Motorola M68000 User's Guide download pdf (Page 79)

Conditional and Arithmetic Instructions 65

FINI: MOVE.L Dl,D0

JSR OUTDEC

JSR NEWLINE

MOVE•L D0,D1

BRA NEXT

YES, SAVE AS NEW VAL

BACK FOR NEXT VAL

SET UP FOR OUTPUT

OUTPUT THE VALUE

GO TO NEXT LINE

Two CMP instructions are used. The first CMP tests for the zero termi

nating value. The second determines if a new input value is larger than

the largest value encountered so far. It is important to remember to ini

tialize this value to something meaningful. In this case, a zero value is

smaller than any value we will encounter.

There are several additional versions of the compare instruction. Al

though any addressing mode, including immediate values, can be used as

the source operand, the destination operand can only be a data register.

The general form of the CMP instruction is:

The CMPA instruction allows the use of an address register as the desti

nation operand. Its form is:

While we may often desire to compare a data or address register with an

immediate value, we sometimes desire to compare a constant with the

contents of a memory location. It is not possible to use the CMP or CMPA

instruction in such a manner. We would first have to copy the contents

of memory to a register and then use the CMP instruction. Fortunately, a

special version of the CMP instruction, CMPI, exists to solve our problem.

Its general form is:

The destination operand is restricted in that it can't be an address register

(use CMPA in this case), or an immediate value. (This latter restriction

really isn’t a restriction, because it makes no sense to compare one

constant with another, since the outcome is always the same.) Let’s assume

that we want to see if the contents of memory location COUNT, a byte

value, is less than 10. These are the instructions we would use:

CMP[.<size>] <ea>,Dn

CMPA(.<size>] <ea>,An

CMPII.<size>] #<data>,<ea>

CMPI.B #10,COUNT

BLT LESSTHAN

1 2 ... 74 75 76 77 78 79 80 81 82 83 84 ... 255 256

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Motorola M68000 User's Guide Page 79