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PCI Interface
1-5
ThunderLAN Overview
1.3.2 Byte Ordering
ThunderLAN follows the
PCI Local Bus Specification
convention when trans-
ferring data on the PCI bus. The PCI bus data is transferred on the PAD[31::0]
lines. PAD31 is the most significant bit, and PAD0 is the least significant bit.
The 32 data lines are enough to transfer four bytes per data cycle. Byte 0 is
the LSbyte and byte 3 is the MSbyte. Byte 0 uses bits 0–7, byte 1 uses bits
8–15, byte 2 uses bits 16–23, byte 3 uses bits 24–31.
Figure 1–2. PCI Bus Byte Assignment
Byte 0Byte 1Byte 2Byte 3
07815162324
31
ThunderLAN uses the full four bytes per data cycle. The only exception is when
the data to be transferred is not octet aligned. In this case, the PCI controller
might not transfer the full four bytes on the first cycle. ThunderLAN deasserts
the IRDY signal only once, if needed, to synchronize the PCI bus to the internal
64-bit architecture. The deassertion of IRDY occurs on the third cycle of the
PCI bus. ThunderLAN does not deassert IRDY for the rest of the transfer un-
less the PCI bus asserts the TRDY signal.