A jump that feels like Mario's

A red square on a strip of ground. Right walks, A jumps. The program is 228 instructions in 513 bytes, written in the same text the autopsy writes its listings in and turned into the cartridge by the same assembler, which checks that the text gives back the cartridge byte for byte.

Play it here The cartridge, to play anywhere that plays NES cartridges: jump.nes

On the screen

Where the numbers came from

Every number in the program was read off Super Mario Bros. by playing it on our console and watching its memory after each frame: how fast the speed across grows while Right is held, how fast the jump starts, and how much is added to the speed down each frame. Two of those depend on A: while A is held and the jump is still rising, the pull is light; let go, or start to fall, and it is much stronger. That is why a tap is a hop and a long press goes high.

The two, played the same way

The same presses, played on this cartridge and on Super Mario Bros., frame for frame.

	this cartridge	Super Mario Bros.
frames of Right to full speed	38	40
full speed, pixels a frame	1.5	1.5
A held 4 frames: frames in the air, pixels risen	27 frames, 34 pixels	27 frames, 34 pixels
A held 20 frames: frames in the air, pixels risen	45 frames, 63 pixels	45 frames, 63 pixels
A held 80 (70) frames: frames in the air, pixels risen	58 frames, 71 pixels	57 frames, 71 pixels

One line in the wrong place

The first version pulled before it moved: each frame it added the pull to the speed and then moved by the new speed, and every jump came out lower than Mario’s. The program below moves first and pulls after, and the table above is what that order gives. What is left: with A held 80 (70) frames, this cartridge gives 58 frames, 71 pixels and Super Mario Bros. gives 57 frames, 71 pixels.

As written

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

;; Up and down: a new press of A on the ground starts a jump.
jump:
    LDA $03
    BNE air
    LDA $02
    EOR #$FF
    AND $01
    AND #$80
    BEQ done
    LDA #$00
    STA $16
    LDA #$FC
    STA $17
    LDA #$01
    STA $03
air:
    CLC
    LDA $14
    ADC $16
    STA $14
    LDA $15
    ADC $17
    STA $15
    LDX #$60
    LDA $01
    AND #$80
    BEQ pull
    LDA $17
    BPL pull
    LDX #$1E
pull:
    TXA
    CLC
    ADC $16
    STA $16
    LDA $17
    ADC #$00
    STA $17
    BMI fall
    CMP #$04
    BCC fall
    LDA #$00
    STA $16
    LDA #$04
    STA $17
fall:
    LDA $17
    BMI done
    LDA $15
    CMP #$B0
    BCC done
    LDA #$B0
    STA $15
    LDA #$00
    STA $14
    STA $16
    STA $17
    STA $03
done:
    RTS

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