1,885
edits
Changes
→Place a Graphic on the Screen
dcb 07,00,00,07
dcb 00,03,03,00
== Etch-a-Sketch<sup>tm</sup> Style Drawing ==
; zero-page variable locations
define ROW $20 ; current row
define COL $21 ; current column
define POINTER $10 ; ptr: start of row
define POINTER_H $11
; constants
define DOT $01 ; dot colour
define CURSOR $04 ; black colour
ldy #$00 ; put help text on screen
print: lda help,y
beq setup
sta $f000,y
iny
bne print
setup: lda #$0f ; set initial ROW,COL
sta ROW
sta COL
draw: lda ROW ; ensure ROW is in range 0:31
and #$1f
sta ROW
lda COL ; ensure COL is in range 0:31
and #$1f
sta COL
ldy ROW ; load POINTER with start-of-row
lda table_low,y
sta POINTER
lda table_high,y
sta POINTER_H
ldy COL ; store CURSOR at POINTER plus COL
lda #CURSOR
sta (POINTER),y
getkey: lda $ff ; get a keystroke
ldx #$00 ; clear out the key buffer
stx $ff
cmp #$43 ; handle C or c
beq clear
cmp #$63
beq clear
cmp #$80 ; if not a cursor key, ignore
bmi getkey
cmp #$84
bpl getkey
pha ; save A
lda #DOT ; set current position to DOT
sta (POINTER),y
pla ; restore A
cmp #$80 ; check key == up
bne check1
dec ROW ; ... if yes, decrement ROW
jmp done
check1: cmp #$81 ; check key == right
bne check2
inc COL ; ... if yes, increment COL
jmp done
check2: cmp #$82 ; check if key == down
bne check3
inc ROW ; ... if yes, increment ROW
jmp done
check3: cmp #$83 ; check if key == left
bne done
dec COL ; ... if yes, decrement COL
clc
bcc done
clear: lda table_low ; clear the screen
sta POINTER
lda table_high
sta POINTER_H
ldy #$00
tya
c_loop: sta (POINTER),y
iny
bne c_loop
inc POINTER_H
ldx POINTER_H
cpx #$06
bne c_loop
done: clc ; repeat
bcc draw
; these two tables contain the high and low bytes
; of the addresses of the start of each row
table_high:
dcb $02,$02,$02,$02,$02,$02,$02,$02
dcb $03,$03,$03,$03,$03,$03,$03,$03
dcb $04,$04,$04,$04,$04,$04,$04,$04
dcb $05,$05,$05,$05,$05,$05,$05,$05,
table_low:
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
; help message on character screen
help:
dcb "A","r","r","o","w",32,"k","e","y","s"
dcb 32,"d","r","a","w",32,"/",32,"'","C","'"
dcb 32,"k","e","y",32,"c","l","e","a","r","s"
dcb 00