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21555 Non-Transparent PCI-to-PCI Bridge User Manual 145
List of Registers
Table 51. Downstream I/O Data and Upstream I/O Data Registers
The Downstream I/O Data register is used for I/O transactions to be initiated on the secondary
bus, and the Upstream I/O Data register is used for I/O transactions to be initiated on the primary
bus. The downstream register can be written from the primary interface only and the upstream
register can be written from the secondary interface only. A downstream transaction is initiated
by a primary interface I/O register access only and an upstream transaction is initiated by a
secondary interface I/O access only.
Bit Name R/W Description
31:0
IO_DATA
(ID)
DID:
R/(WP)
UID:
R/(WS)
Res (Mem)
This register contains the write data driven or the read data returned
from an I/O transaction initiated on the target bus. The Downstream or
Upstream I/O Address register contains the address for this
transaction. The transaction is initiated when this register is written (for
an I/O write) or read (for an I/O read) using an I/O transaction. This
register is treated as a reserved register for all memory accesses. The
byte enables used for this register access are the same byte enables
used for the transaction driven on the target bus. A target retry is
returned to the initiator until the transaction has been completed on
the target bus.
Reset value is 0
Table 52. I/O Own Bits Registers
• Byte Offset: 025:024h
Bit Name R/W Description
0
Downstream
I/O Own Bit
R0TS (P)
R (S)
Indicates ownership of the Downstream I/O Address and
Downstream I/O Data registers.
• When 0, downstream I/O Address and Downstream I/O Data
registers are not owned. When read as a 0 from the primary
interface, this bit is subsequently set to a 1 by the 21555.
• When 1, downstream I/O Address and Downstream I/O Data
registers are owned by a master. When the semaphore method is
used, other masters should not attempt to access these registers
when this bit is a 1. This bit is automatically cleared once the I/O
transaction has completed on the initiator bus.
• Reset value is 0.
7:1 Reserved R Read only as 0.
8
Upstream I/
O Own Bit
R0TS (S)
R (P)
Indicates ownership of the Upstream I/O Address and Upstream I/O
Data registers.
• When 0, upstream I/O Address and Upstream I/O Data registers
are not owned. When read as a 0 from the secondary interface,
this bit is subsequently set to a 1 by the 21555.
• When 1, upstream I/O Address and Upstream I/O Data registers
are owned by a master. When the semaphore method is used,
other masters should not attempt to access these registers when
this bit is a 1. This bit is automatically cleared once the I/O
transaction has completed on the initiator bus.
• Reset value is 0.
15:9 Reserved R Read only as 0.
Offsets Downstream I/O Data Upstream I/O Data
Byte 01B:018h 023:020h