RG82845SL5YQ Datasheet, PDF (97/148 Pages) Intel Corporation – Intel® 845 Chipset: 82845 Memory Controller Hub (MCH) for SDR

System Address Map
R
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
Additional System
Memory Address
Range
PCI Memory Address
Range
System Mem ory
Address Range
DOS Legacy Address
Range
16 GB
4 GB
Hub Interface
Top of Low
Mem ory
AGP
Graphics
Aperture
1 MB
Independently Program m able
Non-overlapping W indows
I/O
Aperture
APICs
sys_addr_map_1
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.
Intel^® 82845 MCH for SDR Datasheet
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