82845PE Datasheet, PDF (107/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

System Address
PCI Memory Address Range (Top of Main Memory to 4 GB)
The address range from the top of main memory to 4 GB (top of physical memory space supported
by the (G)MCH) is normally mapped via the hub interface to PCI.
As an internal graphics configuration (82845GE only), there are two exceptions to this rule:
• Addresses decoded to graphics configuration registers.
• Addresses decoded to the Memory Mapped Range of the Internal Graphics Device.
Both exception cases are forwarded to the Internal Graphics Device.
As an AGP configuration, there are two exceptions to this rule:
• Addresses decoded to the AGP Memory Window defined by the MBASE, MLIMIT,
PMBASE, and PMLIMIT registers are mapped to AGP.
• Addresses decoded to the Graphics Aperture range defined by the APBASE and APSIZE
registers are mapped to the main SDRAM.
Warning:
There are two sub-ranges within the PCI memory address range defined as APIC configuration
space and High BIOS address range. As an Internal Graphics Device (82845GE only), the
memory-mapped range of the Internal Graphics Device must not overlap with these two ranges.
Similarly, as an AGP device, the AGP memory window and graphics aperture window must not
overlap with these two ranges. These ranges are described in detail in the following paragraphs.
APIC Configuration Space (FEC0_0000h–FECF_FFFFh, FEE0_0000h–FEEF_FFFFh)
This range is reserved for APIC configuration space which includes the default I/O APIC
configuration space. The default local APIC configuration space is FEE0_0000h to FEEF_0FFFh.
Processor accesses to the local APIC configuration space do not result in external bus activity since
the local APIC configuration space is internal to the processor. However, an MTRR must be
programmed to make the local APIC range uncacheable (UC). The local APIC base address in each
processor should be relocated to the FEC0_0000h (4GB-20MB) to FECF_FFFFh range so that one
MTRR can be programmed to 64 KB for the Local and I/O APICs. The I/O APIC(s) usually reside
in the ICH4 portion of the chipset or as a stand-alone component(s).
I/O APIC units will be located beginning at the default address FEC0_0000h. The first I/O APIC
will be located at FEC0_0000h. Each I/O APIC unit is located at FEC0_x000h where x is I/O APIC
unit number 0 through F(hex). This address range will be normally mapped to Hub Interface.
Note: There is no provision to support an I/O APIC device on AGP.
The address range between the APIC configuration space and the High BIOS (FED0_0000h to
FFDF_FFFFh) is always mapped to the hub interface.
High BIOS Area (FFE0_0000h–FFFF_FFFFh)
The top 2 MB of the extended memory region is reserved for system BIOS (High BIOS), extended
BIOS for PCI devices, and the A20 alias of the system BIOS. The processor begins execution from
the High BIOS after reset. This region is mapped to the hub interface so that the upper subset of
this region aliases to 16 MB–256 KB range. The actual address space required for the BIOS is less
than 2 MB but the minimum processor MTRR range for this region is 2 MB so that the full 2 MB
must be considered.
Intel^® 82845GE/82845PE Datasheet
107