.MODEL SMALL
.STACK 100H

.DATA
SYNC_CHAR DB 'S'  ; Sync character

.CODE
MAIN PROC

    MOV AX, @DATA    ; Initialize DS
    MOV DS, AX

    ; Initialize 8251 USART in synchronous mode
    MOV DX, 3F8H      ; Load DX with 8251's control port address
    MOV AL, 0AEH     ; Control word to set synchronous mode, 1 stop bit, 8 data bits
    OUT DX, AL

    ; Send sync character
    MOV DX, 3F8H      ; Load DX with 8251's data port address
    MOV AL, SYNC_CHAR ; Load AL with the sync character
    OUT DX, AL        ; Send sync character

    MOV AH, 4CH       ; Exit program
    INT 21H

MAIN ENDP
END MAIN