Datasheet
4−10
4.21 I/O Base Registers 0, 1
The I/O base registers indicate the lower address of a PCI I/O address range. These registers are used by the
controller to determine when to forward an I/O transaction to the CardBus bus and when to forward a CardBus cycle
to the PCI bus. The lower 16 bits of this register locate the bottom of the I/O window within a 64-Kbyte page, and the
upper 16 bits (31−16) are a page register which locates this 64-Kbyte page in 32-bit PCI I/O address space. Bits 31−2
are read/write. Bits 1 and 0 are read-only and always return 00b, forcing I/O windows to be aligned on a natural
doubleword boundary.
NOTE: 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
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
Default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Register: I/O base registers 0, 1
Offset: 2Ch, 34h
Type: Read-only, Read/Write
Default: 0000 0000h
4.22 I/O Limit Registers 0, 1
The I/O limit registers indicate the upper address of a PCI I/O address range. These registers are used by the
controller to determine when to forward an I/O transaction to the CardBus bus and 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 that 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) on doubleword boundaries.
Bits 31−16 are read-only and always return 0000h when read. The page is set in the I/O base register. Bits 1 and 0
are read-only and always return 00b, forcing I/O windows to be aligned on a natural doubleword boundary. Write
transactions to read-only bits have no effect. The controller assumes that the lower 2 bits of the limit address are 11b.
NOTE: The I/O base or the I/O limit register must be nonzero to enable an I/O transaction.
Bit 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
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
Default 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Register: I/O limit registers 0, 1
Offset: 30h, 38h
Type: Read-only, Read/Write
Default: 0000 0000h