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267 lines
7.4 KiB
Plaintext
267 lines
7.4 KiB
Plaintext
// URL: http://acritum.com/winrar/rar-format
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RAR version 3.93 – Technical information
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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THE ARCHIVE FORMAT DESCRIBED BELOW IS ONLY VALID FOR VERSIONS SINCE 1.50
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==========================================================================
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RAR archive file format
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==========================================================================
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Archive file consists of variable length blocks. The order of these
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blocks may vary, but the first block must be a marker block followed by
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an archive header block.
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Each block begins with the following fields:
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HEAD_CRC 2 bytes CRC of total block or block part
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HEAD_TYPE 1 byte Block type
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HEAD_FLAGS 2 bytes Block flags
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HEAD_SIZE 2 bytes Block size
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ADD_SIZE 4 bytes Optional field – added block size
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Field ADD_SIZE present only if (HEAD_FLAGS & 0×8000) != 0
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Total block size is HEAD_SIZE if (HEAD_FLAGS & 0×8000) == 0
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and HEAD_SIZE+ADD_SIZE if the field ADD_SIZE is present – when
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(HEAD_FLAGS & 0×8000) != 0.
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In each block the followings bits in HEAD_FLAGS have the same meaning:
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0×4000 – if set, older RAR versions will ignore the block
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and remove it when the archive is updated.
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if clear, the block is copied to the new archive
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file when the archive is updated;
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0×8000 – if set, ADD_SIZE field is present and the full block
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size is HEAD_SIZE+ADD_SIZE.
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Declared block types:
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HEAD_TYPE=0×72 marker block
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HEAD_TYPE=0×73 archive header
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HEAD_TYPE=0×74 file header
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HEAD_TYPE=0×75 old style comment header
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HEAD_TYPE=0×76 old style authenticity information
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HEAD_TYPE=0×77 old style subblock
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HEAD_TYPE=0×78 old style recovery record
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HEAD_TYPE=0×79 old style authenticity information
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HEAD_TYPE=0x7a subblock
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Comment block is actually used only within other blocks and doesn’t
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exist separately.
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Archive processing is made in the following manner:
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1. Read and check marker block
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2. Read archive header
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3. Read or skip HEAD_SIZE-sizeof(MAIN_HEAD) bytes
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4. If end of archive encountered then terminate archive processing,
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else read 7 bytes into fields HEAD_CRC, HEAD_TYPE, HEAD_FLAGS,
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HEAD_SIZE.
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5. Check HEAD_TYPE.
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if HEAD_TYPE==0×74
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read file header ( first 7 bytes already read )
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read or skip HEAD_SIZE-sizeof(FILE_HEAD) bytes
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if (HEAD_FLAGS & 0×100)
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read or skip HIGH_PACK_SIZE*0×100000000+PACK_SIZE bytes
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else
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read or skip PACK_SIZE bytes
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else
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read corresponding HEAD_TYPE block:
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read HEAD_SIZE-7 bytes
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if (HEAD_FLAGS & 0×8000)
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read ADD_SIZE bytes
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6. go to 4.
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==========================================================================
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Block Formats
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==========================================================================
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Marker block ( MARK_HEAD )
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HEAD_CRC Always 0×6152
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2 bytes
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HEAD_TYPE Header type: 0×72
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1 byte
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HEAD_FLAGS Always 0x1a21
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2 bytes
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HEAD_SIZE Block size = 0×0007
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2 bytes
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The marker block is actually considered as a fixed byte
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sequence: 0×52 0×61 0×72 0×21 0x1a 0×07 0×00
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Archive header ( MAIN_HEAD )
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HEAD_CRC CRC of fields HEAD_TYPE to RESERVED2
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2 bytes
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HEAD_TYPE Header type: 0×73
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1 byte
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HEAD_FLAGS Bit flags:
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2 bytes
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0×0001 – Volume attribute (archive volume)
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0×0002 – Archive comment present
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RAR 3.x uses the separate comment block
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and does not set this flag.
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0×0004 – Archive lock attribute
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0×0008 – Solid attribute (solid archive)
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0×0010 – New volume naming scheme (‘volname.partN.rar’)
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0×0020 – Authenticity information present
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RAR 3.x does not set this flag.
