The horizontal physics are completely different from the NES due to the fact the playfield has to scroll in chunks equal to 1/40th the screen size rather than 1/256th like on NES. Compromises had to be made and as a result the horizontal movement is not as fluid a motion, with
Mariored plumber starting/stopping instantly rather than accelerating and sliding to a stop. However, Sprybug seems to have totally rewritten his engine and perfected the horizontal acceleration in his upcoming game SonicZippy the Porcupine.
Thanks for the explanation. "Red Plumber" and "Zippy" on the 2600..Very cool. I have to come here more often so I'm up on things