KB141203 - Fault Handling Logic Changed for Windows 95

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In Windows 3.1x, the following code logic might have been used by a fault handler:

   pPrevFaultHandler dd ?

      mov     eax, fault_number
      mov     esi, offset32 FaultHandler
      VMMCall Hook_V86_Fault
      mov     pPrevFaultHandler, esi

   BeginProc FaultHandler
      ;;;
      ;;; handler code
      ;;;

      cmp     pPrevFaultHandler, 0
      jz     @F
      jmp     pPrevFaultHandler
   @@:   ret
   EndProc FaultHandler

In Windows 95, this logic should be modified as follows:

   pPrevFaultHandler dd 0

      mov     eax, fault_number
      mov     esi, offset32 FaultHandler
      VMMCall Hook_V86_Fault
   ;   NOTE:  No "mov pPrevFaultHandler, esi" instruction
   ;   esi = 0 if this is the first fault handler
   ;   pPrevFaultHandler will *always* be nonzero.
   ;   if esi = 0, pPrevFaultHandler will be the address
   ;   of the default handler.

      ...
      mov     eax, fault_number
      mov     esi, offset32 FaultHandler
      VMMCall UnHook_V86_Fault

   BeginProc FaultHandler, HOOK_PROC, pPrevFaultHandler
      ;;;
      ;;; handler code
      ;;;

   ;   NOTE: No "cmp pPrevFaultHandler, 0" instruction
      jmp   pPrevFaultHandler
   EndProc FaultHandler

Fault Handling Logic Changed for Windows 95

SUMMARY

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