; ============================================================================= ; CBS Cruncher v5.2 format 3 - Screen 5-8 RAM reference decruncher v0.2 ; ============================================================================= ; One decoder handles Screen 5, 6, 7 and 8 because the packed stream stores ; its actual byte width. It also accepts cropped images and imported BMP/PNG ; data whose DX is not the normal full-screen width. ; ; Input: ; HL = three-byte CBS format-3 image header ; byte 0 = format and destination ; byte 1 = packed bytes per line; 0 means 256 ; byte 2 = number of lines; 0 means 256 ; DE = destination in RAM ; ; New-offset selector: ; 0 = complemented gamma line count; distance = count * bytesPerLine ; 1 = ordinary CBS compact 128-byte distance ; ; Matches may cross a line boundary. The line representation only makes ; distances equal to complete rows cheaper; it does not stop at row ends. ; ; The row width is copied once into the immediate operand at ; CBS_V52_SCREEN_WIDTH_LOAD. The routine must therefore execute from RAM. ; The 16-step multiply runs only when a new whole-line offset is introduced. ; ; This reference version expands into linear RAM and ignores byte 2 after ; skipping it. The RAM-stage/HMMC companion uses this same decoding path, then ; points HL to the completed buffer and performs one separate VDP transfer. ; ; The entry trusts compressor output and performs no format or bounds checks. ; License: zlib ; ============================================================================= CBS_V52_SCREEN58_RAM_TO_RAM: INC HL ; skip format / destination byte LD C,(HL) ; packed bytes per line; 0 = 256 INC HL LD B,00h LD A,C OR A JR NZ,CBS_V52_SCREEN_WIDTH_READY INC B ; BC = 256 CBS_V52_SCREEN_WIDTH_READY: INC HL ; skip line count; HL = payload LD (CBS_V52_SCREEN_WIDTH_LOAD+1),BC LD A,80h ; sentinel control-bit reservoir LD BC,FFFFh ; default negative offset = -1 PUSH BC ; persistent last offset on stack ; The first token is always a literal run. CALL CBS_V52_SCREEN_GET_GAMMA LDIR ; After a literal: ; 0 + gamma(1) + 0 = one-byte match using the previous offset ; 0 + gamma(1) + 1 = end marker ; 0 + gamma(2...) = match using the previous offset ; 1 = match introducing a new offset CBS_V52_SCREEN_AFTER_LITERAL: ADD A,A JR NC,CBS_V52_SCREEN_REPEAT_OFFSET CALL Z,CBS_V52_SCREEN_REFILL_BITS JR C,CBS_V52_SCREEN_NEW_OFFSET CBS_V52_SCREEN_REPEAT_OFFSET: CALL CBS_V52_SCREEN_GET_GAMMA JR C,CBS_V52_SCREEN_COPY_MATCH ADD A,A ; gamma(1): match or EOS? JR NC,CBS_V52_SCREEN_COPY_MATCH CALL Z,CBS_V52_SCREEN_REFILL_BITS JR C,CBS_V52_SCREEN_FINISHED ; Copy BC bytes from DE+negativeOffset to DE. Overlap is intentional. CBS_V52_SCREEN_COPY_MATCH: EX (SP),HL ; HL=negative offset, stack=packed HL PUSH HL ADD HL,DE ; match source LDIR POP HL EX (SP),HL ; restore packed HL and saved offset ; After a match: ; 0 + gamma(2...) = literal run ; 0 + gamma(1) + 0 = one literal byte ; 0 + gamma(1) + 1 = end marker ; 1 = match introducing a new offset CBS_V52_SCREEN_AFTER_MATCH: ADD A,A JR NC,CBS_V52_SCREEN_LITERAL_AFTER_MATCH CALL Z,CBS_V52_SCREEN_REFILL_BITS JR