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slightly faster swapzp, and some comments

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4am 2020-09-04 19:11:30 -04:00
parent 3365ac8a37
commit 39b643de50
1 changed files with 30 additions and 26 deletions
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56
src/fx/fx.hgr.1bit.fizzle.a
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@ -5,12 +5,13 @@
!to "build/FX/BIT.FIZZLE",plain
*=$6000
copymasks = $6100 ; [256 bytes]
addrs = $6200 ; [256 bytes]
copymasks = $6100 ; [256 bytes, page-aligned]
addrs = $6200 ; [256 bytes, page-aligned]
jsr swapzp
ldx #0
lda #1
jsr swapzp ; copy LFSR code to zerp page
inx
lda #1 ; create copymask lookup table
-- ldy #$20
ora #$80
- sta copymasks, x
@ -19,8 +20,9 @@ addrs = $6200 ; [256 bytes]
bne -
asl
bne --
clc
-- ldy #$20
-- ldy #$20 ; create address lookup table
tya
- sta addrs, x
adc #1
@ -29,14 +31,17 @@ addrs = $6200 ; [256 bytes]
bne -
txa
bne --
jsr $0
jsr $0 ; call LFSR code on zero page
lda $4000 ; last lousy byte (because LFSR never hits 0)
sta $2000
; fall through to restore and exit
swapzp ldx #(end-start-1)
- lda $0, x
pha
lda start, x
sta $0, x
pla
- ldy start, x
lda $0, x
sta start, x
sty $0, x
dex
bpl -
rts
@ -45,31 +50,30 @@ start
!pseudopc 0 {
; in: X,Y=0
loop txa
loop1 eor #$B4
tax
loop2 lda addrs, x
sta <dst+2
eor #$60
loop1 eor #$B4 ; LFSR form 0xB400 for period 65535
tax ; X is LFSR high byte, Y is LFSR low byte
loop2 lda addrs, x ; which means X is the index into the base address lookup table
sta <dst+2 ; and Y is the offset from the base address
eor #$60 ; which works out well with the available addressing modes
sta <src+2
lda (<dst+1), y
src eor $FD00, y
and copymasks, x
eor (<dst+1), y
dst sta $FD00, y
src eor $FD00, y ; merge source and destination bits
and copymasks, x ; isolate the bits to replace, zero the rest
eor (<dst+1), y ; unmerge source and destination bits, leaves 'to keep' destination bits intact
dst sta $FD00, y ; write the result
txa
lsr
tax
tya
ror
tay
bcc loop2
bne loop
bcc loop2 ; C set by ror
bne loop ; Z set by tay
bit $C000
bmi exit
txa
bne loop1
exit lda $4000
sta $2000
rts
exit rts
}
end