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21555 Non-Transparent PCI-to-PCI Bridge User Manual 161
List of Registers
7:6
Subtractive
Decode
Enable
R/W
Controls subtractive decoding for downstream and upstream I/O
transactions. When the 21555 is enabled to perform subtractive decoding
in one direction, those transactions are forwarded to the opposite bus with
no address translation.
Possible values are:
• 00: No subtractive decoding is performed on either interface.
• 01: I/O subtractive decoding enabled on primary interface.
• 10: I/O subtractive decoding enabled on secondary interface.
• 11: Illegal. Results are unpredictable.
Reset value is 00
11:8 Page Size R/W
Selects the page size used for the Upstream Memory 2 address range. The
total size of this range is dependent on the page size. Possible page size
values and their encoding are dependent on the LUT Page Size Extension
bit [12] in the Chip Control 0 register.
When the LUT Page Size Extension bit is 0, the encodings are:
• 0h : Disables the Upstream Memory 2 Base address register.
• 1h : 256 bytes
• 2h : 512 bytes
• 3h : 1 KB
• 4h : 2 KB
• 5h : 4 KB
• 6h : 8 KB
• 7h : 16 KB
• 8h : 32 KB
• 9h : 64 KB
• Ah : 128 KB
• Bh : 256 KB
• Ch : 512 KB
• Dh : 1 MB
• Eh : 2 MB
• Fh : 4 MB
When the LUT Page Size Extension bit is 1, the encodings are:
• 0h : Disables the Upstream Memory 2 Base address register.
• 1h : 8 MB
• 2h : 16 MB
• 3h : 32 MB
• 4h to Fh : Disables the Upstream Memory 2 Base address register.
Reset value is 0h
Table 78. Chip Control 1 Register (Sheet 2 of 3)
This register may be preloaded by serial ROM or programmed by the local processor before host
configuration.
• Primary byte offset: CF:CEh
• Secondary byte offset: CF:CEh
Bit Name R/W Description