X-Hacker.org- Turbo Pascal - overlay create program overlays pp 149

 OVERLAY Create Program Overlays pp 149
 Syntax: Overlay [Procedure] [Function] ;
 Type: N/A
 Form: Declaration
 Purpose: Allow multiple sections of code to share the same address space.
 Notes: The overlay system allows the creation of programs that can be much
 larger than available memory permits. The technique is to collect
 a number of subprograms in one or more files separate from the main
 program file, which can then be loaded into the same address space.
 An overlay group is a collection of subprograms in a common file
 that will be loaded into the same address space one at a time.
 A section of memory is allocated for the overlay area and is sized
 to hold the largest of the single subprograms.
 Caveats: Overlay programs in the same group share the same area in memory
 and thus cannot be loaded concurrently. They must therefore not
 call each other. An overlay may call another overlay that resides
 in a separate overlay file, because they will load and execute in
 two different overlay areas of the main program.
 Overlay programs can not be forward declared. This may be
 easily circumvented by forward declaring an ordinary subprogram
 that in turn calls the overlay subprogram.
 Overlay program can not be recursive. This is also circumvented
 by declaring an ordinary recursive subprogram that in turn calls
 the overlay subprogram.
 Run-time errors occurring in overlays are found as usual, and an
 address is issued by the error handling system. This address is
 within the overlay area, and there is no way of knowing which
 subprogram was active at the time of the failure.
 Consecutive Overlays:
 Overlay Procedure One ;
 Begin
 End ;
 Overlay Procedure Two ;
 Begin
 End ;
 Overlay Procedure Three ;
 Begin
 End ;
 These three overlays will be in a common group and reside in a
 common overlay file (FileName.000). None can call the others.
 Non-Consecutive Overlays:
 Overlay Procedure One ; { Overlay.000 file }
 Begin
 End ;
 Overlay Procedure Two ; { Overlay.000 file }
 Begin
 End ;
 Procedure Not_An_Overlay ; { Force a separation of overlays }
 Begin
 End ;
 Overlay Procedure Three ; { Overlay.001 file }
 Begin
 End ;
 The first two subprograms will be in a common overlay group and
 in the first overlay file, Overlay.000.
 The third subprogram will be in a separate overlay group which
 loads in a separate overlay area in memory, and will reside in
 the second overlay file, Overlay.001.