🚀 go-pugleaf

padgett@tccslr.dnet.mmc.com (A. Padgett Peterson) wrote recently

> Enclosed is a DEBUG script that will create a 68 byte .COM file
> (CLEAR.COM) that will zero out memory between the current load
> position and the TOM. To use, ...

The only trouble with this is there is a surprising amount of 
software which leaves active code in memory, but does not bother to 
tell DOS.   

To guard against possible unknown viruses like to Chinese Fish, 
which install themselves in high memory, but do not set the top of 
memory down, we recently added a feature to VET to fill unused 
memory with a diagnostic procedure which gives a warning message, 
and locks the PC, if anything attempts to execute unused memory.  So 
if you run VET, and an unknown virus of this type is already in 
memory, you get the warning as soon as VET calls an interrupt the 
virus has trapped.

Ahh! Another loophole closed!  Unfortunately a big customer 
immediately complained some users could no longer log in.  

We investigated, & found that they were using Microsoft Lan Manager.  
When PROTMAN was run from CONFIG.SYS a block of code was installed 
at 7000:7800, but top of memory (as recorded at offset 2 in the PSP) 
remained 9FFF:0000.  If this code was overwritten by running VET (or 
anything else) before the user logged in, the system would crash 
when the program NBP.EXE was run as part of the log in procedure.

We sent details of this to MicroSoft Support, but, naturally, never 
received any reply.

We added an option to VET to disable this feature.  A significant no. 
of users have encountered similar problems.  Most have been using 
Lan Manager, but other programs may also have been involved.

Roger Riordan                     riordan.cybec@tmxmelb.mhs.oz.au

CYBEC Pty Ltd.                                 Tel: +613 521 0655
PO Box 205, Hampton Vic 3188   AUSTRALIA       Fax: +613 521 0727