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vfs.md

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Virtual Filesystem (VFS) Layer

Overview

The VFS layer provides a POSIX-like file API for the kernel, backed by the FAT32 filesystem driver. It is the single point of entry for all file I/O in the kernel – higher-level code never calls the FAT32 or ATA drivers directly.

 ┌──────────────┐
 │  Kernel code  │   vfs_open / vfs_read / vfs_write / …
 └──────┬───────┘
        │
 ┌──────▼───────┐
 │    VFS layer  │   c_files/src/vfs.c
 │  (fd table)   │
 └──────┬───────┘
        │
 ┌──────▼───────┐
 │ FAT32 driver  │   c_files/src/fat32.c
 └──────┬───────┘
        │
 ┌──────▼───────┐
 │  ATA PIO      │   c_files/src/ata.c
 └───────────────┘

Boot Initialisation

fs_init() is called from kernel_init() after the heap is ready:

  1. ATA probe – ata_init() sends IDENTIFY to the primary master drive.
  2. MBR parse – LBA 0 is read; if it contains a valid MBR (0xAA55), the partition table is scanned for the first FAT32 entry (type 0x0B / 0x0C) or FAT12/16 entry (0x01 / 0x04 / 0x06 / 0x0E).
  3. Super-floppy fallback – If no partition table is found, LBA 0 is treated as the start of a FAT volume directly.
  4. FAT mount – fat32_init() reads the BPB, EBPB and FSInfo sector, then populates the fat32_context_t structure.

If any step fails, fs_init() returns -1 and the kernel continues without a mounted filesystem.

File Descriptor Table

A fixed-size table (VFS_MAX_OPEN_FILES = 16) tracks open files. Each slot stores:

Field Description
in_use Whether the slot is occupied
flags VFS_O_* flags from the open call
position Current byte offset for read/write
entry Copy of the FAT32 directory entry
loc On-disk sector/offset of the dir entry
dirty Metadata needs flush on close

API Reference

Initialisation

int fs_init(void);            // Probe ATA, find FAT partition, mount
fat32_context_t *vfs_get_context(void);  // Raw access (debugging)

File Operations

int vfs_open(const char *path, int flags);   // Returns fd ≥ 0, or -1
int vfs_close(int fd);
int vfs_read(int fd, void *buf, unsigned int count);  // Returns bytes read
int vfs_write(int fd, const void *buf, unsigned int count);
int vfs_seek(int fd, int offset, int whence);  // Returns new position

Metadata

int vfs_stat(const char *path, vfs_stat_t *st);
int vfs_readdir(const char *path, vfs_readdir_cb_t cb, void *userdata);

Directory / File Management

int vfs_mkdir(const char *path);
int vfs_unlink(const char *path);   // Files only (not directories)

Open Flags

Flag Value Description
VFS_O_RDONLY 0x0001 Read-only
VFS_O_WRONLY 0x0002 Write-only
VFS_O_RDWR 0x0003 Read and write
VFS_O_CREAT 0x0100 Create if not exists
VFS_O_TRUNC 0x0200 Truncate existing file to 0
VFS_O_APPEND 0x0400 All writes go to end of file
VFS_O_DIRECTORY 0x1000 Fail if not a directory

Seek Whence

Constant Value Base
VFS_SEEK_SET 0 Beginning of file
VFS_SEEK_CUR 1 Current position
VFS_SEEK_END 2 End of file

Usage Example

#include "vfs.h"

void demo(void) {
    // Create and write
    int fd = vfs_open("/hello.txt", VFS_O_RDWR | VFS_O_CREAT);
    vfs_write(fd, "Hello, world!\n", 14);

    // Seek back and read
    vfs_seek(fd, 0, VFS_SEEK_SET);
    char buf[64];
    int n = vfs_read(fd, buf, sizeof(buf));
    vfs_close(fd);

    // List root directory
    vfs_readdir("/", my_callback, NULL);

    // Create a subdirectory
    vfs_mkdir("/mydir");

    // Delete a file
    vfs_unlink("/hello.txt");
}

Source Files

File Purpose
c_files/includes/vfs.h Public header (types, flags, API)
c_files/src/vfs.c Implementation (fd table, init, all ops)
c_files/includes/fat32.h FAT32 driver header
c_files/src/fat32.c FAT32 driver implementation
c_files/includes/ata.h ATA PIO driver header
c_files/src/ata.c ATA PIO driver implementation