Vanilla Doom - 35 FPS?

[Sonim]

Just read that vanilla Doom is capped at 35 FPS, but what exactly does it mean? Only the game update logic is capped at 35 FPS? If so, then what happens with the render code? Does it redraw the same frame or it just call some sleep function till the next tic?

Edited May 31, 2024 by Sonim

[Redneckerz]

Mandatory Doom Wiki article on this subject. This likely explains your answer:
 

Quote

Because the timer latching mechanism used by the vanilla Doom engine on MS-DOS to maintain vertical synchronization was not present in the Linux Doom code base, the game's main loop naturally runs as fast as it can given the amount of time left after game logic processing. This means that, without modification, the released Doom source builds into a program with an essentially uncapped framerate. The game logic is timed to 35 frames per second, but the renderer may be invoked any number of times per tic, effectively rendering the same scene multiple times in a row.

 

Addition of code to interpolate the positions of actors, sectors, and other game world entities between tics allows a port to take advantage of this and display the game in a way which appears more fluid than the actual simulation will allow without breaking backward compatibility with modifications, DeHackEd patches, or demos. Most use of "uncapped framerate" in the community has come to actually refer to the implementation of this interpolation, and not to the default state of the code base as described above.

 

Unrelated, but i got scared back into existence realizing that the threaded Doom Fandom has a quicker search hit than our lord and savior The DoomWiki.

[Sonim]

So the original Doom game under DOS actually used vsync in order to avoid unnecessary redraw! I didn't know this, but it makes sense performance wise.

[DiavoJinx]

IIRC, monitors using VGA cables in the 90s ran at 70Hz.  So if you do that math, 35 is a nicely computed half of 70.

[maxmanium]

27 minutes ago, DiavoJinx said:

IIRC, monitors using VGA cables in the 90s ran at 70Hz.  So if you do that math, 35 is a nicely computed half of 70.

 

And I believe Wolf3D ran at 70fps so checks out.

[Dark Pulse]

70 Hz monitors is precisely why it was decided to be 35 Hz logic. That also let them set up a buffering system for the video output.