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To give a concrete example around which to discuss endian format issues, consider writing code for a system that contains a DBDMA-like controller. The DMA code includes a descriptor format whose C definition might be as follows:
struct {
UInt8 C; // "command" byte
UInt8 F; // "flags"
UInt16 L; // "length" (count)
UInt32 A; // "address"
UInt64 X; // "field64"
} DMA_Descriptor;
A compiler would assign offsets to the fields of the descriptor as follows:
C 0
F 1
L 2
A 4
X 8
Consider the diagram in Figure A-1, which presents the layout of the descriptor in a format that is neither big-endian nor little-endian. In Figure A-1, the numbers represent byte offsets to the descriptor's fields.
Figure A-1 Neutral descriptor layout
In Figure A-1 the byte offsets are associated with the "beginning" of each field. As discussed in the next sections, the primary difference between big-endian and little-endian addressing has to do with what is defined as the "beginning" of a field.