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164 21555 Non-Transparent PCI-to-PCI Bridge User Manual
List of Registers
Table 80. Generic Own Bits Register
The 21555 implements two generic own bits that can be accessed in either memory or I/O space from either
the primary or secondary interface. These bits may be used as an aid to lock resources in software. When a
bus master reads the Own bit, it returns 1 if it has already been set, or it returns 0 if the Own bit is available
and then automatically sets the bit upon completion of the read. The Own bit is cleared by writing a 1 to the bit.
A read
-only shadow copy of the bit can be read to check the status of an Own bit without causing the bit to
set.
Own bit 0 is bit [0] at CSR offset 0D0h, bits [7:1] are reserved. Own bit 1 is bit [0] at CSR offset 0D1h, bits [7:1]
are reserved. Shadow copies of these Own bits may be found at bits [1:0] at CSR offset 0D2h.
These bits may be used as generic own bits, or semaphores. Setting or clearing these bits has no
direct hardware effect on other
21555 functions. Byte reads of this location are suggested to avoid
unintended side-effects
.
• Byte offsets: 0D2:0D0h
Bit Name R/W Description
0
Generic
Own Bit 0
ROTS/
W1TC
Generic own bit 0. This bit may be used as a semaphore by either
primary or secondary bus masters. When read, current value is
returned, and then the bit is automatically set to a 1 if the value read
is 0, or kept as a 1 if the value read was 1.
Writing a 1 clears this bit to a 0.
Writing a 0 has no effect.
Generic Own Status 0 can be read to check the state of this bit
without side effects.
Reset value is 0
7:1 Reserved R Reserved. Returns 0 when read.
8
Generic
Own Bit 1
ROTS/
W1TC
Generic own bit 1. This bit may be used as a semaphore by either
primary or secondary bus masters. When read, current value is
returned, and then the bit is automatically set to a 1 if the value read
is 0, or kept as a 1 if the value read was 1.
Writing a 1 clears this bit to a 0.
Writing a 0 has no effect.
Generic Own Status 1 can be read to check the state of this bit
without side effects.
Reset value is 0
15:9 Reserved R Reserved. Returns 0 when read.
16
Generic
Own 0
Status
R
Returns the current state of Own Bit 0. When checking the state of
this bit, the lower two bytes of this Dword location should be masked
to prevent unintended side effects.
17
Generic
Own 1
Status
R
Returns the current state of Own Bit 1. When checking the state of
this bit, the lower two bytes of this Dword location should be masked
to prevent unintended side effects.
23:18 Reserved R Reserved. Returns 0 when read.