🚀 go-pugleaf

Please help me to improve my PASCAL code :
It works (nearly ... you can use the convDB only once :-) .... damn it
!  :-)
as advertised, but I would like to improve its
structure / performance / size AND firewalls against bad input
Any suggestions will be appreciated ..... thanks in advance . Mike

PROGRAM bitodec (INPUT,OUTPUT);

{is used to convert numbers from decimal to binary
 as well as from binary to decimal}

USES
  CRT;

VAR
  choice , quit : CHAR;

{---------------------------------------------------------------------------}

PROCEDURE errormsg;

BEGIN
  WRITELN;
  WRITELN ('*** ERRONEUS INPUT ***');
  WRITELN;
END;

{---------------------------------------------------------------------------}

PROCEDURE convBD;
{pre: user is propted to enter a number in base 2 representation
 post: number is converted to decimal representation and printed to
screen}
var
   binarry: ARRAY [1..16] OF INTEGER;
   n , num , counter : INTEGER;
   choice : CHAR;
BEGIN
  WHILE n < 17 DO
  binarry[n] := 0;
  n := 0;
  WRITE ('Enter the number in binary: ');
  REPEAT
    n := n + 1;
    choice := READKEY;
    WRITE (choice);
    IF choice = '1' THEN
      binarry[n] := 1
    ELSE IF choice = '0' THEN
      binarry[n] := 0
    ELSE IF (choice <> #13) THEN BEGIN
    errormsg;
    EXIT;
    END;
  UNTIL NOT (choice IN ['0','1']);
  WRITELN;
  counter := 0;
  num := 0;
  REPEAT
    counter := counter + 1;
    num := (num * 2) + binarry[counter];
  UNTIL counter = (n - 1);
  WRITE ('equals to ',num,' decimal');
  READLN;
END;

{---------------------------------------------------------------------------}

PROCEDURE convDB;
{pre: user is prompted for decimal number
 post: converted to binary and printed to the screen}

VAR
  I , Code , n : INTEGER;
  decstr : STRING[5];
  binout : STRING[16];

BEGIN
  binout := '0000000000000000';{saves unnecessarry string operations
                                and keeps code short}
  WRITE ('Enter the decimal to be converted : ');
  READLN (decstr);
  VAL (decstr , I , Code);
  IF Code <> 0 THEN
    WRITE ('error at position :',Code)
  ELSE
  FOR n := 16 DOWNTO 1 DO BEGIN
    IF ODD(I) THEN
      binout[n] := '1';
      I := I SHR 1;{shift right one}
  END;{FOR}
  WRITE ('Equals to : ',binout);
END{convDB};

{============================================================================}

BEGIN
  REPEAT
    CLRSCR;
    WRITELN ('options :');
    WRITELN ('1  Convert binary to decimal');
    WRITELN ('2  Convert decimal to binary');
    WRITELN ('3  quit');
    WRITELN;
    choice := READKEY;
    IF choice = '1' THEN convBD
    ELSE IF choice = '2' THEN convDB
    ELSE IF choice = '3' THEN HALT(1)
    ELSE errormsg;
    WRITELN;
    WRITE ('Quit ?       ');
    READ (quit);
  UNTIL (quit =  'y') OR (quit = 'Y');
END.{bitodec}