ridiculously fast SHR fizzle

src/fx/fx.shr.fizzle.a

@@ -1,94 +1,73 @@
 ;license:MIT
-;(c) 2019-2020 by qkumba/4am
+;(c) 2019-2020 by qkumba/4am/John Brooks
 
 !cpu 6502
 !to "build/FX/SHR.FIZZLE",plain
-*=$a000
+*=$A000
 
-rnd1=$fe
+addrs=$A100               ; [128 bytes, can be anywhere in main memory but don't cross page boundary]
-rnd2=$ff
 
-         ;init RNG
+         jsr   swapzp     ; copy LFSR code to zero page
 
-         ldy   #1
+         ldx   #$7F       ; create address lookup table
-         sty   rnd1
+         lda   #$9F
-         dey
+         sec
-         sty   rnd2
+-        sta   addrs, x
-
+         sbc   #1
-         ;copy SCB and palette
-
-         lda   #$9d
-         sta   $3d
-         sty   $3c
-         ldx   #$23                  ; copy SCB+palette+this code to auxmem
-         sta   $c005
-@copystuff
-         lda   ($3c),y
-         sta   ($3c),y
-         iny
-         bne   @copystuff
-         inc   $3d
          dex
-         bne   @copystuff
+         bpl   -
+         inx
 
-         ;force write offset 0 otherwise it will be missed
+         sta   $C005      ; pre-copy SHR SCB and palette
+         ldy   #0
+-        lda   $9D00, y
+         sta   $9D00, y
+         lda   $9E00, y
+         sta   $9E00, y
+         lda   $9F00, y
+         sta   $9F00, y
+         iny
+         bne   -
 
-         lda   $2000
+         jsr   $0         ; call LFSR code on zero page
+
+         lda   $2000      ; last lousy byte (because LFSR never hits 0)
          sta   $2000
-         sta   $c004
 
-         ;iterate
+         sta   $C004      ; restore and exit
+swapzp   ldx   #(end-start-1)
+-        lda   $0, x
+         pha
+         lda   start, x
+         sta   $0, x
+         pla
+         sta   start, x
+         dex
+         bpl   -
+         rts
 
-@loop
+start
-         ldx   rnd1
+!pseudopc 0 {
-@loop2
+; in: X,Y=0
-         stx   $26
+loop     txa
-         stx   $3c
+loop1    eor   #$60                  ; LFSR form 0x6000 with period 32767
-         ldx   rnd2
+         tax
-         lsr   rnd2
+loop2    lda   addrs, x
-         ror   rnd1
+         sta   <dst+2
-         bcc   +
+         lda   (<dst+1), y
-
+dst      sta   $FD00, y
-         ;feedback polynomial forms #$4001 for period of 32767
-
-         lda   rnd1
-         eor   #1
-         sta   rnd1
-         lda   rnd2
-         eor   #$40
-         sta   rnd2
-
-         ;don't rewrite SCB or palette
-         ;it will make the fizzle take longer than obviously needed
-
-+        cpx   #$7d
-         bcs   +
          txa
-
+         lsr
-         ;select address
+         tax
-
+         tya
-         adc   #$20
+         ror
-         sta   $27
+         tay
-         sta   $3d
+         bcc   loop2
-
+         bne   loop
-         ;copy pixel from other page to this page
+         bit   $C000
-
+         bmi   exit
-         lda   ($3c),y
+         txa
-         sta   $c005
+         bne   loop1
-         sta   ($26),y
+exit     rts
-         sta   $c004
+}
-
+end
-+
-         ;exit conditions
-
-         ldx   rnd2
-         bne   @loop
-
-         ldx   $c000
-         bmi   @exit
-
-         ldx   rnd1
-         cpx   #1
-         bne   @loop2
-
-@exit    rts