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bztsrc--bootboot/specs/echfs.md
bzt 5b2816a411 Added fs specs
2021-03-21 02:02:06 +01:00

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echidnaFS Specification

Blocks start with block 0, and are addressed as quad-words.

The file system should be addressable using 32-bit addressing too, in a "compatibility mode".

Of course, when addressing using 32-bit values the maximum number of blocks will be 2^32, same thing for file sizes and other values.

block#0 to block#15

Reserved blocks. Always. A bigger block size will result in more reserved space.

block#0 contains the identity table as follows:

times 4 db     ; reserved for jump instruction, or other purposes
db '_ECH_FS_'  ; echidnaFS signature
qword          ; total block count
qword          ; length of the main directory in blocks (usually 5% of the total blocks)
qword          ; bytes per block (MUST be a multiple of 512)
dword          ; reserved
qword[2]       ; UUID of partition

block#16: Allocation table

Contains a chain of qwords. One per physical block, hence the size of the table (in blocks) can be calculated as: (total_blocks * sizeof(uint64_t) + block_size - 1) / block_size

Possible values for an entry:

  • 0x00000000_00000000: "Free-for-use" block.
  • 0x00000000_00000001 -> 0xFFFFFFFF_FFFFFFEF: Mark the next block in the chain.
  • 0xFFFFFFFF_FFFFFFF0: "Reserved block" (the first 16 blocks are always marked as reserved, along with the directory and allocation table blocks).
  • 0xFFFFFFFF_FFFFFFFF: "End-of-chain".

Main directory

The main directory starts from the first block after the end of the allocation table. Its length is specified in the ID table in block#0.
Contains entries representing files, directories, or other objects.

Directory ID values:

  • 0x00000000_00000000: "End-of-directory".
  • 0x00000000_00000001 -> 0xFFFFFFFF_FFFFFFFD: Normal directory entry.
  • 0xFFFFFFFF_FFFFFFFE: Deleted entry.
  • 0xFFFFFFFF_FFFFFFFF: Root directory.

Directory entry structure:

qword         ; Directory ID of the entry (see above)
byte          ; Object type (0=file, 1=directory, rest to be implemented)
times 201 db  ; Zero-terminated ascii string containing the name of the object. max. 200 chars + null byte.
qword         ; 64-bit unix atime
qword         ; 64-bit unix mtime
word          ; Permissions (-------rwxrwxrwx)
word          ; Owner ID
word          ; Group ID
qword         ; 64-bit unix ctime
qword         ; Starting block (if file), directory ID (if directory)
qword         ; File size in bytes (0 if directory, shouldn't matter anyways)