dos33fsprogs/games/tfv/gr_vlin.s

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;=========================================
; vlin
;=========================================
; X, V2 at Y
vlin:
sty TEMPY ; save Y (x location)
vlin_loop:
txa ; a=x (get first y)
and #$fe ; Clear bottom bit
tay ;
lda gr_offsets,Y ; lookup low-res memory address low
sta GBASL ; put it into our indirect pointer
iny
lda gr_offsets,Y ; lookup low-res memory address high
clc
adc DRAW_PAGE ; add in draw page offset
sta GBASH ; put into top of indirect
ldy TEMPY ; load back in y (x offset)
txa ; load back in x (current y)
lsr ; check the low bit
bcc vlin_low ; if not set, skip to low
vlin_high:
lda #$F0 ; setup masks
sta MASK
lda #$0f
bcs vlin_too_slow
vlin_low: ; setup masks
lda #$0f
sta MASK
lda #$f0
vlin_too_slow:
and (GBASL),Y ; mask current byte
sta (GBASL),Y ; and store back
lda MASK ; mask the color
and COLOR
ora (GBASL),Y ; or into the right place
sta (GBASL),Y ; store it
inx ; increment X (current y)
cpx V2 ; compare to the limit
bcc vlin_loop ; if <= then loop
rts ; return