DelCmos is a typical boot sector virus. It is only able to infect a hard disk when you try to boot the machine with an infected diskette in drive A:. At this time the virus infects the Master Boot Record (MBR) of the hard drive, and after that it will go resident to high DOS memory during every boot-up from the hard disk. Once the virus gets resident to memory, it will infect practicly all non-write- protected diskettes used in the machine.
DelCmos allocates two kilobytes of memory while it is active. This can be seen as a decrease in the total amount of DOS memory - it drops from 640kB to 638kB. DelCmos assumes that the machine has full 640kB of DOS memory. This is not always the case, as some systems reserve a kilobyte or two for internal BIOS needs. In this case, DelCmos will just crash the machine every time it's booted after the infection.
DelCmos also assumes the A: drive of the machine to be a 3.5" HD (1.44MB) drive. If it's a 5.25" drive or a 3.5" DD or ED drive, floppies may be corrupted during infection. They can be fixed with the FIXBOOT program.
DelCmos.A contains a routine to overwrite the CMOS SETUP information. DelCmos.B has this activation routine removed; it does nothing except spreads.
DelCmos.A is known to be in the wild in the USA. DelCmos.B was reported to be in the wild in Spain in January 1996.
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.
Description Created: Mikko Hypponen, F-Secure