Replying to a message of Jan Wagemakers to Darryl Gregorash:
 JW>>> Because the following has worked with only MSDOS Himem.sys
 JW>>> loaded. /* mov     ax,0 mov     ds,ax       ;DS = mov    
 JW>>> word ptr [ds:0b8142h],8403h
 DG>> Out of idle curiosity, what happens if you try this without
 DG>> himem.sys loaded?
 JW> The computer locks up. The only thing you can do is pressing
 JW> the reset-button ;-)
Hardly surprising; that offset into the segment is more than 64K. The only 
Intel CPU that would wrap the offset back around to 0 and start over is the 
8086/88. On any other Intel CPU, such an instruction, when in "real" mode, 
will generate a trap D, ie. a segment boundary violation. This stuff is all 
in Ralf Brown's interrupt list.
As I suggested before, Himem.sys probably has a trap D interrupt handler 
which detects the violation, determines what caused it (there is only one 
thing which can cause it in "real" mode), then kicks the CPU into native mode 
to perform the mem operation, before returning to "real" mode and the code 
which generated the segmentation fault.
--- FleetStreet 1.21 NR
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