A name typed from a grid, the way Zelda's is

A screen for typing a name, with a grid of letters we drew ourselves. Play it: the arrows move the cursor, A types the letter under it, and holding an arrow keeps it moving. The program is 268 instructions in 611 bytes, built and checked the same way.

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

On the screen

The cursor sits behind the letters

On the screen where you register your name, The Legend of Zelda’s cursor is a sprite drawn behind the background, so the letter under it shows through, and it blinks. It moves the frame after you press, and if you hold the button it waits a moment and then keeps going, faster. At the edge of the grid it does not stop: it carries on in reading order, so going left from the first letter takes you to the last cell. Every step makes a click, and A writes one tile, the letter, into your name the frame after. This cartridge does the same with a grid, a wait, a beat and a click of its own.

	this cartridge	The Legend of Zelda
frames from a press to the cursor moving	0	1
frames held before it moves again	20	17
then frames between moves	6	9
letters across the grid	10	11
left from the first cell goes to cell	39	43
tiles written for each letter typed, and frames after A	1, 1 after	1, 1 after
the cursor is drawn behind the letters	yes	yes
frames the cursor is shown, then hidden	16, then 16	8, then 8
moves that made a click	13 of 13	11 of 11

As written

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

;; Up, Down, Left and Right (bits 3 to 0 of the pad). A new press moves
;; at once and sets the wait at 20 frames; held, the cursor moves again
;; when the wait runs out, and then every 6.
steer:
    LDA $01
    AND #$0F
    BEQ still
    LDA $02
    AND #$0F
    BNE held
    LDA #$14
    STA $06
    JMP step
held:
    DEC $06
    BNE still
    LDA #$06
    STA $06
step:
    LDA $01
    LSR A
    BCS right
    LSR A
    BCS left
    LSR A
    BCS down
    LDA $40
    SEC
    SBC #$0A
    BCS moved
    ADC #$28
    JMP moved
down:
    LDA $40
    CLC
    ADC #$0A
    CMP #$28
    BCC moved
    SBC #$28
    JMP moved
left:
    LDA $40
    SEC
    SBC #$01
    BCS moved
    LDA #$27
    JMP moved
right:
    LDA $40
    CLC
    ADC #$01
    CMP #$28
    BCC moved
    LDA #$00
moved:
    STA $40
;; The click: a short high note on the first square channel.
    LDA #$9F
    STA $4000
    LDA #$00
    STA $4001
    LDA #$40
    STA $4002
    LDA #$00
    STA $4003
still:
    RTS
;; A new press of A: the letter under the cursor waits to be written at
;; the end of the name, while the name has room. The empty cell types a
;; space: nothing is written and the name moves on.
type:
    LDA $02
    EOR #$FF
    AND $01
    AND #$80
    BEQ full
    LDA $41
    CMP #$08
    BCS full
    CLC
    ADC #$CC
    STA $43
    INC $41
    LDA $40
    JSR tileof
    STA $42

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