: To run the game smoothly on modern computers, players typically take the official data file (DOOM.WAD) from their purchased copy and run it using free, open-source community engines like GZDoom or Chocolate Doom.

While the first episode of the original 1993 game was famously released as free shareware, the complete expanded edition known as The Ultimate Doom is a commercial product that must be purchased.

: If you want to play for free legally, you can download the original 9-level shareware episode from the Doom Wiki Shared Files or play it directly in your web browser via emulation sites like DOS.Zone .

If you tell me what operating system you are using, I can guide you on how to get the classic shareware version running safely.