OBJ file

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OBJ files contain the sprites graphics (equivalent to the C ROMs on cartridge systems). Data is compressed however using a simple RLE compression removing runs with a value of 0x00000000. Once decompressed the format is described here: sprite graphics format.

RLE compression

  • First long word value is the compressed data length (/4-1).
  • The system works by checking single long words at a time.
  • When a zero value is found it is stored and the next value is checked. A fill of zero is created to match the value read unless it is zero.

Example

compressed:

0x00000007, 0x11111111, 0x00000000, 0x00000003, 0x22222222, 0x00000000, 0x00000000, 0x33333333, 0x44444444

decompressed:

0x11111111, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x22222222, 0x00000000, 0x33333333, 0x44444444

This system will not compress data very well unless there are large areas of zero data within the decompressed sprite data. This is due to a single zero value being inflated by 100% to let the decompressor know it is not a run. To my current knowledge only Art of Fighting uses the OBJ format.

For reference below is a 68000 code snippet from bios TOP-SP1 that handles decompresses.

C0BC5A  203C 00E0 0000             MOVE.L   #0xE00000,D0
C0BC60  D0AD 7EF4                  ADD.L    (0x7EF4,A5),D0
C0BC64  2240                       MOVEA.L  D0,A1                      <---- Where to decompress data
C0BC66  206D 7EF8                  MOVEA.L  (0x7EF8,A5),A0
C0BC6A  7000                       MOVEQ    #0x0,D0
C0BC6C  2E18                       MOVE.L   (A0)+,D7                   <---- Get Length
C0BC6E  6738                       BEQ.S    *+0x3A [0xC0BCA8]
C0BC70  22D8                       MOVE.L   (A0)+,(A1)+                <---- Move single long word
C0BC72  6712                       BEQ.S    *+0x14 [0xC0BC86]          <---- Branch if zero.
C0BC74  51CF FFFA                  DBF      D7,*-0x4 [0xC0BC70]        <---- check data end
C0BC78  2009                       MOVE.L   A1,D0
C0BC7A  0480 00E0 0000             SUBI.L   #0xE00000,D0
C0BC80  2B40 7EF4                  MOVE.L   D0,(0x7EF4,A5)
C0BC84  4E75                       RTS

C0BC86  2C18                       MOVE.L   (A0)+,D6                   <---- Get next value
C0BC88  51CE 0014                  DBF      D6,*+0x12 [0xC0BC9E]       <---- Decompress if non zero
C0BC8C  51CF FFE2                  DBF      D7,*-0x1C [0xC0BC70]       <---- Check data end
C0BC90  2009                       MOVE.L   A1,D0
C0BC92  0480 00E0 0000             SUBI.L   #0xE00000,D0
C0BC98  2B40 7EF4                  MOVE.L   D0,(0x7EF4,A5)
C0BC9C  4E75                       RTS

C0BC9E  22C0                       MOVE.L   D0,(A1)+                   <---- Recreate zero run
C0BCA0  51CE FFFC                  DBF      D6,*-0x2 [0xC0BC9E]        <---- and Loop until complete
C0BCA4  51CF FFCA                  DBF      D7,*-0x34 [0xC0BC70]       <---- Check data end

C0BCA8  2009                       MOVE.L   A1,D0
C0BCAA  0480 00E0 0000             SUBI.L   #0xE00000,D0
C0BCB0  2B40 7EF4                  MOVE.L   D0,(0x7EF4,A5)
C0BCB4  4E75                       RTS