C,CBS_V52_SCREEN_NEW_OFFSET CBS_V52_SCREEN_LITERAL_AFTER_MATCH: CALL CBS_V52_SCREEN_GET_GAMMA JR C,CBS_V52_SCREEN_COPY_LITERALS ADD A,A ; gamma(1): literal or EOS? JR NC,CBS_V52_SCREEN_COPY_LITERALS CALL Z,CBS_V52_SCREEN_REFILL_BITS JR C,CBS_V52_SCREEN_FINISHED CBS_V52_SCREEN_COPY_LITERALS: LDIR JR CBS_V52_SCREEN_AFTER_LITERAL ; ----------------------------------------------------------------------------- ; New offset: selector 0 = whole-line distance, selector 1 = compact general. ; ----------------------------------------------------------------------------- CBS_V52_SCREEN_NEW_OFFSET: ADD A,A JR NC,CBS_V52_SCREEN_ROW_OFFSET CALL Z,CBS_V52_SCREEN_REFILL_BITS JR C,CBS_V52_SCREEN_GENERAL_OFFSET CBS_V52_SCREEN_ROW_OFFSET: CALL CBS_V52_SCREEN_GET_NEG_GAMMA INC BC ; BC = negative row count ; Preserve the bit reservoir, output pointer and packed pointer while ; calculating negativeRows * bytesPerLine modulo 65536. PUSH AF PUSH DE PUSH HL CBS_V52_SCREEN_WIDTH_LOAD: LD DE,0000h ; patched from header byte 1 LD HL,0000h ; 16-bit product LD A,16 CBS_V52_SCREEN_MULTIPLY: ADD HL,HL SLA C RL B ; Carry = next multiplier bit JR NC,CBS_V52_SCREEN_MULTIPLY_SKIP ADD HL,DE CBS_V52_SCREEN_MULTIPLY_SKIP: DEC A JR NZ,CBS_V52_SCREEN_MULTIPLY LD B,H LD C,L ; BC = negative byte distance POP HL ; packed pointer POP DE ; output pointer POP AF ; control-bit reservoir EX (SP),HL ; discard old persistent offset POP HL PUSH BC ; install row-scaled negative offset CALL CBS_V52_SCREEN_GET_GAMMA INC BC ; match length = gamma + 1 JR CBS_V52_SCREEN_COPY_MATCH CBS_V52_SCREEN_GENERAL_OFFSET: CALL CBS_V52_SCREEN_GET_NEG_GAMMA INC BC ; BC = negative high offset part RR B ; Carry = bit 8 of negative high LD B,C LD C,(HL) ; complemented low7 + length control INC HL RR B RR C ; BC = negative distance EX (SP),HL ; discard old persistent offset POP HL PUSH BC ; install new negative offset ; Packed-low bit 0 already supplies the first length control bit. CALL CBS_V52_SCREEN_GET_GAMMA_FIRST INC BC JR CBS_V52_SCREEN_COPY_MATCH CBS_V52_SCREEN_FINISHED: POP BC ; discard persistent last offset RET ; ----------------------------------------------------------------------------- ; Interleaved bit refill. ; ----------------------------------------------------------------------------- CBS_V52_SCREEN_REFILL_BITS: LD A,(HL) INC HL RLA RET ; ----------------------------------------------------------------------------- ; Interlaced Elias gamma readers. ; ----------------------------------------------------------------------------- CBS_V52_SCREEN_GET_GAMMA: LD BC,0001h JR CBS_V52_SCREEN_GAMMA_READ_CONTROL CBS_V52_SCREEN_GET_NEG_GAMMA: LD BC,FFFEh CBS_V52_SCREEN_GAMMA_READ_CONTROL: ADD A,A JR NC,CBS_V52_SCREEN_GAMMA_LOOP CALL Z,CBS_V52_SCREEN_REFILL_BITS JR NC,CBS_V52_SCREEN_GAMMA_LOOP CCF ; gamma(1) returns Carry clear RET CBS_V52_SCREEN_GET_GAMMA_FIRST: LD BC,0001h JR NC,CBS_V52_SCREEN_GAMMA_LOOP CCF RET CBS_V52_SCREEN_GAMMA_LOOP: ADD A,A ; data bit CALL Z,CBS_V52_SCREEN_REFILL_BITS RL C RL B ADD A,A ; next control bit JR NC,CBS_V52_SCREEN_GAMMA_LOOP CALL Z,CBS_V52_SCREEN_REFILL_BITS JR NC,CBS_V52_SCREEN_GAMMA_LOOP RET