Intel 21555 user manual Generic Own Bits Register, Rots

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
			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.
0	Generic	ROTS/	Writing a 1 clears this bit to a 0.
	Own Bit 0	W1TC
			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.
			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.
8	Generic	ROTS/	Writing a 1 clears this bit to a 0.
	Own Bit 1	W1TC
			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.
	Generic		Returns the current state of Own Bit 0. When checking the state of
16	Own 0	R	this bit, the lower two bytes of this Dword location should be masked
	Status		to prevent unintended side effects.
	Generic		Returns the current state of Own Bit 1. When checking the state of
17	Own 1	R	this bit, the lower two bytes of this Dword location should be masked
	Status		to prevent unintended side effects.
23:18	Reserved	R	Reserved. Returns 0 when read.

164 21555 Non-Transparent PCI-to-PCI Bridge User Manual