Apple GEOS File Formats

The Apple GEOS disk I/O subsystem is based on ProDOS 8. In order to understand the information provided here both knowledge of ProDOS 8 Technical Reference Manual terminology and the general concept of GEOS sequential files and GEOS VLIR files is necessary.

• The ProDOS 'file_name' is not limited to capital letters, digits, and periods.
• The ProDOS 'file_type' is in the range $80-$8F ($82 for a GEOS 'application').
• The ProDOS 'aux_type' contains the ProDOS block address of a header block.
• The header block counts for the ProDOS 'blocks_used'.
• The header block offsets $000-$0FF contain the the GEOS 'file header'.
• The header block offsets $100-$1FF are unused.

• The format is nearly identical to a ProDOS sapling file.
• The ProDOS 'storage_type' is 2.
• The ProDOS index block points to up to 254 data blocks.
• The ProDOS index block offset $0FF contains the low byte of the ProDOS 'EOF'.
• The ProDOS index block offset $1FF contains the mid byte of the ProDOS 'EOF'.
• The ProDOS index block offset $0FE contains the high byte of the ProDOS 'EOF'.
• The ProDOS index block offset $1FE is 0.

• The format is somewhat similiar to a ProDOS tree file.
• The ProDOS 'storage_type' is 3.
• The file contains up to 127 VLIR records.
• The ProDOS 'EOF' is the sum of all record sizes in bytes.
• The ProDOS master index block points to one ProDOS index block for every record.
• The list of records is terminated with the ProDOS block address $0000.
• The list of records may contain empty records indicated by the ProDOS block address $FFFF.
• The ProDOS master index block offset $0FF contains the low byte of the ProDOS 'EOF'.
• The ProDOS master index block offset $1FF contains the mid byte of the ProDOS 'EOF'.
• The ProDOS master index block offset $0FE contains the high byte of the ProDOS 'EOF'.
• The ProDOS master index block offset $1FE is 0.
• Each ProDOS index block points to up to 254 data blocks.
• Each ProDOS index block offset $0FF contains the low byte of the record size in bytes.
• Each ProDOS index block offset $1FF contains the mid byte of the record size in bytes.
• Each ProDOS index block offset $0FE contains the high byte of the record size in bytes.
• Each ProDOS index block offset $1FE is 0.

The Hitchhiker's Guide To GEOS contains no section on file formats. However several sections contain side notes on Apple GEOS file formats:

An empty record is marked with $ffff in the VLIR index table (stored in the buffer
at fileHeader). An unused record is marked with $0000. Use PointRecord to
check the status of a particular record (unused, empty, or filled).

The Apple version of BlkAlloc builds out a list of allocated blocks in the internal
INDEXBLOCKBUF buffer. This index block is a modified 512-byte ProDOS
sapling index block. The block is divided into two 256-byte portions. The lower
half (bytes 0 through 255) represent the low-bytes of the data blocks and the
upper half (bytes 256 through 511) represent the associated high-bytes. Byte 0
and byte 256+0 combine to form the block number of the first data block, byte 1
and byte 256+1 combine to form the number of the second data block, and so on
for each allocated block. A block pointer of $0000 ($00 high, $00 low) represents
the end of the list.

The last two pointers in the index block—the ones in a standard ProDOS sapling
index that point to the 254th and 255th blocks in the chain—comprise a four-byte
size value in GEOS. The four-byte value, which represents the number of bytes
stored in the allocated blocks, can be accessed using the following offsets into the
index block:
DSIZE0 Low byte
DSIZE1 .
DSIZE2 .
DSIZE3 High byte

Apple GEOS has no FollowChain equivalent because ProDOS links blocks
together, not by pointers within each block, but by a list of blocks in an index
associated with each sequential file and VLIR record This index is called the
index block and, as its name implies, occupies a single block on the disk.

FreeFile first copies the entire directory entry (ENTRY_SIZE bytes) from
DIRENTRY (in main memory) to an internal buffer in auxiliary memory.

If the file is a VLIR file, then FreeFile reads the VLIR index block (the ProDOS
master index block) into memory and frees all the data blocks in each record. The
VLIR index block and all the individual record index blocks are then deleted.
FreeFile finishes by calling PutVBM to flush the VBM cache.

If the file is a sequential file, FreeFile reads the sequential index block into
memory, frees all assoiciated data blocks, then frees the index block itself.
FreeFile finishes by calling PutVBM to flush the VBM cache.

fileHeader: contains 256-byte GEOS file header. (This is a 512-byte
buffer in Apple GEOS, although only 256 bytes are used in
the GEOS file header for compatibility).

fileBytes: total number of bytes in file (as picked up from last from
bytes 254,255,511, and 512 of the master index block.

All ProDOS disks are already GEOS compatible.