Overview

Filesystem Stack

Userland (syscalls 0x20–0x2E)
    │
    ▼
fs/vfs — mount table, path dispatch
    ├── fs/fat12    — floppy FAT12 (read/write)
    └── fs/iso9660  — CD-ROM ISO9660 (read-only)
         │               │
    fat12/block.rs   iso9660/block.rs
    (Floppy/ISA DMA)  (Atapi/PIO)
         │               │
    BlockDevice trait (fs/block.rs)

Both filesystems implement the same BlockDevice trait. All higher-level logic (directory walks, FAT chains, ISO records) is built on top of that single abstraction.

---

BlockDevice Trait (fs/block.rs)

pub trait BlockDevice {
    fn read_sector(&self, lba: u64, buffer: &mut [u8]);
    fn write_sector(&self, lba: u64, buffer: &[u8; 512]);
}

One sector = 512 bytes for FAT12/floppy. ISO9660 uses 2048-byte logical blocks but maps them to the same interface internally. Implementors: Floppy, Atapi (write is a no-op), MemDisk.

---

VFS (fs/vfs/mod.rs)

The VFS is a simple mount table — it does not provide a unified file descriptor layer or inode abstraction. All it does is map path prefixes to filesystem types, and dispatch resolves which filesystem to use for a given absolute path.

Mount Table

pub static VFS: Mutex<VfsTable>

pub struct VfsTable {
    mounts: [VfsMount; MAX_MOUNTS],   // MAX_MOUNTS = 8
    count:  usize,
}

pub struct VfsMount {
    path:     [u8; 32],
    path_len: usize,
    fs_type:  FsType,
}

FsType enum:

Variant	Meaning
None	Empty / unused slot
Root	Root mountpoint (/)
Fat12	FAT12 floppy at /mnt/fat
Iso9660	ISO9660 CD-ROM at /mnt/iso

Mounts at Boot

Set up by init::fs::vfs_init():

Path	FsType	Condition
/	Root	Always
/mnt/fat	Fat12	Always
/mnt/iso	Iso9660	Only if Iso9660::probe() succeeds

Path Resolution

VfsTable::resolve(path) returns (FsType, relative_sub_path) using longest-prefix matching:

1. Iterate all mounts; check if path starts with the mount path.
2. Require exact match or that the next character after the prefix is /.
3. The mount with the longest matching prefix wins.
4. Returns the sub-path after stripping the mount prefix (and a leading /).

Example: path b"/mnt/fat/SUBDIR/FILE.TXT" → (Fat12, b"SUBDIR/FILE.TXT").

Helpers

Function	Description
try_fat12_absolute(path)	Returns Some(rel) if path resolves under the Fat12 mount
try_iso9660_absolute(path)	Returns Some(rel) if path resolves under the Iso9660 mount
mount(path, fs_type)	Add a mount entry
umount(path)	Remove a mount entry by path

These are the primary VFS entry points used by syscall handlers in abi/syscall.rs.

Syscall Dispatch Pattern

Every filesystem syscall uses the same two-step dispatch:

path → try_iso9660_absolute(path)
         Some(rel) → Iso9660::probe()?.resolve(rel)  [read-only]
         None      → vfs_resolve_fat12(path) → Filesystem::new(&floppy)

vfs_resolve_fat12(path) strips the /mnt/fat/ prefix if present, or falls back to the current working directory cluster from SYSTEM_CONFIG.

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Working Directory

The current working directory is stored in SYSTEM_CONFIG (init/config.rs) as two fields:

Field	Type	Description
path	[u8; 32]	String representation (e.g. /mnt/fat/SUBDIR)
path_cluster	u16	FAT12 cluster for the directory (0 = root, 0 for ISO9660)

Changed by syscall 0x2E (chdir), which validates that the target exists as a directory before updating.

---

MemDisk (fs/memdisk/block.rs)

MemDisk wraps a &'static mut [u8] as a BlockDevice. read_sector copies 512 bytes from the slice. write_sector is a no-op. Intended for in-memory disk images but not currently wired into any live code path.

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Limits

Resource	Value
Max VFS mounts	8
Max mount path length	31 bytes
FAT12 sector size	512 bytes
ISO9660 block size	2048 bytes
Max directory entries returned (syscall 0x28)	32
Max directory entries returned (syscall 0x2D)	64
Max VFS mounts listed (syscall 0x2C)	8 × 34-byte entries