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xmas2018: more scroll optimization

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This commit is contained in:
Vince Weaver 2018-12-20 12:46:56 -05:00
parent 067f1c53fc
commit 3e53efc75d
1 changed files with 12 additions and 9 deletions
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21
xmas_2018/merry.s
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@ -306,13 +306,15 @@ scroll_done:
; 40*
; hgr_scroll_line_loop: 25
; high bit 20
; prepare bits: 18
; prepare bits: 16
; output new byte: 21
; increment and loop: 7
; 5 return
;
; (93*40)+7=3727 -- original total
;
; (91*40)+7=3647 -- remove branch in highbit code
; (89*40)+7=3567 -- convert 5 asl to 4 ror
hgr_scroll_line:
ldy #0 ; 2
@ -366,15 +368,16 @@ done_high_bit:
prepare_bits:
; get right byte, bottom 2 bits, shifted left to be in 6+5
lda NEXT ; 3
and #$3 ; 2
asl ; 2
asl ; 2
asl ; 2
asl ; 2
asl ; 2
; this method 2 cycles faster than asl x 5
ror ; 2
ror ; 2
ror ; 2
ror ; 2
and #$60 ; 2
sta NEXT ; 3
;==========
; 18
; 16
output_new:
; get current, mask off bottom 2 bits (no longer needed)