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981 is a virus on Microsoft Windows that was discovered in the United States in May 1992.  Its origin is unknown. This virus is a memory resident infector of .COM and .EXE files, but not COMMAND.COM.

Payload

When the first 981 infected program is executed, the 981 virus will install itself memory resident at the top of system memory but below the 640K DOS boundary. Interrupt 12's return will not be moved. Total system and available free memory, as indicated by the DOS CHKDSK program, will have decreased by 1,952 bytes. Interrupts 21 and 22 will be hooked by the virus in memory.

Once memory resident, the 981 virus will infect .COM and .EXE programs when they are executed or opened for any reason.  .COM files increase in size by 981 bytes, though if the file was originally smaller than 981 bytes, they will become 1,934 bytes in length. .EXE files increase in size by 1,015 to 1,025 bytes. In both cases the virus will be located at the end of the program. This virus will sometimes reinfect previously infected programs, adding an additional 981 bytes to the file. The infected program's date and time in the DOS disk directory listing will not be altered.

One text string can be found in all 981 infected programs:

This program requires MS-DOS 3.00 or later

It is unknown what the 981 virus may do besides replicate.

Variants

Known variant(s) of 981 are:Joe's Demise: Functionally similar to the original virus, this variant has 11 bytes which differ.

Origin:  Unknown  June 1992.

Removal

Delete the infected files.

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