Простой пример DLL

library MyDll;

uses Windows;

function SomeFunc(I: Integer): Integer; stdcall;
begin
  if Odd(I) then 
    Result := 3 * I + 1
  else 
    Result := I div 2;
end;

exports
  SomeFunc;

begin
end.

unit MyDllImport;

interface

uses Windows;

function SomeFunc(I: Integer): Integer; stdcall;

implementation

function SomeFunc; external 'mydll.dll';

end.