Here is a little routine I have that clears a segment. the 'mov'

gives illegal instruction on the second pass through the loop ???

;
; clrseg - Clear any segment.
;       FS = Segment base, ECX = Size in Paragraphs.
;
clrseg  proc     near
        push     eax
        push     ecx
        push     edi
;
        sub      edi,edi           ;clear index
        shl      ecx,2             ;mult by 4 = n'dwords/para

        add      edi,4             ;next...

;
        pop      edi
        pop      ecx
        pop      eax
        ret
clrseg  endp
;

With FS set up correctly, it shouldn't go bang.  I think you'll need
to post the code where you set it up.
It could happily write to FS:0, but not FS:4, if the value at FS:0 is
0, and the value as FS:4 isn't.

Jim

    Bill Lanam