VT8601 Apollo ProMedia
Revision 1.3 September 8, 1999
-48-
Device 0 Bus 0 Host Bridge Registers
7HFKQRORJLHV ,QF
:H&
:H&RRQQHFW
QQHFW
GART / Graphics Aperture Control
The function of the Graphics Address Relocation Table
(GART) is to translate virtual 32-bit addresses issued by an
AGP device into 4K-page based physical addresses for system
memory access. In this translation, the upper 20 bits (A31-
A12) are remapped, while the lower 12 address bits (A11-A0)
are used unchanged.
A one-level fully associative lookup scheme is used to
implement the address translation. In this scheme, the upper
20 bits of the virtual address are used to point to an entry in a
page table located in system memory. Each page table entry
contains the upper 20 bits of a physical address (a "physical
page" address).
For simplicity, each page table entry is 4
bytes. The total size of the page table depends on the GART
range (called the "aperture size") which is programmable in
the VT8601.
This scheme is shown in the figure below.
31
12 11
0
Virtual Page Address
Page Offset
index
TLB Base
Page Table
31
12 11
0
Physical Page Address
Page Offset
Figure 4. Graphics Aperture Address Translation
Since address translation using the above scheme requires an
access to system memory, an on-chip cache (called a
"Translation Lookaside Buffer" or TLB) is utilized to enhance
performance. The TLB in the 82C501 contains 16 entries.
Address "misses" in the TLB require an access of system
memory to retrieve translation data. Entries in the TLB are
replaced using an LRU (Least Recently Used) algorithm.
Addresses are translated only for accesses within the
"Graphics Aperture" (GA). The Graphics Aperture can be any
power of two in size from 1MB to 256MB (i.e., 1MB, 2MB,
4MB, 8MB, etc). The base of the Graphics Aperture can be
anywhere in the system virtual address space on an address
boundary determined by the aperture size (e.g., if the aperture
size is 4MB, the base must be on a 4MB address boundary).
The Graphics Aperture Base is defined in register offset 10 of
device 0. The Graphics Aperture Size and TLB Table Base
are defined in the following register group (offsets 84 and 88
respectively) along with various control bits.
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