4. Addressing > ISA Addressing
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PEB383 User Manual
July 25, 2011
Integrated Device Technology, Inc.
Confidential - NDA Required
VGA I/O Addresses (Address bits 15:10 are not decoded when the VGA 16-Bit Decode bit is 0b) are:
•
Address bits 9:0 = 0x3B0 through 0x3BB and 0x3C0 through 0x3DF (VGA 16-Bit Decode bit is
0b)
•
Address bits 15:0 = 0x03B0 through 0x03BB and 0x03C0 through 0x03DF (VGA 16-bit Decode
bit is 1b)
The VGA Palette Snoop Enable bit is implemented as read-only with a value of zero.
4.6
ISA Addressing
The PEB383 supports ISA addressing through ISA Enable bit in the
“PCI Bridge Control and Interrupt
. The ISA Enable affects only I/O addresses that are in the bridge’s I/O range (as defined by
the I/O Base, I/O Base Upper 16 Bits, I/O Limit, and I/O Limit Upper 16 Bits) and in the first 64 KB of
PCI I/O Space (0000 0000h to 0000 FFFFh). If this bit is set and the I/O address meets the stated
constraints, the PEB383 blocks the forwarding of I/O transactions downstream if the I/O address is in
the top 768 bytes of each naturally aligned 1-KB block. If the ISA Enable bit is clear, the PEB383
forwards downstream all I/O addresses in the address range defined by the I/O Base and I/O Limit
registers.
If the ISA Enable bit is set, I/O transactions on the PCI bus in the top 768 bytes of any 1-KB address
block within the first 64 KB of PCI I/O space is forwarded upstream, even if the address is between the
I/O base and I/O limit addresses.
illustrates this mapping for a 4-KB range.
The ISA Enable bit only affects the I/O address decoding behavior of the bridge. It does not affect the
bridge's prefetching, posting, ordering, or error handling behavior.