dos33fsprogs/ootw/gr_twoscreen_scroll.s

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;=========================================================
; gr_twoscreen_scroll, 40x48 version
;=========================================================
; offset is in BG_SCROLL, must be multiple of 2
;
; two screens, top is at $1000, bottom at $BC00
; copy lines Y-48 from $1000 to $c00
; copy lines 0 - (48-y) from $BC00 to $c00
gr_twoscreen_scroll:
lda #0
sta TEMPY ; dest
ldy BG_SCROLL
cpy #48
beq gr_twoscreen_bottom ; no top to draw
gr_twoscreen_top:
; calculate source
lda gr_offsets+1,Y
clc
adc #($10-4)
sta gr_twoscreen_smc_src+2
lda gr_offsets,Y
sta gr_twoscreen_smc_src+1
tya
pha
; calculate destination
lda TEMPY
tay
lda gr_offsets+1,Y
clc
adc #($c-4)
sta gr_twoscreen_smc_dst+2
lda gr_offsets,Y
sta gr_twoscreen_smc_dst+1
ldx #0
gr_twoscreen_line_loop:
gr_twoscreen_smc_src:
lda $1000,X
gr_twoscreen_smc_dst:
sta $c00,X
inx
cpx #40
bne gr_twoscreen_line_loop
inc TEMPY
inc TEMPY
pla
tay
iny
iny
cpy #48
bne gr_twoscreen_top
;===============================
; now copy the bottom from $BC00
lda BG_SCROLL
beq done_twoscreen_bottom ; if 0, no bottom
gr_twoscreen_bottom:
ldy #0
gr_twoscreen_bottom_loop:
; calculate source
lda gr_offsets+1,Y
clc
adc #($bc-4)
sta gr_twoscreen_bottom_smc_src+2
lda gr_offsets,Y
sta gr_twoscreen_bottom_smc_src+1
tya
pha
; calculate destination
lda TEMPY
tay
lda gr_offsets+1,Y
clc
adc #($c-4)
sta gr_twoscreen_bottom_smc_dst+2
lda gr_offsets,Y
sta gr_twoscreen_bottom_smc_dst+1
ldx #0
gr_twoscreen_bottom_line_loop:
gr_twoscreen_bottom_smc_src:
lda $BC00,X
gr_twoscreen_bottom_smc_dst:
sta $c00,X
inx
cpx #40
bne gr_twoscreen_bottom_line_loop
inc TEMPY
inc TEMPY
pla
tay
iny
iny
cpy BG_SCROLL
bne gr_twoscreen_bottom_loop
done_twoscreen_bottom:
rts ; 6