Too many sprites on a line flicker rather than vanish, the way Mario's do

Ten figures standing on one line, two more than the console can draw there. Play it and press Select to turn the order on and off: off, the same two are missing every frame; on, they all flicker. The program is 141 instructions in 341 bytes, built and checked the same way.

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

On the screen

The console draws eight sprites on a line

The picture chip looks through sprite memory as it draws each line and takes the first eight sprites it finds there; any more on that line are simply left out. A game that writes its sprites in the same order every frame loses the same ones every frame, and they are never seen. Super Mario Bros. moves an enemy’s sprites to a different place in sprite memory each frame, round a short cycle, so that if a line ever has too many, a different one goes missing each time and they all flicker instead. In the run we measured it never had more than eight on a line, so nothing was left out; the turning is there all the same. This cartridge shows the problem and the cure side by side.

	this cartridge	Super Mario Bros.
most sprites on one line	10	6
sprites left out on that line each frame	2	0
figures never drawn, written in the same order every frame	2	it always turns
figures never drawn, with the order turning	0	0
frames before a figure's sprites are back in the same place in sprite memory	10	3

As written

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

;; Select turns the turning on or off; while it is on, each frame starts
;; one figure further on, round all ten.
order:
    LDA $02
    EOR #$FF
    AND $01
    AND #$20
    BEQ same
    LDA $40
    EOR #$01
    STA $40
same:
    LDA $40
    BEQ kept
    INC $41
    LDA $41
    CMP #$0A
    BCC kept
    LDA #$00
    STA $41
kept:
    RTS
;; Each figure, two sprites (head over body), written into sprite memory
;; at its place in this frame's order: the figure written first takes
;; the first two sprites, and so on round.
figures:
    LDX #$00
figure:
    TXA
    SEC
    SBC $41
    BCS placed
    ADC #$0A
placed:
    ASL A
    ASL A
    ASL A
    TAY
    LDA #$77
    STA $0200,Y
    LDA #$7F
    STA $0204,Y
    LDA #$06
    STA $0201,Y
    LDA #$07
    STA $0205,Y
    LDA #$00
    STA $0202,Y
    STA $0206,Y
    LDA across,X
    STA $0203,Y
    STA $0207,Y
    INX
    CPX #$0A
    BNE figure
    RTS

Before this one: A status bar that stays still, the way Mario's does

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