Boot-sector viruses infect computer systems by copying code either to the
boot sector on a floppy disk or the partition table on a hard disk. During
startup, the virus is loaded into memory. Once in memory, the virus will
infect any non-infected disks accessed by the system. Examples of boot-
sector viruses are Michelangelo and Stoned.
Boot-sector viruses are spread to computer systems by booting, or
attempting to boot, from an infected floppy disk. Even if the disk does not
contain the MS-DOS system files needed to successfully boot, an attempt to
boot from an infected disk will load the virus into memory. The virus hooks
itself into memory as if it were a device driver. The virus moves the
Interrupt 12 return, allowing itself to remain in memory even after a warm
boot. The virus will then infect the first hard disk in the system.
Because the virus moves the Interrupt 12 return, the MS-DOS system memory
will be 2K (2048 bytes) smaller than normal. This can be verified by
running the MS-DOS CHKDSK command.
For example, if your system has 640K, CHKDSK will report:
655360 Total Bytes Memory
If the system is infected with a boot-sector virus, CHKDSK will
653312 Total Bytes Memory
Some systems use 1K (1024 bytes) of memory for the BIOS. Other systems use
2K (2048 bytes) of memory for shadow RAM. You must take this into account
before CHKDSK can be used as an accurate measure of whether or not a system
is infected with a virus. Please refer to the hardware manufacturer to see
if the system uses part of the MS-DOS 640K of memory.
Once a system is infected with a boot-sector virus, any non-write-protected
disk accessed by this system will become infected. For example, simply
doing a DIR command on a floppy disk will cause the disk to become infected
with the virus.
Note: MS-DOS version 5.0 disks are shipped without a notch; therefore, they
are write-protected. The chances of these disks containing a virus are
close to none. The MS-DOS 5.0 disk files are compressed, so the actual file
sizes are different. You can determine a compressed file by the underscore
character (_) that is the last character of the filename extension. To
expand a compressed file, use the EXPAND utility on Disk 5 (5.25-inch disk
set) or Disk 3 (3.5-inch disk set).