I'm stuck with my code for M68000 using Easy68k simulator. I'm getting some text, I'm storing the text into registry A0, then getting an option to transform it to uppercase and looping through A0.

I'm not sure about the looping part. I'm trying to transform the character into uppercase, but my final output after all the transformations seems to be 0. I'm stuck, can you point me in the right direction?

CR  EQU       $0D
LF  EQU       $0A

START: ORG    $1000
* load dialog WRITE in memory
    LEA WRITE,A1
    MOVE #0,D2 ;D2 stores transformed string

* show prompt WRITE
    MOVE #14,D0
    TRAP #15
* read input WRITE
    MOVE #2,D0 ;lee el input de la cadena
    TRAP #15
    MOVE A1,A0

*Show prompt CHOOSE
SR_CHOOSE:
    LEA CHOOSE,A1     
    MOVE #14,D0
    TRAP #15
    
* Read input CHOOSE
    MOVE #4,D0 ;read num input
    TRAP #15
    CMP.B #1,D1
    BEQ LOOP_UPPER
    BLE SR_CHOOSE

LOOP_UPPER:
    CMP.B   #0, (A0)  ; Compare byte at (A0) with zero (null terminator)
    BEQ     FINISHED
    SUB.B   #32, (A0) ; Convert to uppercase
    MOVE    A0, D2
    TST.B   (A0)+      ; Move to the next character and check for null terminator
    BNE     LOOP_UPPER

*Show transformed text*
FINISHED: 
    MOVE D2,A1  ;Set A1 to D2
    MOVE #14,D0
    TRAP #15

    SIMHALT

*Mensajes
WRITE: DC.W    'ESCRIBE ALGO: ',CR,LF,0
CHOOSE: DC.W    'Transforma a 1-MAYUSCULAS, 2-MINUSCULAS: ',CR,LF,0
        END     START

1. write text, i.e. hello
2. choose transform option 1 (UPPERCASE, for now)
3. loop through text and transform
4. expected: HELLO, actual: 0