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Conditional and Arithmetic Instructions 63

values such as ASCII characters. One way to perform a comparison would

be to subtract the two numbers and then test the flags to determine their

relationship. If the two values are equal to each other, the zero flag will be

set. The carry flag, sign flag, and so on, can also be tested to determine

other inequalities. The particular flags that must be tested depends on

whether the comparison is between signed or unsigned quantities. In

fact, it is important to test for overflow as well. This may result in a

requirement to test two or more flags.

A disadvantage in performing a subtraction to compare two numbers

is the fact that the destination operand will be changed. Since the desti

nation operand must be one of the values to be compared, its value will

be destroyed. We could, of course, make a copy. The following exam

ple determines if the contents of register DO are equal to the contents of

HOVB.L D0,D2

SUB.L D1,D2

BEQ EQUAL

•

EQUAL: . EQUAL

There is a better way to compare numbers. A special compare instruc

tion, CMP, works almost like the SUB instruction except that the result

of the subtraction is not actually saved—in other words, the operands

remain unchanged. All the flags that would be set or reset by the SUB

instruction are also set or reset by the CMP instruction. The destination

operand must be a data register, but the source operand can be a regis

ter, variable, or constant. In fact, the source operand can use any of the

addressing modes that will be discussed in Chapter 6.

The previous example can be rewritten using the CMP instruction.

CMP.L Dl ,DO

BEQ EQUAL

EQUAL: !

Since we are using the CMP instruction and not an SUB, the value of DO

does not have to be copied.

In general we will desire to compare both signed and unsigned num

bers. The standard inequalities all have unique conditional branches that

are used with the CMP instruction. For inequalities other than equal or

MAKE A COPY OF DO

SUBTRACT Dl PROM D2

BRANCH IF THEY ARE =

NOT EQUAL

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Motorola M68000 Betriebsanweisung Seite 77