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xmas2019: update snow
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Vince Weaver
parent
35f6558cb7
commit
6787c7a58c
119
xmas_2019/random16.s
Normal file
119
xmas_2019/random16.s
Normal file
@ -0,0 +1,119 @@
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; 16-bit 6502 Random Number Generator (cycle-invariant version)
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; Linear feedback shift register PRNG by White Flame
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; http://codebase64.org/doku.php?id=base:small_fast_16-bit_prng
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; The Apple II KEYIN routine increments SEEDL:SEEDH
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; while waiting for keypress
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;SEEDL = $4E
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;SEEDH = $4F
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XOR_MAGIC = $7657 ; "vW"
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;=============================
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; random16
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;=============================
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; takes:
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; not 0, cs = 6(r16)+12(lnz)+5(nop)+ 19(deo) = 42
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; not 0, cc = 6(r16)+14(lnz)+2(nop)+ 20(neo) = 42
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; $0000 = 6(r16)+ 6(loz)+11nops+ 19(deo) = 42
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; $8000 = 6(r16)+ 6(loz)+ 4(ceo) + 6nops+ 20(neo) = 42
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; $XX00 cc = 6(r16)+ 6(loz)+4(ceo)+2(cep) +4nops+ 20(neo) = 42
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; $XX00 cs = 6(r16)+ 6(loz)+4(ceo)+4(cep) +3nops+ 19(deo) = 42*
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random16:
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lda SEEDL ; 3
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beq low_zero ; $0000 and $8000 are special values ; 3
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;==========
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; 6
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lownz:
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; -1
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asl SEEDL ; Do a normal shift ; 5
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lda SEEDH ; 3
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rol ; 2
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bcs five_cycle_do_eor ; 3
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;===========
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; 12
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bcc two_cycle_no_eor ; 3
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;==========
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; 12+3-1 = 14
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;===================================================================
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eleven_cycle_do_eor:
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nop ; 2
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nop ; 2
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nop ; 2
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five_cycle_do_eor:
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nop ; 2
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three_cycle_do_eor:
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sta SEEDH ; nop ; 3
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do_eor:
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; high byte is in A
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eor #>XOR_MAGIC ; 2
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sta SEEDH ; 3
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lda SEEDL ; 3
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eor #<XOR_MAGIC ; 2
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sta SEEDL ; 3
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eor_rts:
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rts ; 6
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;===========
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; 19
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;=========================================================================
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six_cycles_no_eor:
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nop ; 2
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four_cycle_no_eor:
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nop ; 2
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two_cycle_no_eor:
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nop ; 2
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no_eor:
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nop ; 2
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nop ; 2
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nop ; 2
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nop ; 2
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sta SEEDH ; 3
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jmp eor_rts ; 3+6
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;===========
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; 20
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;======================================================================
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;======================================================================
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low_zero:
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lda SEEDH ; 3
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beq eleven_cycle_do_eor ; High byte is also zero ; 3
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; so apply the EOR
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;============
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; 6
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ceo:
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; -1
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; wasn't zero, check for $8000
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asl ; 2
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beq six_cycles_no_eor ; if $00 is left after the shift; 3
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; then it was $80
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;===========
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; 4
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; else, do the EOR based on the carry
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cep:
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; -1
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bcc four_cycle_no_eor ; 3
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;============
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; 2
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bcs three_cycle_do_eor ; 2+3-1 = 4
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@ -9,11 +9,19 @@ GBASL = $26
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GBASH = $27
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BASL = $28
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BASH = $29
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SEEDL = $4E
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SEEDH = $4F
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HGR_COLOR = $E4
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SNOWX = $F0
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HGR = $F3E2
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NUMFLAKES = 10
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.include "hardware.inc"
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@ -33,23 +41,23 @@ display_loop:
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ldx #0
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erase_loop:
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lda snow_y,X
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lda snow_y,X ; get Y
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lsr
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lsr
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and #$fe
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; lsr
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lsr ; divide by 8
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tay
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clc
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lda hgr_offsets,Y
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lda hgr_offsets_l,Y
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adc snow_x,X
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sta GBASL
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sta GBASL ; point GBASL to right location
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lda snow_y,X
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asl
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asl
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and #$1f
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clc
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adc hgr_offsets+1,Y
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adc hgr_offsets_h,Y
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sta GBASH
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ldy #0
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@ -57,7 +65,7 @@ erase_loop:
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sta (GBASL),Y
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inx
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cpx #8
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cpx #NUMFLAKES
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bne erase_loop
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;==========================
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@ -75,10 +83,30 @@ move_snow:
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just_inc:
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jsr random16
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lda SEEDL
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and #$f
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beq snow_left
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cmp #$1
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beq snow_right
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inc snow_y,X
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jmp snow_no
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snow_right:
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inc snow_offset,X
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jmp snow_no
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snow_left:
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dec snow_offset,X
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snow_no:
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done_inc:
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inx
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cpx #8
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cpx #NUMFLAKES
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bne move_snow
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@ -90,11 +118,10 @@ draw_loop:
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lda snow_y,X
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lsr
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lsr
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; lsr
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and #$fe
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lsr
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tay
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clc
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lda hgr_offsets,Y
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lda hgr_offsets_l,Y
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adc snow_x,X
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sta GBASL
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@ -103,7 +130,7 @@ draw_loop:
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asl
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and #$1f
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clc
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adc hgr_offsets+1,Y
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adc hgr_offsets_h,Y
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sta GBASH
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ldy #0
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@ -111,7 +138,7 @@ draw_loop:
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sta (GBASL),Y
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inx
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cpx #8
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cpx #NUMFLAKES
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bne draw_loop
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lda #100
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@ -121,15 +148,23 @@ draw_loop:
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snow_x:
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.byte 2,4,6,8,10,12,14,16
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.byte 2,4,6,8,10,12,14,16,18,20
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snow_offset:
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.byte 0,1,2,3,4,5,6,7
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.byte 0,1,2,3,4,5,6,7,0,1
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snow_y:
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.byte 0,0,0,0,0,0,0,0
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.byte 0,0,0,0,0,0,0,0,0,0
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hgr_offsets:
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.word $2000,$2080,$2100,$2180,$2200,$2280,$2300,$2380
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.word $2028,$20A8,$2128,$21A8,$2228,$22A8,$2328,$23A8
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.word $2050,$20D0,$2150,$21D0,$2250,$22D0,$2350,$23D0
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hgr_offsets_h:
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.byte >$2000,>$2080,>$2100,>$2180,>$2200,>$2280,>$2300,>$2380
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.byte >$2028,>$20A8,>$2128,>$21A8,>$2228,>$22A8,>$2328,>$23A8
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.byte >$2050,>$20D0,>$2150,>$21D0,>$2250,>$22D0,>$2350,>$23D0
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hgr_offsets_l:
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.byte <$2000,<$2080,<$2100,<$2180,<$2200,<$2280,<$2300,<$2380
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.byte <$2028,<$20A8,<$2128,<$21A8,<$2228,<$22A8,<$2328,<$23A8
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.byte <$2050,<$20D0,<$2150,<$21D0,<$2250,<$22D0,<$2350,<$23D0
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.include "random16.s"
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