Motorola M68000 User's Guide download pdf (Page 134)

120 Assembly Language Programming for the 68000 Family

TST.B - 1 (AO) HAVE WE HIT A NULL?

BNE STRCMP1 NOW MORE BYTES LEFT

STRRET: MOVEM.L (SP)+,A0-A1 RESTORE REGS

RTS

This subroutine sets the condition code register according to the last two

bytes compared, if they were unequal; the zero condition is set if the

strings match. We can then determine not only the equality or inequality

of the strings, but also their alphabetical order. Fortunately, the ASCII

character set is ordered properly from A to Z. These last two subroutines

are minor variations of the techniques used in Chapter 6.

The standard I/O subroutines introduced in Chapter 4 only provide

a mechanism to input or output a single character. We must be able to

input and output a complete character string. Two subroutines, INS and

OUTS, will be used to input and output null-terminated strings. OUTS

merely calls PUTC for each character in the string until the null is found.

* OUTS - OUTPUT A NULL TERMINATED STRING TO THE SCREEN

* aO -> STRING

OUTS: MOVEM.L AO/DO,-(SP)

0UTS1: CLR.L DO

MOVE.B (AO)+/D0

BEQ OUTSRET

JSR PUTC

BRA OUTS1

OUTSRET:MOVEM.L (SP)+,AO/DO

RTS

SAVE REGISTERS

CLEAR HIGH ORDER BYTES OF DO

MOVE A CHARACTER INTO DO

QUIT IF NULL

OUTPUT THE CHARACTER

LOOP FOR ANOTHER

RESTORE REGISTERS

Inputting a character string is almost as simple. There is one minor

detail to keep in mind: the size of the array that we are reading the string

into is of a certain size. For this example, the maximum size data string

is a total of NDATA characters long. Since the string must be terminated

with a null, we must reserve one of these characters for the null byte.

Therefore, we can only read a total of NDATA-1 characters from the

keyboard. The INS subroutine will read characters using GETC until a

carriage return is entered or NDATA-1 characters have been entered.

* INS - INPUT A STRING UNTIL CR OR NDATA-1 CHARACTERS

* AO -> STRING

INS: MOVEM.L A0/D0-D1,- (SP) SAVE REGISTERS

MOVE.W #NDATA-2,D1 SET UP LOOP COUNT

INS1: JSR GETC GET A CHARACTER

MOVE.B D0,(A0)+ STORE IT IN STRING

CMP.B #CR,D0 CR?

DBEQ Dl,INS1 LOOP UNTIL COUNT RUNS OUT OR CR

1 2 ... 129 130 131 132 133 134 135 136 137 138 139 ... 255 256

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Motorola M68000 User's Guide Page 134

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