Why does endianism raise its ugly head so starkly when we look at the notion of bit fields? Illustrate your answer with reference to the 68020’s bit field instructions.

Question

What will be an ideal response?

Answer 1

The bits fields used by the 68020 are numbered with the least?significant bit on the left; for example, the first

bit of the bit field 10001110 is 1 in bit position 0 (and not position 7). However, we number the bits of a byte in

the 68020 world in the reverse order. The first bit (least?significant bit) of the byte 00011111 is 1. Consequently,

bit field bit numbering and byte bit numbering are in reverse order. For example, in the string 00100000 the

bits set to 1 is bit 5 of the byte and bit 2 of the bit field.



Consider the diagram below. This shows four bytes in memory. As you can see, the most?significant bit of each

byte, bit 7, is on the left, whereas the most?significant bit of the bit field is the 0 in bit 1 of byte 803.