User guide

10.4 Byte Swapping
The Digital Alpha VME 4 interface provides hardware to support byte-swapping
for transfers to and from the VMEbus. Four modes of swapping are supported.
The swap mode is defined for each inbound or outbound page by the related
scatter-gather entry.
10.4.1 DC7407 Byte Swapping
The swap mode for each scatter-gather entry is defined by 3 bits, SWP<2:0>. Bits
<1:0> define mode 0 through 3 and SWP<2> enables D64 swapping, which is only
used in D64 block mode data transfers.
The four swap modes are described in Figure 10–17 with the D64 swap cases
illustrated with mode 3. The following table defines the swap modes.
Table 10–12 Swap Modes
Mode Type of Swap Description
0 No Swap No bytes are swapped, and in transferring bytes from the little
endian PCI to the big endian VMEbus, the address of any byte
as seen on the two buses remains the same.
1 Byte Swap The bytes within words are swapped.
2 Word Swap The words within longwords are swapped.
3 Longword Swap Combination of modes 1 and 2. Byte 11 in a longword becomes
byte 00, 10 becomes 01, 01 becomes 10, and 00 becomes 11.
D64 Swap Used only in D64 block mode transfers. Swaps the order that
the longwords are taken from or put into memory over the PCI
bus. For example, when enabled with a mode 3 swap, byte
000 in a quadword becomes byte 111, that is, the binary byte
address is inverted.
10–26 VME Interface