AMD-761 datasheet(97/264 Pages) AMD

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Chapter 2

AMD-761™ System Controller Programmer’s Interface

24081D—February 2002

AMD-761™ System Controller Software/BIOS Design Guide

Preliminary Information

Programming Notes

Bit Definitions

AGP Virtual Address Space Size (Dev0:F0:0xAC)

Bit

Name

Function

31–17

Reserved

Vga_IA_En

ISA Address Aliasing Enable

When set, this bit forces the AMD-761™ system controller to alias ISA addresses, which

means that address bits [15:10] are not used in decoding. When clear, no ISA aliasing is

performed and address bits [15:10] are used for decoding.

15–4

Reserved

3–1

VA_Size

Virtual Address Size

This field defines the virtual address space size to be allocated to GART by the system

BIOS. Prior to the execution of the system BIOS memory mapping software, system BIOS

gets the amount of GART virtual address space required by the graphics controller. It sets

these bits to the required value. Changing these bits automatically changes bits [30:25] in

the host-PCI bridge (device 0) AGP Virtual Address Space register, offset 0x10 (see

“Dev0:F0:0x10” on page 37).

The size of GART virtual address space is always greater than or equal to the amount of

physical system memory allocated to AGP in non-contiguous 4-Kbyte blocks. The amount

of physical memory allocated to AGP is determined by operating system software.

[3]

[2]

[1]

VA_Size

32 Mbytes

64 Mbytes

128 Mbytes

256 Mbytes

512 Mbytes

1 Gbytes

2 Gbytes

GART_En

GART Enable

When clear, GART is not valid in this system. System BIOS does not allocate virtual address

space for GART because the host-PCI bridge (device 0) AGP virtual address space, offset

0x10 (see “Dev0:F0:0x10” on page 37) is set to 0. The PCI-PCI bridge (device 1) capabilities

pointer is set to point to the next item in the linked list or null if there is no other item. This

bit is set by BIOS PCI enumeration routines.

When set, GART is valid in this system. System BIOS allocates virtual address space for

GART based upon the value in bits [3:1] above.

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