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58 lines
2.4 KiB
Markdown
58 lines
2.4 KiB
Markdown
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# echidnaFS Specification
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**Blocks start with block 0, and are addressed as quad-words.**
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The file system should be addressable using 32-bit addressing too, in a "compatibility mode".
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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.
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## block#0 to block#15
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Reserved blocks. Always. A bigger block size will result in more reserved space.
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**block#0** contains the **identity table** as follows:
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```x86asm
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times 4 db ; reserved for jump instruction, or other purposes
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db '_ECH_FS_' ; echidnaFS signature
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qword ; total block count
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qword ; length of the main directory in blocks (usually 5% of the total blocks)
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qword ; bytes per block (MUST be a multiple of 512)
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dword ; reserved
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qword[2] ; UUID of partition
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```
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## block#16: Allocation table
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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`
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Possible values for an entry:
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* `0x00000000_00000000`: "Free-for-use" block.
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* `0x00000000_00000001 -> 0xFFFFFFFF_FFFFFFEF`: Mark the next block in the chain.
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* `0xFFFFFFFF_FFFFFFF0`: "Reserved block" (the **first 16 blocks** are always marked as **reserved**, along with the directory and allocation table blocks).
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* `0xFFFFFFFF_FFFFFFFF`: "End-of-chain".
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## Main directory
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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**.\
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Contains entries representing files, directories, or other objects.
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Directory ID values:
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* `0x00000000_00000000`: "End-of-directory".
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* `0x00000000_00000001 -> 0xFFFFFFFF_FFFFFFFD`: Normal directory entry.
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* `0xFFFFFFFF_FFFFFFFE`: Deleted entry.
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* `0xFFFFFFFF_FFFFFFFF`: Root directory.
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Directory entry structure:
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```x86asm
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qword ; Directory ID of the entry (see above)
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byte ; Object type (0=file, 1=directory, rest to be implemented)
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times 201 db ; Zero-terminated ascii string containing the name of the object. max. 200 chars + null byte.
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qword ; 64-bit unix atime
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qword ; 64-bit unix mtime
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word ; Permissions (-------rwxrwxrwx)
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word ; Owner ID
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word ; Group ID
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qword ; 64-bit unix ctime
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qword ; Starting block (if file), directory ID (if directory)
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qword ; File size in bytes (0 if directory, shouldn't matter anyways)
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```
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