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0×0040 – Recovery record present
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0×0080 – Block headers are encrypted
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0×0100 – First volume (set only by RAR 3.0 and later)
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other bits in HEAD_FLAGS are reserved for
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internal use
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HEAD_SIZE Archive header total size including archive comments
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2 bytes
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RESERVED1 Reserved
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2 bytes
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RESERVED2 Reserved
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4 bytes
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File header (File in archive)
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HEAD_CRC CRC of fields from HEAD_TYPE to FILEATTR
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2 bytes and file name
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HEAD_TYPE Header type: 0×74
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1 byte
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HEAD_FLAGS Bit flags:
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2 bytes
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0×01 – file continued from previous volume
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0×02 – file continued in next volume
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0×04 – file encrypted with password
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0×08 – file comment present
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RAR 3.x uses the separate comment block
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and does not set this flag.
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0×10 – information from previous files is used (solid flag)
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(for RAR 2.0 and later)
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bits 7 6 5 (for RAR 2.0 and later)
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0 0 0 – dictionary size 64 KB
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0 0 1 – dictionary size 128 KB
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0 1 0 – dictionary size 256 KB
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0 1 1 – dictionary size 512 KB
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1 0 0 – dictionary size 1024 KB
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1 0 1 – dictionary size 2048 KB
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1 1 0 – dictionary size 4096 KB
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1 1 1 – file is directory
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0×100 – HIGH_PACK_SIZE and HIGH_UNP_SIZE fields
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are present. These fields are used to archive
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only very large files (larger than 2Gb),
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for smaller files these fields are absent.
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0×200 – FILE_NAME contains both usual and encoded
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Unicode name separated by zero. In this case
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NAME_SIZE field is equal to the length
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of usual name plus encoded Unicode name plus 1.
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If this flag is present, but FILE_NAME does not
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contain zero bytes, it means that file name
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is encoded using UTF-8.
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0×400 – the header contains additional 8 bytes
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after the file name, which are required to
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increase encryption security (so called ‘salt’).
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0×800 – Version flag. It is an old file version,
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a version number is appended to file name as ‘;n’.
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0×1000 – Extended time field present.
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0×8000 – this bit always is set, so the complete
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block size is HEAD_SIZE + PACK_SIZE
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(and plus HIGH_PACK_SIZE, if bit 0×100 is set)
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HEAD_SIZE File header full size including file name and comments
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2 bytes
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PACK_SIZE Compressed file size
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4 bytes
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UNP_SIZE Uncompressed file size
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4 bytes
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HOST_OS Operating system used for archiving
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1 byte 0 – MS DOS
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1 – OS/2
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2 – Win32
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3 – Unix
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4 – Mac OS
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5 – BeOS
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FILE_CRC File CRC
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4 bytes
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FTIME Date and time in standard MS DOS format
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4 bytes
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UNP_VER RAR version needed to extract file
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1 byte
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Version number is encoded as
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10 * Major version + minor version.
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METHOD Packing method
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1 byte
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0×30 – storing
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0×31 – fastest compression
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0×32 – fast compression
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0×33 – normal compression
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0×34 – good compression
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0×35 – best compression
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NAME_SIZE File name size
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2 bytes
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ATTR File attributes
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4 bytes
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HIGH_PACK_SIZE High 4 bytes of 64 bit value of compressed file size.
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4 bytes Optional value, presents only if bit 0×100 in HEAD_FLAGS
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is set.
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HIGH_UNP_SIZE High 4 bytes of 64 bit value of uncompressed file size.
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4 bytes Optional value, presents only if bit 0×100 in HEAD_FLAGS
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is set.
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FILE_NAME File name – string of NAME_SIZE bytes size
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SALT present if (HEAD_FLAGS & 0×400) != 0
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8 bytes
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EXT_TIME present if (HEAD_FLAGS & 0×1000) != 0
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variable size
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other new fields may appear here.
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==========================================================================
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Application notes
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==========================================================================
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1. To process an SFX archive you need to skip the SFX module searching
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for the marker block in the archive. There is no marker block sequence (0×52
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0×61 0×72 0×21 0x1a 0×07 0×00) in the SFX module itself.
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2. The CRC is calculated using the standard polynomial 0xEDB88320. In
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case the size of the CRC is less than 4 bytes, only the low order bytes
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are used.
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