dos33fsprogs/games/ootw/gr_plot.s

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2020-06-21 05:38:07 +00:00
;================================
; plot routine
;================================
; Xcoord in X
; Ycoord in A
; color in COLOR
plot:
stx XPOS
lsr ; shift bottom bit into carry ; 2
bcc plot_even ; 2nt/3
plot_odd:
ldx #$f0 ; 2
bcs plot_c_done ; 2nt/3
plot_even:
ldx #$0f ; 2
plot_c_done:
stx MASK ; 3
asl ; shift back (now even) ; 2
tay
lda gr_offsets,Y ; lookup low-res memory address ; 4
clc ; 2
adc XPOS ; 3
sta GBASL ; 3
iny ; 2
lda gr_offsets,Y ; 4
adc DRAW_PAGE ; add in draw page offset ; 3
sta GBASH ; 3
ldy #0 ; 2
plot_write:
lda MASK ; 3
eor #$ff ; 2
and (GBASL),Y ; 5
sta COLOR_MASK ; 3
lda COLOR ; 3
and MASK ; 3
ora COLOR_MASK ; 3
sta (GBASL),Y ; 5
rts ; 6