Title:      "Putting the Byte on a Virus"
Reference:  PC World July 1990
Author:     Bob Billingsley
Files:      NOVIRUS.DOC, NOVIRUS.BAT

To guard against virus attacks, insert the batch file commands from
NOVIRUS.BAT into AUTOEXEC.BAT. The commands compare COMMAND.COM with
a copy of itself, and can therefore catch a virus that's clever
enough not to change COMMAND.COM's length. Booting takes a little
longer--about 3 seconds more on my 12 MHz AT clone--a small price to
pay for extra protection from a wicked virus.

To put this technique into action, COPY COMMAND.COM to COMMAND.XXX in
your boot drive's root directory. Add the batch file commands to
AUTOEXEC.BAT after a PATH statement that lists the directory where
FC.EXE is stored. If your DOS version has COMP.EXE instead of FC.EXE,
change the third line to echo N | comp command.com command.xxx | find
"ok">chkcomd1.$$$, and insert another line, echo, after the fifth
line.

From then on, every time you boot, the bytes in COMMAND.COM will be
compared to the original file copy. If the comparison fails, the
batch file will display a message and pause. In that event, press
<Ctrl>-C to return to the DOS prompt and copy a fresh COMMAND.COM
from your DOS master disk onto your hard drive.

echo off
echo Comparing COMMAND.COM and COMMAND.XXX
fc command.com *.xxx | find "no" >chkcomd1.$$$
copy chkcomd1.$$$ chkcomd2.$$$ >nul
del chkcomd1.$$$
if exist chkcomd2.$$$ goto okay
echo !! Comparison failed -- Possible Virus !!
goto end
:okay
del chkcomd2.$$$
:continue
rem Batch file continues here...
:end

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