Speed

Speed, abreviated as (SPD) is an ingame Variable that indicates how many units Mario's moving. Vertical Mario speed is called Vertical Speed (VS) and Horizontal Mario speed, Horizontal Speed (HS). If the Speed value is Negative, it means that Mario is moving Downwards in VS, or Backwards (Prefixed both cases with an "M").

Gravity effect and Terminal Speed

Mario falling at Terminal Speed (M75) in PSS

When Airborne, mario's Vertical Speed is constantly decreasing by 4 every frame, as Gravity is applying on him a Constant of -4 per frame. Additionally, Mario's Vertical Terminal Speed is of -75 (M75 as shown ingame), so he will not fall faster than that. VS is also unused when mario is on the ground, so his VS is set to 0 as soon as he touch the ground, and remain that way until Mario is Airborne again.

However, there are movements that don't reset Mario's VS to 0 when he touches the ground (See Vertical Speed Conservation).