pi7c8150b Pericom Semiconductor Corporation, pi7c8150b Datasheet - Page 42

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

06-0044
4.2.2
4.3
The I/O limit register consists of an 8-bit field at configuration offset 1Dh and a 16-bit field
at offset 32h. The top 4 bits of the 8-bit field define bits [15:12] of the I/O limit address.
The bottom 4 bits read only as 1h to indicate that 32-bit I/O addressing is supported. Bits
[11:0] of the limit address are assumed to be FFFh, which naturally aligns the limit address
to the top of a 4KB I/O address block. The 16 bits contained in the I/O limit upper 16 bits
register at configuration offset 32h define AD[31:16] of the I/O limit address. All 16 bits
are read/write. After primary bus reset or chip reset, the value of the I/O limit address is
reset to 0000 0FFFh.
Note: The initial states of the I/O base and I/O limit address registers define an I/O range
of 0000 0000h to 0000 0FFFh, which is the bottom 4KB of I/O space. Write these registers
with their appropriate values before setting either the I/O enable bit or the master enable bit
in the command register in configuration space.
ISA MODE
PI7C8150B supports ISA mode by providing an ISA enable bit in the bridge control
register in configuration space. ISA mode modifies the response of PI7C8150B inside the
I/O address range in order to support mapping of I/O space in the presence of an ISA bus in
the system. This bit only affects the response of PI7C8150B when the transaction falls
inside the address range defined by the I/O base and limit address registers, and only when
this address also falls inside the first 64KB of I/O space (address bits [31:16] are 0000h).
When the ISA enable bit is set, PI7C8150B does not forward downstream any I/O
transactions addressing the top 768 bytes of each aligned 1KB block. Only those
transactions addressing the bottom 256 bytes of an aligned 1KB block inside the base and
limit I/O address range are forwarded downstream. Transactions above the 64KB I/O
address boundary are forwarded as defined by the address range defined by the I/O base
and limit registers.
Accordingly, if the ISA enable bit is set, PI7C8150B forwards upstream those I/O
transactions addressing the top 768 bytes of each aligned 1KB block within the first 64KB
of I/O space. The master enable bit in the command configuration register must also be set
to enable upstream forwarding. All other I/O transactions initiated on the secondary bus are
forwarded upstream only if they fall outside the I/O address range.
When the ISA enable bit is set, devices downstream of PI7C8150B can have I/O space
mapped into the first 256 bytes of each 1KB chunk below the 64KB boundary, or anywhere
in I/O space above the 64KB boundary.
MEMORY ADDRESS DECODING
PI7C8150B has three mechanisms for defining memory address ranges for forwarding of
memory transactions:
Memory-mapped I/O base and limit address registers
Prefetchable memory base and limit address registers
VGA mode
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ASYNCHRONOUS 2-PORT PCI-TO-PCI BRIDGE
APRIL 2006 – Revision 2.02
PI7C8150B