🚀 go-pugleaf

Help me.
 
I am writing a device driver (as a DLL).  Part of the device driver is an
Interrupt routine that needs to access the DLL's Data Segment.
 
I have been having trouble using FreeSelector in my device driver.  I
have to play with Selectors because I use self-modifying code to
allow my interrupt routine to access its Data segment.
 
Is this the best way to get an Interrupt Routine to access a Data
Segment?
 
What I do is the following:
 
In an initialization routine (see below), I modify the interrupt routine by
writing the Data Segment Selector in.  In order to write data to
a Code segment (a no-no in protected mode), I first called
(AllocCStoDSAlias) to allocate a Data Selector for the Code Segment,
used it to modify the code, then called FreeSelector() to dispose of it.
 
*******
 
In the initialization routine: 
  (DS points to library's data seg, and this code is in the same
  segment as the interrupt code)
 
    push    ds                          ; save Viper Data Selector
 
    Arg     <cs>                        ; get IntCode Code Selector
    cCall   AllocCStoDSAlias            ; create Data Selector copy in ax
                                        
    mov     ds,ax                       ; ds now has Data Selector to IntCode
    pop     ax                          ; get Viper Data Selector
    mov     ds:[WORD PTR ViperStoreDS], ax ; modify code
 
    Arg     <ds>
    cCall   FreeSelector                ; free allocated selector
 
    mov     ds,ax                       ; restore Viper Data Selector
 
*******
 
At the beginning of my interrupt routine I have:
 
    mov     ax,0000h           ; !!! this part of code gets modified !!!
ViperStoreDS EQU  $-2          ; label to part of code to modify
    mov     ds,ax              ; load ds with Data Seg Selector
 
*******
 
The problem with FreeSelector() is that is causes a GP fault!?! (at
FreeSelector+0061).
 
Has anyone else had this experience?  What am I overlooking?
 
Thanks,
   Marni
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