MSG1 DB "x = ","$"                ; string to display for (1)
MSG2 DB "y = ","$"                ; string to display for (1)
MSG3 DB 0DH,0AH,"x+y = ","$"      ; string for sum format
MSG4 DB 0DH,0AH,"x-y = ","$"      ; string for sub format
MSG5 DB 0DH,0AH,"Error","$"       ; error display message
START:                            ; initialize index
    MOV DI,00H                    ; keep position for XLAT
    MOV BX,200H
    MOV SI,04H
READ_HEX_NUMBER:
    MOV DH,02H                    ; counter for pair of 2HEX remaining
READ_FIRST:
    MOV AH,08H
    INT 21H
    CMP AL,0DH                    ; if ENTER is pressed
    JE ERROR                      ; then goto ERROR
    CMP AL,30H                    ; if digit is valid then read again
    JL READ_FIRST
    CMP AL,39H                    ; if digit is valid the goto STORE_NUM
    JLE STORE_NUM1
    CMP AL,41H
    JL READ_FIRST
    CMP AL,46H
    JLE STORE_NUM1
    CMP AL,61H
    JL READ_FIRST
    CMP AL,66H
    JG READ_FIRST

STORE_NUM1:                        ; stores digit in array
    CMP AL,60H
    JLE SKIP_LINE
    SUB AL,20H
SKIP_LINE:
    MOV [BX+DI],AL                ; increase counter of array position
    INC DI
READ_SECOND:

    MOV AH,08H                    ; same procedure as for first digit
    INT 21H
    CMP AL,0DH                    ; if ENTER is pressed
    JE ERROR                      ; then goto ERROR
    CMP AL,30H
    JL READ_SECOND
    CMP AL,39H
    JLE CONTINUE
    CMP AL,41H
    JL READ_SECOND
    CMP AL,46H
    JLE CONTINUE
    CMP AL,61H
    JL READ_SECOND
    CMP AL,66H
    JG READ_SECOND


CONTINUE:                         ; store second digit in array
    CMP AL,60H
    JLE SKIP_LINE2
    SUB AL,20H
SKIP_LINE2:
    MOV [BX+DI],AL                ; increase counter of array position
    INC DI
    DEC DH                        ; decrease remaining numbers left to read
    CMP DH,00H                    ; if 2 numbers got read then continue
    JNZ READ_FIRST                ; else read 2nd number
    MOV DX, offset MSG1           ; display string MSG1
    MOV AH,09H
    INT 21H
    MOV AL,00H                    ; display 1st digit of 1st number
    XLAT
    MOV DL,AL
    MOV AH,02H
    INT 21H
    MOV AL,01H                    ; display 2nd digit of 1st number
    XLAT
    MOV DL,AL
    MOV AH,02H
    INT 21H
    MOV DL,20H                    ; print "space" character
    MOV AH,02H
    INT 21H
    MOV DX, offset MSG2           ; display string MSG2
    MOV AH,09H
    INT 21H
    MOV AL,02H                    ; display 1st digit of 2nd number
    XLAT
    MOV DL,AL
    MOV AH,02H
    INT 21H
    MOV AL,03H                    ; display 2nd digit of 2nd number
    XLAT
    MOV DL,AL
    MOV AH,02H
    INT 21H
    MOV AH,08H
    INT 21H
    CMP AL,0DH                    ; if ENTER is pressed after 2 numbers
    JE PRINT_SUM_SUB              ; then print sum and difference of numbers
                                  ; else if there are more than 2 numbers
WAIT_FOR_ENTER:                   ; then wait for ENTER to display error message
    MOV AH,08H
    INT 21H
    CMP AL,0DH
    JE ERROR
    JMP WAIT_FOR_ENTER

