of Low
Memory
1 MB
4 GB
Hub Interface
AGP
Graphics
Aperture
I/O
Aperture
APICs
Independently Programmable
Non-overlapping Windows
sys_addr_map_1
Additional System
Memory Address
Range
16 GB
These address ranges are always mapped to system memory, regardless of the system
configuration. Memory may be taken out of the system memory segment for use by System
Management Mode (SMM) hardware and software. The Top of Low Memory (TOM) register
defines the top of system memory.
Note that the address of the highest 16 MB quantity of valid memory in the system is placed into
the GBA15 register. For memory populations <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.