
EPC-7 Hardware Reference
The EPC-7 contains programmable byte-swapping hardware to allow programs to
read or write VMEbus memory in either byte order. When using the E-page to access
the VMEbus, the order is selected by bit 5 (BORD) in the VME modifier register
(8151). When using direct memory mapping, the order is address-range dependent
(e.g., E0000000-E0FFFFFF accesses the A24 space with big endian byte ordering,
and 20000000-20FFFFFF accesses the A24 space with little endian byte ordering).
When performing a single byte (D08) access, the byte order makes no difference.
However, word (D16) or double-word (D32) accesses may require byte-swapping.
When little-endian is selected, bytes pass straight through unchanged. Little endian
should only be used when reading or writing data between two Intel processor
systems. The results of using little-endian byte ordering to transfer a double-word
integer between an Intel processor and a Motorola processor are shown below.
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
= 76543210h
= 10325476h
Figure 16. Little-Endian Byte Order.
7
7
Since the 486 processor uses Addr as the least-significant byte and the Motorola
processor uses Addr as the most-significant byte, the processor receiving the data gets
a "scrambled" value.
When big-endian is selected, the bytes are swapped between the 486 and VME. See
the diagram below.
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