george-emu/roms/demo.asm

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

    .org $8000

n = $01 ; temporary storage for data stack operations

temp = $20 ; scratchpad page

cursor = $300
cursor_x = cursor
cursor_y = cursor + 1

rand_index = $200

reset:
    sei
    ldx #0; initialize data stack pointer

init:
    cli

main:
    jsr print
    jsr rand_draw
    jmp main

newline: ; sets cursor to start of next line
    stz cursor_x
    lda cursor_y
    cmp #28
    bne .end
    stz cursor_y
    rts
    .end:
        inc cursor_y
        rts

text:
    .byte 1,2,3,2,4
    .asciiz "- george loves u <3"

random_y: 
    .byte 25,12,0,20,4,25,5,13
    .byte 20, 1, 1, 22, 12, 19, 19, 19
    .byte 9, 0, 4, 18, 13, 14, 4, 16
    .byte 8, 17, 21, 14, 23, 21, 9, 0
    .byte 14, 10, 14, 2, 26, 18, 15, 23
    .byte 12, 2, 5, 4, 25, 20, 27, 4
    .byte 28, 21, 3, 22, 11, 25, 2, 25
    .byte 13, 17, 17, 24, 8, 8, 20, 21
    .byte 11, 24, 27, 25, 8, 12, 7, 0
    .byte 27, 12, 19, 27, 10, 3, 19, 2
    .byte 2, 23, 22, 5, 26, 28, 4, 16
    .byte 18, 7, 10, 9, 6, 19, 9, 2
    .byte 14, 8, 14, 18, 18, 2, 13, 0
    .byte 15, 26, 3, 23, 17, 12, 18, 11
    .byte 4, 16, 17, 22, 9, 25, 3, 15
    .byte 28, 3, 6, 14, 25, 5, 21, 8
    .byte 15, 18, 15, 5, 28, 6, 15, 4
    .byte 10, 1, 16, 24, 6, 9, 22, 3
    .byte 17, 18, 10, 19, 27, 11, 22, 16
    .byte 22, 17, 15, 6, 23, 11, 11, 11
    .byte 4, 15, 5, 25, 19, 1, 8, 26
    .byte 21, 20, 17, 27, 11, 3, 11, 20
    .byte 15, 28, 0, 6, 14, 23, 20, 21
    .byte 17, 20, 16, 15, 19, 6, 21, 19
    .byte 15, 27, 1, 22, 7, 0, 5, 2
    .byte 14, 24, 15, 4, 20, 16, 1, 14
    .byte 4, 16, 4, 8, 13, 26, 3, 9
    .byte 12, 25, 5, 0, 7, 17, 14, 20
    .byte 2, 26, 2, 27, 18, 23, 5, 8
    .byte 4, 21, 10, 11, 28, 22, 6, 6
    .byte 10, 13, 23, 12, 20, 28, 20, 1
    .byte 27, 19, 25, 6, 1, 10, 1

random_x:
    .byte 42, 59, 11, 5, 18, 0, 26, 1
    .byte 61, 16, 1, 51, 36, 47, 23, 1
    .byte 16, 50, 46, 4, 55, 31, 15, 2
    .byte 45, 21, 59, 53, 15, 43, 0, 2
    .byte 64, 31, 38, 41, 25, 12, 12, 3
    .byte 30, 13, 64, 44, 21, 8, 48, 3
    .byte 46, 1, 2, 33, 4, 32, 59, 28
    .byte 4, 24, 58, 53, 21, 41, 30, 2
    .byte 56, 53, 31, 10, 42, 12, 9, 54
    .byte 14, 14, 24, 29, 43, 60, 54, 26
    .byte 5, 53, 17, 55, 27, 46, 31, 3
    .byte 26, 44, 63, 30, 10, 34, 62, 48
    .byte 42, 47, 51, 7, 55, 32, 14, 21
    .byte 15, 26, 52, 37, 48, 0, 13, 2
    .byte 50, 20, 35, 32, 8, 41, 2, 24
    .byte 18, 9, 52, 22, 52, 12, 19, 32
    .byte 29, 46, 34, 58, 54, 51, 43, 57
    .byte 62, 10, 12, 57, 36, 39, 4, 30
    .byte 38, 9, 30, 32, 33, 57, 3, 25
    .byte 21, 36, 59, 30, 19, 39, 9, 60
    .byte 34, 50, 52, 37, 34, 42, 3, 33
    .byte 40, 19, 2, 26, 10, 38, 46, 30
    .byte 3, 1, 19, 16, 26, 58, 42, 49
    .byte 63, 1, 63, 41, 0, 21, 41, 19
    .byte 21, 45, 44, 52, 20, 5, 11, 64
    .byte 1, 62, 16, 5, 5, 8, 58, 56
    .byte 16, 26, 6, 37, 19, 16, 25, 29
    .byte 64, 59, 16, 6, 41, 28, 8, 51
    .byte 54, 5, 19, 28, 13, 38, 52, 35
    .byte 42, 13, 34, 33, 61, 61, 7, 27
    .byte 38, 33, 9, 57, 10, 30, 8, 4
    .byte 46, 3, 39, 46, 62, 20, 48, 7


; increments the cursor line by line, looping to (0, 0) after (63, 28)

inc_cursor:
    lda cursor_x
    cmp #63
    beq .newline
    inc cursor_x 
    rts
    .newline:
        lda cursor_y
        cmp #28
        beq .newscreen
        stz cursor_x
        inc cursor_y
        rts
    .newscreen:
        stz cursor_y
        stz cursor_x
        rts

; zeroes out the display, resets cursor to 0,0

clear: 
    lda #0
    ldy #0

   .loop:
        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 .loop
    stz cursor
    stz cursor + 1
    rts


; prints string from cursor position, stopping at end of string or at 256 chars, whichever comes first
; $6000 + (64*Y) + X
; THIS WILL WRITE OUT OF BOUNDS IF THE CURSOR IS OUT OF BOUNDS/STRING IS TOO LONG

; TODO: figure out a simple way of writing arbitrary length strings

print: 
   jsr cursor_addr
   ldy #0
   ; y_overflow = temp + 5
   .loop:
       lda text, y
       beq .end
       sta (temp), y
       iny
       bra .loop
   .end:
   rts


; calculates real vram address from cursor (x, y)

cursor_addr:
    stz temp
    stz temp + 1
    lda cursor_y
    beq .add_x ; if y's zero just add x
    .y_mult:
        ; multiply by 64
        clc
        asl
        rol temp + 1 
        asl
        rol temp + 1 
        asl  
        rol temp + 1 
        asl  
        rol temp + 1 
        asl  
        rol temp + 1 
        asl  
        rol temp + 1 
        sta temp
    .add_x:
        clc
        lda cursor_x
        adc temp
        sta temp 
        lda #0
        adc temp + 1
        sta temp + 1
        clc

        lda #$60
        adc temp + 1
        sta temp + 1
    rts

    
rand_draw:
    ldx rand_index
    lda random_x, x
    sta cursor_x
    lda random_y, x
    sta cursor_y
    jsr cursor_addr
    inc rand_index
    rts
    

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

    .include "math.inc"

    .org $fffc
    .word reset
    .word isr