User:Superleaf1995/lowFS: Difference between revisions
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(just realized there is already a fs with that name :/) |
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Is a table-filesystem designed by me (Superleaf1995) because why not? |
Is a table-filesystem designed by me (Superleaf1995) because why not? |
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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. |
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. |
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This filesystem also tries to be simple as possible, and be compatible with 16-bit hosts as well as 64-bit ones. |
This filesystem also tries to be simple as possible, and be compatible with 16-bit hosts as well as 64-bit ones. |
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== Bootsector == |
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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 label '_LF_' (ASCII). It should always be in big endian. Followed by a table: |
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<source lang="c"> |
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struct lowFS_megatable { |
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uint8_t attribute; // Attribute byte |
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uint64_t nlb; // Number of LBAs |
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uint8_t version; // lowFS version |
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}; |
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</source> |
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Attribute: |
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{| {{wikitable}} |
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|- |
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! Bit |
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! Purpose |
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|- |
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| 0 |
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| If set, the disk is in big endian, if clear, the disk is in little endian |
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|- |
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| 1 |
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| If set, disk has bad sectors |
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|- |
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| 2-7 |
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| TBD |
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|} |
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== Entry == |
== Entry == |
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The entry header is basically like a FAT entry, however, this is a header ''for entries'' not for files. |
The entry header is basically like a FAT entry, however, this is a header ''for entries'' not for files. |
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<source lang="c"> |
<source lang="c"> |
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struct |
struct lowFS_entry_header { |
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uint8_t reserved; // Should always be 0xCC |
uint8_t reserved; // Should always be 0xCC |
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uint8_t entry_type; // Type of entry |
uint8_t entry_type; // Type of entry |
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uint64_t previous; // Previous entry header in the chain (pointer) |
uint64_t previous; // Previous entry header in the chain (pointer) |
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uint64_t next_entry; // Next entry header in the chain (pointer) |
uint64_t next_entry; // Next entry header in the chain (pointer) |
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uint64_t size; // Size of entry ( |
uint64_t size; // Size of entry (lowFS_entry struct not included). If set to 0, the host must know the size parsing entry type |
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}; |
}; |
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</source> |
</source> |
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If entry_type is set to '''ENTRY_TYPE_FILEMETADATA''' (0xF0): |
If entry_type is set to '''ENTRY_TYPE_FILEMETADATA''' (0xF0): |
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<source lang="c"> |
<source lang="c"> |
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struct |
struct lowFS_entry_filemetadata { |
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uint64_t filename_len; // length of filename |
uint64_t filename_len; // length of filename |
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uint64_t len; // length of file in bytes |
uint64_t len; // length of file in bytes |
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uint8_t filename[filename_len]; // variable size, kernel should alloc memory for incoming filename |
uint8_t filename[filename_len]; // variable size, kernel should alloc memory for incoming filename |
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uint8_t encrypt; // If it's nonzero, the file is encrypted with some TBD encryption system |
uint8_t encrypt; // If it's nonzero, the file is encrypted with some TBD encryption system |
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}; |
}; |
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</source> |
</source> |
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If entry_type is set to '''ENTRY_TYPE_FILEPART''' (0xF2): |
If entry_type is set to '''ENTRY_TYPE_FILEPART''' (0xF2): |
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<source lang="c"> |
<source lang="c"> |
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struct |
struct lowFS_entry_filepart { |
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uint64_t clen; // Length of the bytes of the file |
uint64_t clen; // Length of the bytes of the file |
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uint8_t content[clen]; // variable size, kernel should alloc memory for incoming file content |
uint8_t content[clen]; // variable size, kernel should alloc memory for incoming file content |
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uint16_t checksum; // verifies that the data in the file is correct (First byte of content ANDed by the last one) |
uint16_t checksum; // verifies that the data in the file is correct (First byte of content ANDed by the last one) |
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}; |
}; |
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</source> |
</source> |
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If entry type is set to '''ENTRY_TYPE_DIRECTORYMETADATA''' (0xF4): |
If entry type is set to '''ENTRY_TYPE_DIRECTORYMETADATA''' (0xF4): |
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<source lang="c"> |
<source lang="c"> |
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struct |
struct lowFS_entry_directorymetadata { |
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uint64_t dirname_len; // length of dirname |
uint64_t dirname_len; // length of dirname |
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uint8_t dirname[dirname_len]; // variable size, kernel should alloc memory for incoming dirname |
uint8_t dirname[dirname_len]; // variable size, kernel should alloc memory for incoming dirname |
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<source lang="c"> |
<source lang="c"> |
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struct |
struct lowFS_checksum { |
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uint16_t val; // 0x53BC |
uint16_t val; // 0x53BC |
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uint16_t bf; // Previous 2 bytes |
uint16_t bf; // Previous 2 bytes ANDed with 0x53BC |
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uint32_t fp; // FILEPART size high bytes |
uint32_t fp; // FILEPART size high bytes |
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}; |
}; |
Revision as of 12:25, 22 June 2020
lowFS stands for Universal FileSystem.
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.
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 label '_LF_' (ASCII). It should always be in big endian. Followed by a table:
struct lowFS_megatable {
uint8_t attribute; // Attribute byte
uint64_t nlb; // Number of LBAs
uint8_t version; // lowFS version
};
Attribute:
Bit | Purpose |
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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 |
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
};