transfer a byte to com port (registers)

.model small
.stack 100h

.data

errwrite db "Error: can't send byte",0Dh,0Ah,'$'
errread db "Error: can't read byte",0Dh,0Ah,'$'
output db "Result: $"
byte1 db 'V'
byte2 db ?

.code

initialize proc
    ; COM1
    mov al,80h
    mov dx,3FBh
    out dx,al

    mov dx, 3F8h
    mov al, 00h
    out dx, al
    mov al, 0Dh
    mov dx, 3F9h
    out dx, al

    mov dx, 3FCh
    mov al, 00001011b
    out dx, al

    mov al, 0000011b
    mov dx, 3FBh
    out dx, al

    mov dx, 3F9h
    mov al, 00001011b
    out dx, al

    ; COM2

    mov al, 80h
    mov dx, 2FBh
    out dx,al

    mov dx, 2F8h
    mov al, 00h
    out dx, al
    mov al, 0Dh
    mov dx, 2F9h
    out dx, al

    mov dx, 2FCh
    mov al, 00001011b
    out dx, al

    mov al, 0000011b
    mov dx, 2FBh
    out dx, al

    mov dx, 2F9h
    mov al, 00001011b
    out dx, al

    ret
initialize endp

checkCOM1 proc
    xor al,al
    mov dx,3FDh ; LSR status of com-port
    in al,dx
    test al,10h ; test 5th bit if not set error
    jnz writeError
    ret
checkCOM1 endp

writeError proc
   mov ah,9
   mov dx,offset errwrite
   int 21h
   ret
writeError endp

checkCOM2 proc
    xor al,al
    mov dx,2FDh
    in al,dx
    test al,10b
    jnz readError
    ret
checkCOM2 endp

readError proc
   mov ah,9
   mov dx,offset errread
   int 21h
   ret
readError endp

sendByte proc
    mov dx,3F8h
    mov al,byte1
    out dx,al
    mov ah, 86h
    mov cx, 10
    mov dx, 0
    int 15h
    ret
sendByte endp

readByte proc
    mov dx,2F8h
    in al,dx
    mov byte2,al
    ret
readByte endp

exit proc
    mov ax,4C00h
    int 21h
    ret
exit endp

start:
    mov ax,@data
    mov ds,ax
    call initialize
    call checkCOM1
    call sendByte
    mov al,2
    call checkCOM2
    call readByte
    mov dx,offset output
    mov ah,9h
    int 21h
    mov ah,02h
    mov dl, byte2
    int 21h
    call exit

end start