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4.22 CardBus I/O Limit Registers 0, 1
These registers indicate the upper address of a PCI I/O address range. They are used by the PCI7x21/PCI7x11
controller to determine when to forward an I/O transaction to the CardBus bus, and likewise, when to forward a
CardBus cycle to PCI. The lower 16 bits of this register locate the top of the I/O window within a 64-Kbyte page, and
the upper 16 bits are a page register which locates this 64-Kbyte page in 32-bit PCI I/O address space. Bits 15−2
are read/write and allow the I/O limit address to be located anywhere in the 64-Kbyte page (indicated by bits 31−16
of the appropriate I/O base register) on doubleword boundaries.
Bits 31−16 are read-only and always return 0s when read. The page is set in the I/O base register. Bits 15−2 are
read/write and bits 1−0 are read-only, returning 00 or 01 when read, depending on the value of bit 12 (IO_LIMIT_SEL)
in the general control register (PCI offset 86h, see Section 4.31). Writes to read-only bits have no effect.
These I/O windows are enabled when either the I/O base register or the I/O limit register is nonzero. By default, the
I/O windows are not enabled to pass the first doubleword of I/O to CardBus.
Either the I/O base register or the I/O limit register must be nonzero to enable any I/O transactions.
Bit 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Name I/O limit registers 0, 1
Type R R R R R R R R R R R R R R R R
Default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Name I/O limit registers 0, 1
Type RW RW RW RW RW RW RW RW RW RW RW RW RW RW R R
Default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 X
Register: I/O limit registers 0, 1
Offset: 30h, 38h
Type: Read-only, Read/Write
Default: 0000 000Xh
4.23 Interrupt Line Register
The interrupt line register is a read/write register used by the host software. As part of the interrupt routing procedure,
the host software writes this register with the value of the system IRQ assigned to the function.
Bit 7 6 5 4 3 2 1 0
Name Interrupt line
Type RW RW RW RW RW RW RW RW
Default 1 1 1 1 1 1 1 1
Register: Interrupt line
Offset: 3Ch
Type: Read/Write
Default: FFh