ID Number: Q66054
5.10 5.10a
MS-DOS
Summary:
The following code illustrates the creation of an overlay with an
assembly module. The first procedure _main in the root module calls
the procedure rt2 in the overlaid module.
It is possible to create overlays while linking with the medium- and
large-memory model libraries.
More Information:
The Overlay Manager is a part of a higher-level language's run-time
library (C for instance). While linking, this library must be used if
the code for the Overlay Manager is to be linked with the program.
Page 285 of the "Microsoft Macro Assembler 5.1 CodeView and Utilities
Guide" explains the use of overlays with assembly-language modules.
Only modules with a 32-bit (long) call/ret instruction can be
overlaid. Routines called should be declared as FAR. The linker is
unable to produce overlay modules that can be called indirectly with
function pointers. The root module is the resident (nonoverlaid)
portion of the program, and the overlaid module is the transient
portion.
There are two segments created by the linker. The Overlay_Area and the
Overlay_Data. The size of the Overlay_Area (class "code") is
determined by the Overlay_End statement that points to the end of the
Overlay_Area segment. The Overlay_Data is placed in DGROUP. The two
segments can be viewed in a MAP file.
LINK sets the Overlay_Area to fit the largest overlay. Only the code
is overlaid. The Overlay_Data is initialized by LINK with information
about the executable file and information useful to the run-time
Overlay Manager (for example, number of overlays).
LINK replaces all FAR calls from root to overlay, or overlay to
overlay with the following information:
1. A software interrupt (usually INT 3FH, which can be overridden
the /OVERLAYINTERRUPT option on the LINK line).
2. The overlay number.
3. The offset into the overlay segment.
This information provides the overlay manager with a mechanism to do
the following:
1. Load a specified overlay into memory.
2. Transfer control to a specified offset within an overlay.
If you need to modify the behavior of the Overlay Manager, the source
code is available as part of the Microsoft C run-time source code,
which can be ordered through Microsoft Sales and Service.
Code Example
------------
;main module (1.asm)
assume cs:cseg,ds:dseg,ss:sseg
DSEG segment word public
line02 db 'In the Root module'
db (13) ;CR
db (10) ;LF
db (36) ;$ String Terminator
DSEG ends
EXTRN rt2:far
cseg segment byte public
public _main ;necessary as the run-time library
;following the entry point at _astart
;calls _main (in C startup module crt0.asm)
_main proc FAR
mov ax,DSEG
mov ds,ax
mov ah,09 ;String output Function 09H
mov dx,seg line02
mov ds,dx
mov dx,offset line02
int 21h ;Invoking DOS Interrupt 21H
call rt2 ;Call to the Overlay Procedure
mov ah,4Ch ;Function 4CH return to DOS
int 21h
_main endp
cseg ends
sseg segment stack
db 20 dup (0)
sseg ends
end
------------------------------------
;overlaid module (2.asm)
;assume cs:c1seg,ds:d1seg
D1SEG segment word public
line03 db 'In the Overlaid module'
db (13)
db (10)
db (36)
D1SEG ends
public rt2
c1seg segment byte public
rt2 proc FAR
mov ax,D1SEG
mov ds,ax
mov ah,09
mov dx,seg line03
mov ds,dx
mov dx,offset line03
int 21h
ret
rt2 endp
c1seg ends
end
------------------------------------
The following are commands to assemble and link:
MASM /MX 1.asm;
MASM /MX 2.asm;
LINK /NOI /NOD 1.obj + (2.obj),,,slibcer;
Notes
-----
Consider the following:
1. Either use Segmented Linker version 5.01.20 or later, or OverLay
Linker version 3.64 or later.
2. Use the appropriate run-time library from C version 6.00 and C
versions 5.x (medium or large).
3. Use /MX and /NOI to preserve case sensitivity.
Output from .EXE occurs in the following:
1. In the Root module
2. In the Overlaid module