dos33fsprogs/demos/hellmood_memories/checkers_small2.s

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; Checkers, based on the code in Hellmood's Memories
; 42 bytes
; could be shorter if you're not picky about colors
; by deater (Vince Weaver) <vince@deater.net>
; Zero Page
BASL = $28
BASH = $29
H2 = $2C
COLOR = $30
X1 = $F0
X2 = $F1
Y1 = $F2
Y2 = $F3
TEMP = $FA
TEMPY = $FB
FRAME = $FC
TEMPX = $FD
; Soft Switches
KEYPRESS= $C000
KEYRESET= $C010
SET_GR = $C050 ; Enable graphics
FULLGR = $C052 ; Full screen, no text
PAGE0 = $C054 ; Page0
PAGE1 = $C055 ; Page1
LORES = $C056 ; Enable LORES graphics
; ROM routines
PLOT = $F800 ; plot, horiz=y, vert=A (A trashed, XY Saved)
SETCOL = $F864
TEXT = $FB36 ;; Set text mode
BASCALC = $FBC1
SETGR = $FB40
HOME = $FC58 ;; Clear the text screen
WAIT = $FCA8 ;; delay 1/2(26+27A+5A^2) us
HLINE = $F819
.zeropage
; orig = 42
; make 32x32
checkers:
;===================
; init screen
jsr SETGR ; 3
; bit FULLGR ; 3
;====
; 6
checkers_forever:
inc FRAME ; 2
ldx #39 ; 2
yloop:
ldy #39 ; 2
xloop:
sec ; 1
tya ; 1
sbc FRAME ; 2
sta X2 ; 2
txa ; 1
sbc #0 ; 2
eor X2 ; 2
ora #$DB ; 2
adc #1 ; 2
sta COLOR
; jsr SETCOL ; 3
txa ; A==Y1 ; 1
jsr PLOT ; (X2,Y1) ; 3
dey ; 1
bpl xloop ; 2
dex ; 1
bpl yloop ; 2
bmi checkers_forever ; 2