Register Description
Bit
Description
Upper Programmable Base Address bits (R/W). These bits are used to locate the range size
selected via lower bits 27:4.
31:28
Default = 0000b
Lower “Hardwired”/Programmable Base Address bits. These bits behave as a “hardwired” or
as a programmable depending on the contents of the APSIZE register as defined below:
27
r/w
r/w
r/w
r/w
r/w
r/w
0
26
r/w
r/w
r/w
r/w
r/w
0
25
r/w
r/w
r/w
r/w
0
24
r/w
23
r/w
22
r/w
0
Aperture Size
4 MB
r/w
r/w
r/w
0
8 MB
0
16 MB
0
0
0
32 MB
0
0
0
0
0
0
0
64 MB
27:22
0
0
128 MB
256 MB
0
0
0
Bits 27:22 are controlled by the bits 5:0 of the APSIZE register in the following manner:
If bit APSIZE[5]=0 then APBASE[27]=0 and if APSIZE[5]=1 then APBASE[27]=r/w (read/write).
The same applies correspondingly to other bits.
Default for APSIZE[5:0]=000000b forces default APBASE[27:22] =000000b (i.e., all bits respond
as “hardwired” to 0). This provides a default to the maximum aperture size of 256 MB. The
82443BX specific BIOS is responsible for selecting smaller size (if required) before PCI
configuration software runs and establishes the system address map.
21:4
3
Hardwired to “0”. This forces minimum aperture size selected by this register to be 4MB.
Prefetchable (RO). This bit is hardwired to “1” to identify the Graphics Aperture range as a
prefetchable ( i.e., the device returns all bytes on reads regardless of the byte enables), and the
82443BX may merge processor writes into this range without causing errors.
Type (RO). These bits determine addressing type and they are hardwired to “00” to indicate that
address range defined by the upper bits of this register can be located anywhere in the 32-bit
address space.
2:1
0
Memory Space Indicator (RO). Hardwired to “0” to identify aperture range as a memory range.
3.3.11
SVID—Subsystem Vendor Identification Register (Device 0)
Offset:
Default:
Access:
Size:
2C–2Dh
0000h
Read/Write Once
16 bits
Bit
Description
Subsystem Vendor ID (R/WO). This value is used to identify the vendor of the subsystem. The
default value is 00h. This field should be programmed during boot-up. After this field is written
once, it becomes read only.
15:0
82443BX Host Bridge Datasheet
3-15
ETC [ ETC ]
