A pause that holds the world still, the way Mario's does

The sound lesson’s square and tune, with a pause on Start. Play it, walk right, and press Start; try pressing it again straight away. The program is 313 instructions in 717 bytes, built and checked the same way.

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

On the screen

A pause draws nothing

Pausing Super Mario Bros. puts nothing on the screen: no word, no box. The game simply stops running its world, and the console keeps showing the last picture it was given. The music is cut on the same frame and a short sound plays instead. For a while after a press, Start is ignored, so a button that bounces or a second press that comes too soon cannot undo it. Unpausing plays the sound again, Mario moves at once, and the music comes back only when that wait is over. This cartridge pauses the same way, with a wait, a chime and a tune of its own. Only that shape is taken from the game; what either sound plays is not compared.

	this cartridge	Super Mario Bros.
frames from Start to everything holding still	0	1
nothing moved while paused, with Right held and A pressed	yes	yes
frames Start is ignored after a press	32	43
frames in when a second Start was tried, and did nothing	15	20
writes to the picture while paused	0	0
two pictures taken while paused are identical	yes	yes
every sound cut on the press	yes	yes
notes in the pause sound, and frames between them	3, 8 apart	4, 6 apart
frames from Start to moving again	0	1
frames from unpausing until the music plays again	32	43

As written

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

;; Start: a new press flips the pause, unless one came in the last 32
;; frames. Either way the sound is cut, the chime starts, and the tune's
;; next note is due as soon as it may play.
pause:
    LDA $54
    BEQ ready
    DEC $54
    RTS
ready:
    LDA $02
    EOR #$FF
    AND $01
    AND #$10
    BEQ none
    LDA $53
    EOR #$01
    STA $53
    LDA #$20
    STA $54
    LDA #$00
    STA $4015
    STA $51
    STA $52
    STA $56
    LDA #$03
    STA $4015
    STA $55
none:
    RTS
;; The chime: three short notes on the second square channel, 8 frames
;; apart, rising.
chime:
    LDA $55
    BEQ quiet
    DEC $56
    BPL quiet
    LDA #$07
    STA $56
    DEC $55
    LDX $55
    LDA #$9C
    STA $4004
    LDA #$00
    STA $4005
    LDA chimenote,X
    STA $4006
    LDA #$00
    STA $4007
quiet:
    RTS

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