0100 ; ATARI GRAPHICS OVERLAY›0110 ; PLOTTING ROUTINE›0120 ; (C) 1984 JEFF BRENNER›0130 ;›0140 ; FORM: A=USR(PLOT,X,Y)›0150 ;›0160 COLOR = $CB›0170 LOW =   $CC›0180 HIGH =  $CD›0190 X   =   $CE›0200 Y   =   $CF›0210 ;›0220     *=  $0600   ;RELOCATABLE›0230 ;›0240 PLOT PLA ›0250     PLA ›0260     PLA ›0270     CMP #40     ;HORIZ POS>39 OR <0›0280     BCS RET1›0290     ADC #24›0300     STA X›0310     PLA ›0320     PLA ›0330     CMP #192    ;VERT POS>191 OR <0›0340     BCS RET2›0350     ADC #32›0360     STA Y›0370     LDX HIGH›0380     LDA X›0390     LSR A       ;DIVIDE BY EIGHT›0400     LSR A›0410     LSR A›0420     TAY ›0430     BEQ OVER›0440     LDX HIGH›0450 LOOP INC HIGH   ;INC HIGH BYTE›0460     DEY ›0470     BNE LOOP›0480 OVER ASL A      ;FIND REMAINDER›0490     ASL A›0500     ASL A›0510     SEC ›0520     SBC X›0530     EOR #255›0540     TAY ›0550     LDA #128    ;FIND CORRECT BIT PATTERN›0560     INY ›0570     BEQ NEXT›0580 LOOP1 LSR A›0590     DEY ›0600     BNE LOOP1›0610 NEXT LDY COLOR  ;COLOR=1 TO PLOT›0620     BEQ ERASE   ;IF 0 - ERASE PIXEL›0630     LDY Y       ;OFFSET FOR VERT POS›0640     ORA (LOW),Y ;MERGE BIT PATTERN›0650     SEC ›0660     BCS CONT›0670 ERASE LDY Y›0680     EOR #255›0690     AND (LOW),Y›0700 CONT STA (LOW),Y ;SUBTRACT BIT PATTERN›0710 RETURN STX HIGH ;FROM CURRENT TO ERASE›0720     RTS ›0730 RET1 PLA ›0740     PLA ›0750 RET2 PLA ›0760     PLA ›0770     RTS         ;RETURN TO BASIC›0780     .END ›