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;Z80 depacker for megalz V4 packed files   (C) fyrex^mhm
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; DESCRIPTION:
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;
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; Depacker is fully relocatable, not self-modifying,
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;it's length is 110 bytes starting from DEC40.
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;Register usage: AF,AF',BC,DE,HL. Must be CALL'ed, return is done by RET.
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;Provide extra stack location for store 2 bytes (1 word). Depacker does not
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;disable or enable interrupts, as well as could be interrupted at any time
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;(no f*cking wicked stack usage :).
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; USAGE:
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;
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; - put depacker anywhere you want,
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; - put starting address of packed block in HL,
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; - put location where you want data to be depacked in DE,
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;   (much like LDIR command, but without BC)
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; - make CALL to depacker (DEC40).
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; - enjoy! ;)
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; PRECAUTIONS:
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;
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; Be very careful if packed and depacked blocks coincide somewhere in memory.
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;Here are some advices:
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;
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; 1. put packed block to the highest addresses possible.
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;     Best if last byte of packed block has address #FFFF.
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;
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; 2. Leave some gap between ends of packed and depacked block.
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;     For example, last byte of depacked block at #FF00,
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;     last byte of packed block at #FFFF.
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;
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; 3. Place nonpackable data to the end of block.
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;
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; 4. Always check whether depacking occurs OK and neither corrupts depacked data
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;     nor hangs computer.
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;
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DEC40
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        LD      A,0X80
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        EX      AF,AF'
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MS      LDI
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M0      LD      BC,0X2FF
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M1      EX      AF,AF'
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M1X     ADD     A,A
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        JR      NZ,M2
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        LD      A,(HL)
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        INC     HL
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        RLA
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M2      RL      C
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        JR      NC,M1X
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        EX      AF,AF'
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        DJNZ    X2
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        LD      A,2
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        SRA     C
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        JR      C,N1
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        INC     A
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        INC     C
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        JR      Z,N2
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        LD      BC,0X33F
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        JR      M1
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X2      DJNZ    X3
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        SRL     C
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        JR      C,MS
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        INC     B
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        JR      M1
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X6
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        ADD     A,C
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N2
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        LD      BC,0X4FF
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        JR      M1
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N1
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        INC     C
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        JR      NZ,M4
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        EX      AF,AF'
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        INC     B
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N5      RR      C
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        RET     C
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        RL      B
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        ADD     A,A
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        JR      NZ,N6
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        LD      A,(HL)
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        INC     HL
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        RLA
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N6      JR      NC,N5
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        EX      AF,AF'
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        ADD     A,B
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        LD      B,6
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        JR      M1
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X3
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        DJNZ    X4
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        LD      A,1
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        JR      M3
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X4      DJNZ    X5
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        INC     C
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        JR      NZ,M4
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        LD      BC,0X51F
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        JR      M1
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X5
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        DJNZ    X6
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        LD      B,C
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M4      LD      C,(HL)
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        INC     HL
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M3      DEC     B
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        PUSH    HL
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        LD      L,C
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        LD      H,B
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        ADD     HL,DE
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        LD      C,A
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        LD      B,0
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        LDIR
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        POP     HL
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        JR      M0
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END_DEC40
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