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VXIbus Interface
486
Address
M o t o r o l a
Address
Addr Addr+1 Addr+2 Addr+3
Addr Addr+1 Addr+2
Addr+3
LSB
MSB
10 32
54
76
10
32 54 76
Addr
Addr+1
Addr Addr+1
LSB
MSB
10 32
10 32
D16
Access
D32
Access
Figure 17. Big-Endian Byte-swapping.
When using big-endian byte ordering, care must be taken to assure that the VME
address is aligned on a boundary; for D16 accesses the VME address must be on a
word boundary (address evenly divisible by 2) and for D32 accesses the VME
address must be on a double-word boundary (evenly divisible by 4). If this is not
done, the results will be "scrambled" data. Although the VMEbus address must be
boundary-aligned to match the data width (word or double-word), the 486 address
does not need to be boundary-aligned.
Another consideration is the compiler being used. Some compilers produce two
16-bit accesses when a 32-bit access is desired. When this occurs, again the data will
be "scrambled."
7 7
When transferring a 32-bit floating-point number, special care must be taken to
assure that both processors use the same floating-point format; that both systems
expect the mantissa and exponent in the same byte locations. As long as this is
correct, transferring a floating-point number will work correctly. Since transferring a
64-bit floating-point number is not supported in hardware, two 32-bit transfers must
be used with little-endian byte order and then byte-swapping must be accomplished in
software.
Byte swapping applies only to EPC-7 initiated (master) accesses; it does not apply
to slave accesses ( from other VMEbus masters to the EPC-7's DRAM).
The EPConnect software provides a means of selecting the byte ordering during
memory-copy operations.
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