A moving platform that carries you, the way Mario's lifts do

A hole in the ground with a lift going back and forth over it. Play it, step onto the lift and let go of the pad: the square rides it across and back, and walks off its far end onto the other side. The program is 420 instructions in 1,122 bytes, built and checked the same way.

Play it here Change it on the desk The cartridge, to play anywhere that plays NES cartridges: platforms.nes

On the screen

Standing on something that moves means moving with it

In Super Mario Bros. a lift is an object like an enemy, kept in the same slots and moved a step at a time. When Mario stands on one, the game moves him by the lift’s own step on the same frame, so he keeps his place on it without anything being pressed. We watched a lift in 1-2 carry him and checked every step. This cartridge does it the same way, in the order that makes it work: the lift moves first, then the square on it by the same, and only then whatever the pad asks for.

	this cartridge	Super Mario Bros.
which way the lift moves	across	vertically
pixels a step, and frames between steps	1, every 1	1, every 1
pixels it travels end to end	48	none: it leaves the bottom of the screen and comes back in at the top
frames the player stood on it	158	77
steps that moved the player by the same, of those taken while standing with nothing pressed its way for a second	68 of 68	72 of 72
the player's height above the lift held steady	yes	yes

As written

The part of the program this lesson is about. The whole program is a plain text file: platforms.s

;; Across: Right adds 10/256 a frame up to 1.5 pixels a frame; without
;; it the speed falls by the same until it is 0. Then the right edge's
;; two corners are looked up: in a block, the square goes back to touch
;; it and its speed across is taken away.
;; The lift: a pixel a frame along its path, turning at either end (x
;; 80, where the hole begins, and x 128, where its far end meets the far
;; side). A square standing on it is moved the same pixel on the same
;; frame, before anything the pad asks for; and moved by the step the
;; lift just took, so the turn comes after the carry (turned first, the
;; square went one way while the lift went the other, once at each end).
lift:
    LDA $30
    CLC
    ADC $31
    STA $30
    LDA $32
    BEQ carried
    LDA $11
    CLC
    ADC $31
    STA $11
carried:
    LDA $30
    CMP #$50
    BEQ turn
    CMP #$80
    BNE moved
turn:
    LDA #$00
    SEC
    SBC $31
    STA $31
moved:
    RTS

Before this one: Ground, a block and a wall, solid the way Mario's are

Next: A coin from a block and the score, the way Mario's are

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