2's compliment in assembly

this is a part of my calculator code.This code will convert the input values to ASCII value.My problem is when i input negative values, it will not result to desired output.I want to modify this code if ever the input values is negative.I don't know how to modify this program in order for it to convert negative values to 2's compliment.can u help me?

CONVERT_ASSCII PROC NEAR

    MOV AH , BYTE PTR RESULT        ;MOVE FIRST TWO BYTE OF RESULT TO AX
    MOV AL , BYTE PTR RESULT + 1

    ;AT FIRST CHECK IF AX IS POSITIVE OR NEGETIVE
    TEST EAX , 8000H                    ;CHECK THE LAST BIT . IF THATS 1 THEN AX IS NEG , OTHERWISE THATS POSITIVE
    MOV EDI , 0                     ;AT FIRST SET OUR FLAG TO ZERO
    JZ EAX_POSITIVE                 ;AX IS POSITIVE
    NEG EAX                         ;CALCULATE NEGETIVE OF AX 
    MOV EDI , 1                     ;DI INDICATES THAT AX IS NEG

EAX_POSITIVE :
    MOV ECX , 10
    LEA ESI , ASSCII_NUM
    ADD ESI , 29                        ;MOVE TO THE LAST SPACE

    MOV EBP , 0                     ;THIS IS THE COUNTER OF CHARS

    ASSCII_LOOP : MOV EDX , 0
                  DIV ECX 
                  OR DL , 30H       ;MAKE REMINDER ASSCII
                  MOV [ESI] , DL        ;PUT ASSCII IN ASSCII_NUM
                  DEC ESI
                  INC EBP           ;ADD ONE TO THE CHAR'S COUNTER
                  CMP EAX , 0       ;IF AX > 0 GOTO 
                  JA ASSCII_LOOP    ;ASSCII_LOOP

    CMP EDI , 0                     ;CHECK IF THAT WAS A NEGETIVE NUMBER
    JZ REST                         ;IF THATS NOT NEGETIVE GOTO REST
    MOV DL , '-'
    MOV [ESI] , DL                  ;ADD A MINES SIGN TO THE STRING
    DEC ESI
    INC EBP

REST :
    LEA EDI , ASSCII_NUM

    ;MOVE THE ASSCII CODE TO IT'S RIGHT PLCAE IN ASSCII_NUM
    ORDER_ASSCII : INC ESI
                   MOV AL , BYTE PTR [ESI] 
                   MOV BYTE PTR [EDI] , AL
                   INC EDI
                   DEC EBP
                   CMP EBP , 0
                   JA ORDER_ASSCII

    MOV CL , '$'           
    MOV BYTE PTR [EDI] , CL         ;AT LAST PUT A DOLLOR SIGN AT THE END OF ASSCII_NUM

    RET
CONVERT_ASSCII ENDP