Motorola M68000 User's Guide download pdf (Page 143)

CHAPTER 10

LOGICAL, SHIFT AND

ROTATE INSTRUCTIONS

In this chapter we will examine a group of instructions that can ma

nipulate the individual bits of a byte, word, or longword by performing

logical operations such as AND and OR. We will also see how shift and

rotate instructions can change the positions of all the bits in a byte or

word in interesting ways.

Truth Tables

There are four logical operations that have corresponding 68000 in

structions: NOT, AND, OR, and EOR (exclusive or). These four logical

operations can be described by the use of a truth table. AND, OR, and

EOR are operations that require two operands, while NOT only requires

one. When used as 68000 instructions, these logical operations act on

all

the bits of the operands in parallel. However, the action on individual

bits or bit pairs is the same. We can therefore describe each operation

with a truth table consisting of at most a pair of bits.

The NOT logical operation essentially reverses ones and zeros. In

other words, a 1 becomes a 0 and a 0 becomes a 1. The truth table looks

like this:

Operand

0 1

Result 1 0

NOT

The AND operation takes two operands. The resulting bit is a 1 only if

both the operand bits are 1.

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Motorola M68000 User's Guide Page 143