Binary to decimal in Delphi 7

I’m trying to do a simple console program where the user enters a binary string and gets a decimal number in return. I don’t need to check if the binary string contains anything else than 0s and 1s. I already managed to do decimal to binary but can’t do it the other way.

I tried some code I found on SO and Reddit but most of the time I get an I/O error (run‑time error 105).

Here’s my decimal to binary implementation:

program dectobin;

{$APPTYPE CONSOLE}

uses
  SysUtils,
  Crt32;

var
 d,a :Integer;
 str :String;

begin
   str:='';
   Readln(a);
   while a>0 do begin
    d:=a mod 2;
    str:=concat(IntToStr(d),str);
    a:=a div 2;
  end;
  Writeln(str);
  Readln;
end.

Basics of positional number systems (PNS)

• The number of unique digits in a PNS is its base (or radix)
  • The decimal system (base = 10) has ten digits, 0..9
  • The binary system (base = 2) has two digits, 0..1
• The position of a digit in a number determines its weight:
  • the rightmost digit has a weight of units (1), (base)^0 (decimal 1, binary 1)
  • the second (from right) digit has a weight of (base)^1 (decimal 10, binary 2)
  • the third (from right) digit has a weight of (base)^2 (decimal 100, binary 4)
  • and so on ....

Note that the weight is always base * weight of previous digit (in a right to left direction)

General interpretation of a string of numbers in any positional number system

  assign a variable 'result' = 0
  assign a variable 'weight' = 1 (for (base)^0 )
  repeat
    read the rightmost (not yet read) digit from string
    convert digit from character to integer
    multiply it by weight and add to variable 'result'
    multiply weight by base in prep. for next digit
  until no more digits

After previous, you can use e.g. IntToStr() to convert to decimal string.