ERROR:
    MOV DX, offset MSG5           ; display "Error" message on screen
    MOV AH,09H
    INT 21H
    JMP FINISH
PRINT_SUM_SUB:
    MOV AL,00H
    XLAT
    CALL HEXPROD                  ; call hexprod to keep values for hex numbers
    MOV CL,04H                    ; define shift value for ROL
    MOV DL,CH
    ROL DL,CL                     ; take first ASCII value and convert it
    MOV AL,01H
    XLAT
    CALL HEXPROD
    MOV CL,04H
    ADD DL,CH                     ; take second ASCII code to complete first number
    MOV AL,02H
    XLAT
    CALL HEXPROD
    MOV CL,04H
    ADD DH,CH
    ROL DH,CL
    MOV AL,03H                    ; start translating second hex number
    XLAT
    CALL HEXPROD
    MOV CL,04H
    ADD DH,CH
    MOV BL,DH
    MOV BH,00H                    ;first number saved in BX
    MOV DH,00H                    ;second number saved in DX
    MOV CX,BX
    ADD BX,DX
    MOV AX,BX
    CALL HEX_TO_DECIMAL
    AND DX,00FFH
    SUB DX,CX
    MOV CX,DX
    MOV DX, offset MSG3            ; display MSG3 string
    MOV AH,09H
    INT 21H
    MOV BX,300H                    ; take from array output values
    MOV AL,00H
    XLAT
    CMP AL,00H                     ; if left most digit is zero then dont display it
    JE SKIP1                       ; and move to 2nd digit
    ADD AL,30H
    MOV DL,AL
    MOV AH,02H
    INT 21H
    JMP NEXT_NUM2
SKIP1:                             ; if both 1st digit and 2nd digit is zero
    MOV AL,01H                     ; then dont display it and move to 3rd digit
    XLAT
    CMP AL,00H
    JE SKIP7
NEXT_NUM2:
    MOV AL,01H
    XLAT
    CMP AL,00H
    ADD AL,30H
    MOV DL,AL
    MOV AH,02H
    INT 21H
SKIP7:
    MOV AL,02H
    XLAT
    ADD AL,30H
    MOV DL,AL
    MOV AH,02H
    INT 21H
    MOV DX, offset MSG4            ; display MSG4 string
    MOV AH,09H
    INT 21H
    CMP CX,00H
    JGE CONT3                      ; if result < 0
    NEG CX                         ; convert to positive
    MOV DL,2DH                     ; output minus sign
    MOV AH,02H
    INT 21H
CONT3:
    MOV AX,CX
    CALL HEX_TO_DECIMAL            ; same logic as before to print the sub
    MOV BX,300H
    MOV AL,00H
    XLAT
    CMP AL,00H
    JE SKIP5
    ADD AL,30H
    MOV DL,AL
    MOV AH,02H
    INT 21H
    JMP NEXT_NUM1
SKIP5:
    MOV AL,01H
    XLAT
    CMP AL,00H
    JE SKIP6
NEXT_NUM1:
    MOV AL,01H
    XLAT
    ADD AL,30H
    MOV DL,AL
    MOV AH,02H
    INT 21H
SKIP6:
    MOV AL,02H
    XLAT
    ADD AL,30H
    MOV DL,AL
    MOV AH,02H
    INT 21H
FINISH:
    MOV AH,02H
    MOV DL,0AH                     ; print "\n" character
    INT 21H
    MOV AH,02H
    MOV DX,0DH                     ; print carriage return character
    INT 21H
    MOV AH,02H
    MOV DL,0AH                     ; print "\n" character
    INT 21H
    MOV AH,02H
    MOV DX,0DH                     ; print carriage return character
    INT 21H
    JMP START

HEX_TO_DECIMAL:                    ; convert HEX number to decimal
    PUSH BX
    MOV SI,00H
    MOV DH,64H
    MOV BX,300H                    ; store each decimal digit in array
    DIV DH                         ; divide by 100 to get the right most digit as remainder
    MOV [BX+SI],AL
    AND AX,0FF00H
    XCHG AH,AL
    MOV DH,0AH                     ; divide by 10 the rest of the previous division
    DIV DH                         ; to get the left most digit as quotient
    INC SI                         ; and the middle digit as remainder
    MOV [BX+SI],AL
    INC SI
    MOV [BX+SI],AH
    POP BX
    RET
HEXPROD:                           ; convert ASCII to HEX
    MOV CL,AL                      ; CL contains the value of ASCII
    MOV CH,00H                     ; CH contains the value of HEX -> result
    MOV AL,30H
HEXSTART:
    CMP AL,CL
    JE HEXFOUND
    INC CH
    INC AL
    JNC SKIP0
    CMC
SKIP0:
    DAA
    CMP AL,40H
    JNE AVOID
    INC AL
    JNC SKIP3
    CMC
SKIP3:
    DAA
AVOID:
    JMP HEXSTART
HEXFOUND:
    RET

    END