Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 146)

132 Assembly Language Programming for the 68000 Family

EOR has the most interesting property in its use with a mask. Each bit

set to 1 in the mask corresponds to a bit in the destination that we desire

to complement. In other words, 1 flips to 0 or 0 flips to 1.

Shifts

The logical instructions can be used to manipulate individual bits or

groups of bits within a byte, word, or longword. However, the positions

of the bits that are manipulated remain the same. We sometimes desire to

treat all of the bits of a byte or word as a group and change their positions.

One can imagine a virtually unlimited number of possible operations.

Since it would not be practical to implement a machine instruction for

every possible reorganization of the bits of a word or byte, two of the

most useful operations are implemented, shifts and rotates.

The bits of a word or byte are normally numbered as follows:

Byte

5 4 3 2 1 0

IS 14 13

10 9

Word

B 7

6 5

4 3 2

1 0

31 30 29 2B

27 26

Longword

6 5

4 3 2

1 0

The bits of a word or byte can be shifted either to the left or to the right.

For each shift of one position, all the bits move to the left or right, each

bit replacing the bit that was previously occupying that position. This

looks like the following:

1 2 ... 141 142 143 144 145 146 147 148 149 150 151 ... 255 256

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Motorola M68000 Betriebsanweisung Seite 146