transfer a byte to com port (14h)

.model small
.stack 100h

.data

writeerr db "Error: can't send byte",0Dh,0Ah,'$'
readerr db "Error: can't read byte",0Dh,0Ah,'$'

.code

jmp start

initialize proc
   xor ax,ax
   mov al,10100011b ; 8 bits in symbol ; 1 stop bit ; no parity ; 2400 bod
   mov dx,0 ; COM1 address 0000h
   int 14h
   ret
initialize endp

sendByte proc
   mov al,'D' ; byte to send
   mov ah,1 ; write function 01h of 14h
   mov dx,0 ; COM1 address 0000h
   int 14h
   test al,80h ; catch error
   jnz writeError
   ret
sendByte endp

writeError proc
   mov ah,9
   mov dx,offset writeerr
   add dx,2
   int 21h
   ret
writeError endp

readByte proc
    mov ah,2 ; read function 02h of 14h
    mov dx,1 ; COM2 address 0001h
    int 14h
    test al,80h ; catch error
    jnz readError
    ret
readByte endp

readError proc
   mov ah,9
   mov dx,offset readerr
   add dx,2
   int 21h
   ret
readError endp

print proc
   mov ah,02h
   mov dl,al ; moving read byte from al to dl
   int 21h
   ret
print endp

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

start:

   call initialize ; Initializing COM Port for writing
   call sendByte ; Send byte
   call readByte ; Read byte
   call print ; Write a byte to stdout
   call exit

end start