george-emu/src/roms/keyboard.asm

; .setcpu "65C02"
    .include "./macro.inc"

    .org $8000

n = $01 ; temporary storage for data stack operations
key_buffer = $200

reset:
    sei
    ldx #0; initialize data stack pointer

initdisplay:
    lda #0
    ldy #0

cleardisplay:
    sta $6000,y
    sta $6100,y
    sta $6200,y
    sta $6300,y
    sta $6400,y
    sta $6500,y
    sta $6600,y
    sta $6700,y ; this goes slightly over but it's fine
    iny
    bne cleardisplay
    cli

main:
    ; jsr read_keys
    ; lda key_buffer
    lda $4400
    sta $6000
    jmp main

; first, let's do the simplest case of just the letter a being pressed
; 1. check row 2 ($4402, where the a key is) for pressed keys
; 2. if any keys are pressed, check if the key is a (bit 0)
; 3. if the key is not a, store zero in the key buffer
; 4. if the key is a, store the ascii for a in the key buffer 
; 5. return

; so u don't have to scroll: key_buffer = $200
; key buffer just shows the current state of the keyboard
; it is not a queue of keys to print

read_keys:
    lda $4402 ; check row 2
    beq check_key ; if there're any keys, check which
    clear_buffer:
        stz key_buffer
        rts
    check_key: ; if there is a key pressed, check if it's a
        ror
        bmi store_key
        rts
    store_key:
        ; in a sec we'll set up ascii table lookup,
        ; for now let's just load the ascii byte for a
        lda #$61
        sta key_buffer
rts

draw:
    push_coords #0, #0
    push_char #$af
    jsr draw_char
    rts

draw_char: ; draw a character c at (x, y) (n1: x n2: y n3: c -- )
    lda 0, x ; load a with character to draw
    pop ; and pop it off the stack
    jsr get_char_address ; calculate where to put the character in memory
    sta (0, x) ; store a at the address pointed to on the stack
    rts

get_char_address:   ; gets vram address for a character at (x, y),
                    ; (n1: x n2: y -- n: $6000 + x + (64 * y))
    ;jsr push_lit ; push 64 onto stack, low byte first
    ;.byte 64
    ;.byte 0
    pha
    lda #64
    push ; doing this instead until `push_lit` is fixed
    sta 0, x
    stz 1, x
    jsr mult ; multiply 64 with y (n2)
    jsr plus ; add result with x (n1)

    ;jsr push_lit ; push vram address onto the stack
    ;.byte $00
    ;.byte $60
    lda #$60
    push
    sta 1, x
    stz 0, x
    jsr plus ; add vram start address to result

    pla
    rts

fill: ; fills an area from (x1, y1) to (x2, y2) will character c, (n1: c n2: x1 n3: y1 n4: x2 n5: y2 -- )
    jsr get_char_address

irq: 
    rts

isr: ; interrupt service routine
    pha
    phx
    phy
    jsr irq
    ply
    plx
    pla
    rti

    .include "math.inc"

    .org $fffc
    .word reset
    .word isr