251 lines
3.1 KiB
ArmAsm
251 lines
3.1 KiB
ArmAsm
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; NuCrunch 1.0
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; Christopher Jam
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; May 2018
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;
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.export decrunch_next_group
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.export decrunch
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.export decrunch_end
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.macro getByte1
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lda (zps),y
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inc zps+0
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bne *+4
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inc zps+1
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.endmacro
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.macro getBit1
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.local nomore
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asl zbs1
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bne nomore
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getByte1
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sec
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rol
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sta zbs1
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nomore:
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.endmacro
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; get head of a pair of bits from the bitpair stream
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; (must getBit2t precisely once before invoking again)
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.macro getBit2h
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.local nomore
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asl zbs2
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bne nomore
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getByte1
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sec
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rol
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sta zbs2
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nomore:
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.endmacro
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; same, but preserving A/ trashing X.
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.macro getBit2hpa
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.local nomore
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asl zbs2
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bne nomore
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tax
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getByte1
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sec
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rol
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sta zbs2
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txa
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nomore:
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.endmacro
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; get tail of a pair of bits from the bitpair stream
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.macro getBit2t
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asl zbs2
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.endmacro
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; get head of a quad of bits from the quad stream
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; (must getBit4t precisely three times before invoking again)
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.macro getBit4h
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.local nomore
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asl zbs4
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bne nomore
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getByte1
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sec
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rol
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sta zbs4
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nomore:
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.endmacro
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; get tail of a quad of bits from the quad stream
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.macro getBit4t
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asl zbs4
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.endmacro
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; note, trashes X. Also, carry is clear when done
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.macro getExpGoulombTail
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.local ndone
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ndone:
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getBit2hpa
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rol
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getBit2t
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bcs ndone
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.endmacro
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.macro getExpGoulombTail_odd_aligned
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.local ndone
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ndone:
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getBit2t
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rol
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getBit2hpa
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bcs ndone
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.endmacro
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decrunch_zpa=$e0 ;9 bytes required
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zbs1 = decrunch_zpa+$00 ; 1 byte
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zbs2 = decrunch_zpa+$01 ; 1 byte
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zbs4 = decrunch_zpa+$02 ; 1 byte
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zpc = decrunch_zpa+$03 ; 2 bytes
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zps = decrunch_zpa+$05 ; 2 bytes
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zpd = decrunch_zpa+$07 ; 2 bytes
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offsetm1 = zpc ; these are aliased, as never need both
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decrunch:
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stx zps+0
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sta zps+1
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ldy #0
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sty zbs1
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sty zbs2
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sty zbs4
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decrunch_next_group:
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ldy #0
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next_segment:
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jsr get_byte
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sta zpd+0
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jsr get_byte
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sta zpd+1
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decode_literal:
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; get count [ExpGoulomb0+1] in x
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ldx#1
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getBit1
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bcc ret1
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lda#1
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getExpGoulombTail
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tax
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ret1:
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literal_loop:
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lda (zps),y
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sta (zpd),y
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iny
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dex
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bne literal_loop
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; carry is clear either from bcc above or _getExpGoulombTail above
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tya
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adc zps
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sta zps
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bcc *+5
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inc zps+1
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clc
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tya
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adc zpd
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sta zpd
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bcc *+4
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inc zpd+1
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ldy#0
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; literal is always followed by copy
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decode_copy:
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getBit2h
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bcc short_offset
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lda#1
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getExpGoulombTail_odd_aligned
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adc#255
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sta offsetm1+1
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getByte1
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sta offsetm1
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jmp got_high
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short_offset:
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lda#0
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sta offsetm1+1
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;ExpGoulomb k=3
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getBit4h
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lda#1
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bcc no_tail
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getExpGoulombTail_odd_aligned
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no_tail:
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adc#255
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getBit4t
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rol
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getBit4t
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rol
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getBit4t
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rol
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sta offsetm1
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got_high:
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ldx#1
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getBit2t
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bcc length_two
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lda#1
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getExpGoulombTail
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tax
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cpx#255
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beq end_of_segment ; copy length of 256 marks end of segment
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length_two:
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; note carry is clear at this point; good as we want to subtract (offsetm1+1)
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lda zpd
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sbc offsetm1
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sta zpc
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lda zpd+1
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sbc offsetm1+1
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sta zpc+1
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lda (zpc),y
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sta (zpd),y
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copy_loop:
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iny
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lda (zpc),y
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sta (zpd),y
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dex
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bne copy_loop
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tya
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; carry will be set from SBC above
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adc zpd
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sta zpd
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bcc *+4
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inc zpd+1
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ldy#0
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getBit1
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bcs jmp_decode_copy
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jmp decode_literal
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jmp_decode_copy:
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jmp decode_copy
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get_byte:
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getByte1
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end_of_file:
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rts
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end_of_segment:
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lda offsetm1
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cmp#0
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beq end_of_file
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jmp next_segment
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decrunch_end:
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