Q11979: How Windows Resolves Far Calls When Movable Flag Is Used

Article: Q11979
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
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SUMMARY
=======

The information listed below addresses the following topics relating to using
the middle model of compilation (-AM) when a module's code segment has been
renamed using the -NT switch, and the segment is declared movable in the module
definition (.DEF) file:

1. Locking of the code segment by Windows

2. The handling of FAR calls

MORE INFORMATION
================

Windows does not necessarily keep the code segment locked. The Microsoft C
Compiler uses the BP register as a "frame pointer". Local variables and
parameters are always accessed using offsets from the BP register. The BP
register is initially even, and the Windows stack is word aligned. When a FAR
call is made, BP is increased by one. If the code segment is discarded, the
stack is walked and patched. By determining if BP is odd or even, Windows can
tell whether the call is FAR or NEAR.

When a long return address is on the stack, it has a pushed DS and BP, and
because the BP register is increased by one for FAR frames, FAR frames may be
detected by walking the task chain and BP stack-frame chains.

When the Windows prolog is set up, it does the following:

     extern far pascal funcname();

     cProc funcname,<FAR,PASCAL>
     "cBegin"
     Prolog: push ds ; Fixed/Moveable Multiple Data Segment Support
     pop ax
     nop
     inc bp ; Far Frame Marker/Moveable Code Support
     push bp
     mov bp,sp
     push ds ; Data Context Switch Code
     mov ds,ax ; "

     ...

     "cEnd"
     Epilog: sub bp,2
     mov sp,bp
     pop ds ; Data Context Switch Code
     pop bp
     dec bp ; Far Frame Cleanup

     ret

Additional query words: 3.00 3.10 no32bit

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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