User:Superleaf1995/lowFS
This page or section is a stub. You can help the wiki by accurately contributing to it. |
lowFS stands for Low Filesystem, simple huh?.
Is a table-filesystem designed by me (Superleaf1995) because why not?
In this filesystem each entry points to a portion of a file, this allows loading parts of the file progressively. If any of the tables is damaged, a checksum every 8 entries is made to ensure nothing is wrong. lowFS assumes the disk is a plain array of bytes.
This filesystem also tries to be simple as possible, and be compatible with 16-bit hosts as well as 64-bit ones.
Contents |
Bootsector
There are three ways to identify a lowFS disk: checking byte 0x02, checking the last 4 bytes of sector 4 or checking the last 4 bytes of the disk. It should contain the characters 'LFFL' (ASCII). Followed by a table:
struct lowFS_megatable { uint8_t attribute; // Attribute byte uint8_t version; // lowFS version union { struct lowFS_floppy { uint8_t heads; uint8_t bytes_per_sector; uint16_t sectors_per_track; }; struct lowFS_non_floppy { uint32_t n_lba; }; }table; uint16_t reserved; }__attribute__((packed));
Attribute:
Bit | Purpose |
---|---|
0 | If set, the disk is in big endian, if clear, the disk is in little endian |
1 | If set, disk has bad sectors |
2-7 | TBD |
- If all bits are set, the endianess is little, the disk has no bad sectors (assumed) and there are no checksums
Entry
The entry header is basically like a FAT entry, however, this is a header for entries not for files.
struct lowFS_entry_header { uint8_t reserved; // Should always be 0xCC uint8_t entry_type; // Type of entry uint32_t name_len; // Length of the name (variable, upon the OS how to handle) uint64_t nn_entry; // Next part of this entry (pointer) uint64_t previous; // Previous entry header in the chain (pointer) uint64_t next_entry; // Next entry header in the chain (pointer) uint64_t size; // Size of entry (lowFS_entry struct not included). If set to 0, the host must know the size parsing entry type };
- nn_entry should be set to 0 if this is the final part of the entry.
- next_entry should be set to 0 if this is the final part of the chain of entries.
Entry types
Following the entry_t header, follows the contents of the entry. Notice that these varies in size, but always should align to 8-bytes.
If entry_type is set to ENTRY_TYPE_FILEMETADATA (0xF0):
struct lowFS_entry_filemetadata { uint64_t filename_len; // length of filename uint64_t len; // length of file in bytes uint8_t filename[filename_len]; // variable size, kernel should alloc memory for incoming filename uint8_t encrypt; // If it's nonzero, the file is encrypted with some TBD encryption system uint8_t perm; // use for unix permissions };
- Filename should be ended with a NULL character
If entry_type is set to ENTRY_TYPE_FILEPART (0xF2):
struct lowFS_entry_filepart { uint64_t clen; // Length of the bytes of the file uint8_t content[clen]; // variable size, kernel should alloc memory for incoming file content uint16_t checksum; // verifies that the data in the file is correct (First byte of content ANDed by the last one) };
If entry type is set to ENTRY_TYPE_DIRECTORYMETADATA (0xF4):
struct lowFS_entry_directorymetadata { uint64_t dirname_len; // length of dirname uint8_t dirname[dirname_len]; // variable size, kernel should alloc memory for incoming dirname uint8_t perm; // use for unix permissions };
Checksum
The checksum is in total, 8-bytes.
The checksum is a simple 2-byte value 0x53BC. This byte is placed after 8 entries (directorymetadata,filepart,etc). This word should be used for hosts that confides that the disk is OK, in this case, it should skip the following 6-bytes.
In case the host wants to read the 6-byte value, it should AND the aforementioned 2-byte value with the previous 2 bytes before the checksum bytes. Regardless of it being a entry header or a regular entry or not. The result should match the checksum. If a FILEPART entry is before the checksum, the following 4-bytes are the higher 4-bytes of the size of the file. Otherwise it's the 4-checksum bytes.
After the checksum, the entries continue as normal.
A checksum cannot be after the last entry or entry header in a chain.
struct lowFS_checksum { uint16_t val; // 0x53BC uint16_t bf; // Previous 2 bytes ANDed with 0x53BC uint32_t fp; // FILEPART size high bytes };