How Windows Resolves Far Calls When Movable Flag Is Used

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

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