LISTING4.TXT

• *
• 68000 ASSEMBLER COURSE ON ATARI ST *
• *
• by The Fierce Rabbit (from 44E) *
• *
• Second series *
• *
• Listing number 4 *
• Program self-modification

        INCLUDE   "B:\START.S"      startup routine
        MOVE.L    #MESSAGE1,-(SP)   introducing
        MOVE.W    #9,-(SP)          the program
        TRAP      #1
        ADDQ.L    #6,SP

START MOVE.W #7,-(SP) waiting for a key press

        TRAP      #1
        ADDQ.L    #2,SP
        CMP.W     #" ",D0           space?
        BEQ       END               yes, so goodbye
        MOVE.W    #$4E71,VARIANT    places a NOP (by default)
        CMP.W     #"A",D0           press 'A'?
        BNE       HERE              no
        BSR       ROUTINE           yes, so long subroutine
        BRA       START             and start over

HERE MOVE.W #$4E75,VARIANT places a RTS instead

        BSR       ROUTINE           of the NOP and goes to subroutine
        BRA       START             then starts over

END MOVE.W #0,-(SP)

        TRAP      #1

• --------------------------------------------*

ROUTINE MOVE.L #MESSAGE2,-(SP)

        MOVE.W    #9,-(SP)
        TRAP      #1
        ADDQ.L    #6,SP

VARIANT NOP

        MOVE.L    #MESSAGE3,-(SP)
        MOVE.W    #9,-(SP)
        TRAP      #1
        ADDQ.L    #6,SP
        RTS

• --------------------------------------------*

        SECTION DATA

MESSAGE1 DC.B 27,"E","Press A for a long message,",13,10

        DC.B      "space to exit, another key",13,10
        DC.B      "for a shorter message",13,10,0
        EVEN

MESSAGE2 DC.B 13,10,"Here's the short message...",0

        EVEN

MESSAGE3 DC.B "Oops, no, it's the long one!!!",0

        EVEN
        SECTION BSS
        DS.L      100

STACK DS.L 1

        END

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