Byte-Swapping; Byte-Ordering Schemes - Xycom XVME-678 Manual

Chapter
4 -
Programming
4.8 BYTE-SWAPPING
Byte-swapping is the method the XVME-678/688 uses to transfer data to and from the VMEbus, despite
their different byte-ordering schemes. The CPU processor on the XVME-678/688 follows the Intel byte-
ordering scheme, while the VMEbus follows the Motorola byte-ordering scheme. The difference is
illustrated below.
4.8.1 Byte-Ordering Schemes
The Intel family of processors stores data with the least significant byte located at the lowest address and
the most significant byte at the highest address. The Motorola family stores data exactly opposite, with
the least significant byte located at the highest address and the most significant byte at the lowest address.
This fundamental difference is illustrated in Figure 4-11, which shows a 32-bit quantity stored at address
"M" by both architectures:
4-26
Address
INTEL
MOTOROLA
Low Byte
M
High Byte
•
M+1
•
•
•
•
M+2
•
•
•
High Byte M+3
Low Byte
Figure 4-11. Byte-Ordering Schemes
NOTE
The two architectures differ only in the way they store data into memory,
not in the way they place data on the shared data bus.