Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

Linking to Object Files
To call routines from a separately compiled object file, first link the object file to your application using the 
$L
 (or
$LINK
) compiler directive. For example,
{$L BLOCK.OBJ}
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links BLOCK.OBJ into the program or unit in which it occurs. Next, declare the functions and procedures that you
want to call:
procedure MoveWord(var Source, Dest; Count: Integer); external;
procedure FillWord(var Dest; Data: Integer; Count: Integer); external;
Now you can call the 
MoveWord
 and 
FillWord
 routines from BLOCK.OBJ.
On the Win32 platform, declarations like the ones above are frequently used to access external routines written in
assembly language. You can also place assembly-language routines directly in your Delphi source code.