When it is loaded into memory, it hooks INT 21h to infect any executable that is run, but not every program would be infected.
The virus occupies 3,424 bytes in memory.
The virus activates on April 1st, December 30th and 31st.
Disk cracked prank
When an infected program is run on April 1st, the virus clears the screen, beeps once and displays the following message:
System diagnostic failure: cylinder #1 of your hard disk is cracked. There is a peril of hard disk explosion. Only an immediate formatting can help. Confirm format now [Y/N]?
If the user presses Y, the virus adds the following message:
Say GOOD BYE to all data on your hard disk! Start formatting ...
If other keys are pressed, the virus adds the following message:
Wrong. Your hard disk NEEDS to be formatted. Say GOOD BYE to all data on your hard disk! Start formatting ...
The virus also includes a feature of fake disk activity, making the disk activity light turns on constantly which looks like the hard disk is really formatting.
After that if ESC key is pressed, it adds the message:
Relax ! It's a joke. Your hard disk is OK. Press the ENTER key.
And then it returns to the host program after an Enter key. This can show that Mask is one of the few earliest Joke Programs on DOS, and one of the first joke viruses.
New Year Greeting
When an infected program is run on December 30th or 31st, the virus clears the screen, beeps once and displays an ASCII Christmas tree and the following message at the center of screen:
Happy New Year, buddy ! Yours Truly The Mask 2
And the host program continues after an Enter key.