define  SCINIT  $ff81 ; initialize/clear screendefine  CHRIN  $ffcf ; input character from keyboarddefine  CHROUT  $ffd2 ; output character to screendefine  SCREEN  $ffed ; get screen sizedefine  PLOT    $fff0 ; get/set cursor coordinatesdefine RIGHT  $81define LEFT  $83define ENTER  $0ddefine BACKSPACE $08define NUM1    $15;define  NUM2    $16;jsr SCINITldy #$00jsr firstNumPrint ; ask for input for first numberjsr getNum; get the first numberjsr storeFirstNum  ; then store the first numberldy #$00jsr secondNumPrint  ; ask for input for second numberjsr getNum; get the second numberjsr storeSecondNum  ; store the second numberldy #$00jsr resultPrintString  ; print a string 'Result'jsr printResult ; print the resultjmp mainLoop  ; go back to the first stepgetNum:     sec     jsr PLOT     ldx #$15     clc     jsr PLOTgetNumLoop:     sec     jsr PLOT     jsr CHRINcharCheck:      cmp #BACKSPACE ; if user enter backspace, it erase the #$15 digit     beq move_back     cmp #RIGHT ; if user enter right arrow, it goes to the first digit     beq move_right     cmp #LEFT ; if user enter left arrow, it goes to the second digit     beq move_left     cmp #ENTER ; if user enter enter, it goes to the next process     beq moveprintNum:     cmp #$30     bcc getNumLoop     clc     cmp #$3a     bcs getNumLoop     jsr CHROUT     sec     jsr PLOT     cpx #$17     bne getNumLoop     dex     clc     jsr PLOT     jmp getNumLoopmove_back: cpx #$15 beq getNumLoop jsr CHROUT jmp getNumLoopmove_left:      cpx #$15 ; first digit     beq getNumLoop     jsr CHROUT     jmp getNumLoopmove_right:  cpx #$16 ; second digit     beq getNumLoop     jsr CHROUT     jmp getNumLoopmove:     sec     jsr PLOT     ldx #$15 ; first digit     clc     jsr PLOT     sec     jsr PLOT     clc     sbc #$2F ; to calculate it, it should be subtracted by #$2f     asl     asl     asl     asl     pha     ldx #$16     clc     jsr PLOT     sec     jsr PLOT     clc     sbc #$2F ; to calculate it, it should be subtracted by #$2f     pha     ldx #$00     iny     clc     jsr PLOT     sec     jsr PLOT     pla     tax     pla     rtsstoreFirstNum:     sta NUM1     txa     eor NUM1     sta NUM1     rtsstoreSecondNum:     sta NUM2     txa     eor NUM2     sta NUM2     rtsprintResult:     sec     jsr PLOT     ldx #$15     clc     jsr PLOT     sec     jsr PLOT     sed     lda NUM1     adc NUM2     cld     pha     bcc outputAddition     ldx #$14     clc     jsr PLOT     sec     jsr PLOT     lda #$31     jsr CHROUToutputAddition:     lsr     lsr     lsr     lsr     clc     adc #$30 ; as the received number does not fit for ASCII, it needs to add      jsr CHROUT     pla     and #$0F     clc     adc #$30 ; as the received number does not fit for ASCII, it needs to add      jsr CHROUT     sec     jsr PLOT     ldx #$00     iny     clc     jsr PLOT     rtsfirstNumPrint:     lda firstNum,y     beq goback_main     jsr CHROUT     iny     bne firstNumPrintsecondNumPrint: lda secondNum,y        beq goback_main        jsr CHROUT        iny        bne secondNumPrintresultPrintString: lda result,y        beq goback_main        jsr CHROUT        iny        bne resultPrintStringgoback_main:     rtsfirstNum:dcb "E","N","T","E","R",32,"F","I","R","S","T",32,"N","U","M","B","E","R",":",32,32,"0","0"dcb 00secondNum:dcb "E","N","T","E","R",32,"S","E","C","O","N","D",32,"N","U","M","B","E","R",":",32,"0","0"dcb 00result:dcb "R","E","S","U","L","T",":"dcb 00