Rooms that slide in the way Zelda's do

The square in a grid of rooms, seen from above. The picture stands still while it walks; out of the left or right side, the next room slides in and carries it along. The program is 354 instructions in 759 bytes, built and checked the same way.

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

On the screen

A room at a time

Watching The Legend of Zelda’s memory showed one byte that changes each time Link leaves a screen: it went down by 1 for a step left and by 16 for a step up, so the land is a grid 16 rooms wide, numbered along its rows. At the edge, the picture slides a whole screen at a steady rate and Link is carried with it. This cartridge keeps a room number the same way and slides the same way. How each draws the room that is coming in is where they differ, and the table says how, from the log of every write each made to the picture chip.

	this cartridge	The Legend of Zelda
the room's number, before and after a step left	119 to 118	119 to 118
frames from the number changing to the picture moving	1	30
frames the slide takes	64	64
pixels moved, and pixels a frame	256, 4 a frame	256, 4 a frame
written before the picture moves	none	23 rows of 32 tiles across, 2 rows of 24 tiles across
written while it slides	32 columns of 30 tiles down	32 columns of 22 tiles down, 1 row of 24 tiles across
frame of the slide the first column is written in	1	32
frames between columns	2	1

Where the two part ways

The Legend of Zelda writes 23 rows of 32 tiles across, 2 rows of 24 tiles across before the picture moves, which takes it 30 frames, and writes nothing for the first half of the slide; its columns come from frame 32 of the slide on, one a frame. This cartridge writes nothing beforehand and draws the incoming room through the whole slide, a column every second frame, each just ahead of the edge coming into sight. What The Legend of Zelda’s rows are for has not been read yet.

Not yet

Up and down stop at the edge here: the slide is across only. In the game a step up slides the screen down; that is the next cut.

As written

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

;; Sliding: on every second step a column of the new room, the next to
;; come into sight (from its right side going left, from its left going
;; right); then the scroll moves 4 pixels toward the new room, the square
;; goes with the picture, and after 64 steps the new room is the one on
;; the screen.
slide:
    LDA $34
    AND #$01
    BNE stepped
    LDA $34
    LSR A
    LDX $32
    BNE fromleft
    EOR #$1F
fromleft:
    JSR prepcol
    LDA $35
    EOR #$01
    ASL A
    ASL A
    ORA $39
    STA $39
    LDA #$01
    STA $38
stepped:
    LDA $32
    BNE east
    SEC
    LDA $36
    SBC #$04
    STA $36
    LDA $37
    SBC #$00
    AND #$01
    STA $37
    LDA $11
    CMP #$EC
    BCS carried
    CLC
    ADC #$04
    STA $11
    JMP carried
east:
    CLC
    LDA $36
    ADC #$04
    STA $36
    LDA $37
    ADC #$00
    AND #$01
    STA $37
    LDA $11
    CMP #$04
    BCC carried
    SEC
    SBC #$04
    STA $11
carried:
    INC $34
    LDA $34
    CMP #$40
    BNE sliding
    LDA $35
    EOR #$01
    STA $35
    STA $37
    LDA #$00
    STA $36
    STA $31
sliding:
    RTS

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