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Kernel/FileSystem/FATFS: Support FAT12 file system clusters

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Taj Morton 2023-07-30 17:31:28 -07:00 • committed by Andrew Kaster
parent 1f70a728f0
commit 2995ee5858
2 changed files with 40 additions and 19 deletions
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54
Kernel/FileSystem/FATFS/Inode.cpp
View file

@ -80,13 +80,14 @@ ErrorOr<Vector<BlockBasedFileSystem::BlockIndex>> FATInode::compute_block_list()
break; break;
} }
u32 fat_offset = cluster * cluster_size(); u32 fat_offset = fat_offset_for_cluster(cluster);
u32 fat_sector_index = fs().m_parameter_block->common_bpb()->reserved_sector_count + (fat_offset / fs().m_device_block_size); u32 fat_sector_index = fs().m_parameter_block->common_bpb()->reserved_sector_count + (fat_offset / fs().m_device_block_size);
u32 entry_offset = fat_offset % fs().m_device_block_size; u32 entry_offset = fat_offset % fs().m_device_block_size;
TRY(fs().raw_read(fat_sector_index, fat_sector_buffer)); TRY(fs().raw_read(fat_sector_index, fat_sector_buffer));
cluster = cluster_number(*fat_sector, entry_offset); // Look up the next cluster to read, or read End of Chain marker from table.
cluster = cluster_number(*fat_sector, cluster, entry_offset);
} }
return block_list; return block_list;
@ -99,9 +100,9 @@ u32 FATInode::end_of_chain_marker() const
// of the chain has been reached for a given entry. // of the chain has been reached for a given entry.
switch (fs().m_fat_version) { switch (fs().m_fat_version) {
case FATVersion::FAT12: case FATVersion::FAT12:
return 0xff8; return 0xFF8;
case FATVersion::FAT16: case FATVersion::FAT16:
return 0xfff8; return 0xFFF8;
case FATVersion::FAT32: case FATVersion::FAT32:
return 0x0FFFFFF8; return 0x0FFFFFF8;
default: default:
@ -109,28 +110,49 @@ u32 FATInode::end_of_chain_marker() const
} }
} }
size_t FATInode::cluster_size() const size_t FATInode::fat_offset_for_cluster(u32 cluster) const
{ {
switch (fs().m_fat_version) { switch (fs().m_fat_version) {
case FATVersion::FAT12: case FATVersion::FAT12: {
VERIFY(false); // In FAT12, a cluster entry is stored in a byte, plus
break; // the low/high nybble of an adjacent byte.
//
// CLSTR: 0 1 2 3 4 5
// INDEX: [0 1 2], [3 4 5], [6 7 8]
// Every 2 clusters are represented using 3 bytes.
return (cluster * 3) / 2;
} break;
case FATVersion::FAT16: case FATVersion::FAT16:
return 2; return cluster * 2; // Each cluster is stored in 2 bytes.
case FATVersion::FAT32: case FATVersion::FAT32:
return 4; return cluster * 4; // Each cluster is stored in 4 bytes.
default: default:
VERIFY(false); VERIFY_NOT_REACHED();
} }
} }
u32 FATInode::cluster_number(KBuffer const& fat_sector, u32 entry_offset) const u32 FATInode::cluster_number(KBuffer const& fat_sector, u32 entry_cluster_number, u32 entry_offset) const
{ {
u32 cluster = 0; u32 cluster = 0;
switch (fs().m_fat_version) { switch (fs().m_fat_version) {
case FATVersion::FAT12: case FATVersion::FAT12: {
VERIFY(false); u16 fat12_bytes_le = 0;
// Two FAT12 entries get stored in a total of 3 bytes, as follows:
// AB CD EF are grouped as [D AB] and [E FC] (little-endian).
// For a given cluster, we interpret the associated 2 bytes as a little-endian
// 16-bit value ({CD AB} or {EF CD}), and then shift/mask the extra high or low nybble.
ByteReader::load<u16>(fat_sector.bytes().offset(entry_offset), fat12_bytes_le);
cluster = LittleEndian { fat12_bytes_le };
if (entry_cluster_number % 2 == 0) {
// CD AB -> D AB
cluster &= 0x0FFF;
} else {
// EF CD -> E FC.
cluster = cluster >> 4;
}
break; break;
}
case FATVersion::FAT16: { case FATVersion::FAT16: {
u16 cluster_u16_le = 0; u16 cluster_u16_le = 0;
ByteReader::load<u16>(fat_sector.bytes().offset(entry_offset), cluster_u16_le); ByteReader::load<u16>(fat_sector.bytes().offset(entry_offset), cluster_u16_le);
@ -147,7 +169,7 @@ u32 FATInode::cluster_number(KBuffer const& fat_sector, u32 entry_offset) const
break; break;
} }
default: default:
VERIFY(false); VERIFY_NOT_REACHED();
} }
return cluster; return cluster;
} }
@ -291,7 +313,7 @@ u32 FATInode::first_cluster(FATVersion const version) const
} }
// The space occupied in a directory entry by `first_cluster_high` (0x14) // The space occupied in a directory entry by `first_cluster_high` (0x14)
// is reserved in FAT12/16, and may be used to store file meta-data. // is reserved in FAT12/16, and may be used to store file meta-data.
// As a result, do not include it. // As a result, do not include it on FAT12/16 file systems.
return m_entry.first_cluster_low; return m_entry.first_cluster_low;
} }

5
Kernel/FileSystem/FATFS/Inode.h
View file

@ -30,8 +30,7 @@ public:
private: private:
FATInode(FATFS&, FATEntry, NonnullOwnPtr<KString> filename); FATInode(FATFS&, FATEntry, NonnullOwnPtr<KString> filename);
// Number of bytes used to store a cluster within the table. size_t fat_offset_for_cluster(u32 cluster) const;
size_t cluster_size() const;
// Returns cluster number value that indicates the end of the chain // Returns cluster number value that indicates the end of the chain
// has been reached. Any cluster value >= this value indicates this // has been reached. Any cluster value >= this value indicates this
@ -39,7 +38,7 @@ private:
u32 end_of_chain_marker() const; u32 end_of_chain_marker() const;
// Reads the cluster number located at the offset within the table. // Reads the cluster number located at the offset within the table.
u32 cluster_number(KBuffer const& fat_sector, u32 entry_offset) const; u32 cluster_number(KBuffer const& fat_sector, u32 entry_cluster_number, u32 entry_offset) const;
static constexpr u8 end_entry_byte = 0x00; static constexpr u8 end_entry_byte = 0x00;
static constexpr u8 unused_entry_byte = 0xE5; static constexpr u8 unused_entry_byte = 0xE5;