pi7c8150b Pericom Semiconductor Corporation, pi7c8150b Datasheet - Page 41

• Manufacturer Part Number: pi7c8150b
• Description: Asynchronous 2-port Pci Bridge
• Manufacturer: Pericom Semiconductor Corporation

06-0044
4.2.1
This section provides information on the I/O address registers and ISA mode. Section 4.4
provides information on the VGA modes.
To enable downstream forwarding of I/O transactions, the I/O enable bit must be set in the
command register in configuration space. All I/O transactions initiated on the primary bus
will be ignored if the I/O enable bit is not set. To enable upstream forwarding of I/O
transactions, the master enable bit must be set in the command register. If the master-
enable bit is not set, PI7C8150B ignores all I/O and memory transactions initiated on the
secondary bus.
The master-enable bit also allows upstream forwarding of memory transactions if it is set.
CAUTION
If any configuration state affecting I/O transaction forwarding is changed by a
configuration write operation on the primary bus at the same time that I/O transactions are
ongoing on the secondary bus, PI7C8150B response to the secondary bus I/O transactions
is not predictable. Configure the I/O base and limit address registers, ISA enable bit, VGA
mode bit, and VGA snoop bit before setting I/O enable and master enable bits, and change
them subsequently only when the primary and secondary PCI buses are idle.
I/O BASE AND LIMIT ADDRESS REGISTER
PI7C8150B implements one set of I/O base and limit address registers in configuration
space that define an I/O address range per port downstream forwarding. PI7C8150B
supports 32-bit I/O addressing, which allows I/O addresses downstream of PI7C8150B to
be mapped anywhere in a 4GB I/O address space.
I/O transactions with addresses that fall inside the range defined by the I/O base and limit
registers are forwarded downstream from the primary PCI bus to the secondary PCI bus.
I/O transactions with addresses that fall outside this range are forwarded upstream from the
secondary PCI bus to the primary PCI bus.
The I/O range can be turned off by setting the I/O base address to a value greater than that
of the I/O limit address. When the I/O range is turned off, all I/O trans-actions are
forwarded upstream, and no I/O transactions are forwarded downstream. The I/O range has
a minimum granularity of 4KB and is aligned on a 4KB boundary. The maximum I/O
range is 4GB in size. The I/O base register consists of an 8-bit field at configuration
address 1Ch, and a 16-bit field at address 30h. The top 4 bits of the 8-bit field define bits
[15:12] of the I/O base address. The bottom 4 bits read only as 1h to indicate that
PI7C8150B supports 32-bit I/O addressing. Bits [11:0] of the base address are assumed to
be 0, which naturally aligns the base address to a 4KB boundary. The 16 bits contained in
the I/O base upper 16 bits register at configuration offset 30h define AD[31:16] of the I/O
base address. All 16 bits are read/write. After primary bus reset or chip reset, the value
of the I/O base address is initialized to 0000 0000h.
I/O base and limit address registers
The ISA enable bit
The VGA mode bit
The VGA snoop bit
Page 41 of 108
ASYNCHRONOUS 2-PORT PCI-TO-PCI BRIDGE
APRIL 2006 – Revision 2.02
PI7C8150B