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Storage and Filesystem Stack

TilekarOS features a modular storage stack that abstracts hardware details from the application layer. This allows the same code to work with ramdisks, hard drives, or network storage.

1. The Modular Architecture

TilekarOS uses a tiered approach to storage:

Layer Component Description
High VFS (vfs.c) Virtual File System (Abstracts FAT, Ext2, etc.)
FAT (fat.c) File Allocation Table (FAT12 implementation)
Buffer Cache (buffer.c) Caches disk sectors in RAM to speed up I/O
Low Device Layer (devices.c) Unified interface for Block/Char hardware
Drivers (ata.c, ramdisk.c) Hardware-specific code (Supports PIO and DMA) 
graph TD
    App[User Application] -->|Syscalls| VFS[VFS Layer]
    VFS -->|Mount| FAT[FAT12 Driver]
    FAT -->|Request Sector| BC[Buffer Cache]
    BC -->|Read/Write| DEV[Device Registry]
    DEV -->|Block I/O| ATA[ATA Disk Driver]
    DEV -->|Block I/O| RD[Ramdisk Driver]

2. Virtual File System (VFS)

Source Files: vfs.c, vfs.h

The VFS provides a common interface for file operations like open, read, and write.

Key Concepts:

  • vnode_t: Represents a file or directory on any filesystem.
  • file_t: Represents an open instance of a file (contains current seek position).
  • Mounting: Attaching a filesystem driver to a specific path.

3. FAT12 Filesystem

Source Files: fat.c, fat.h

TilekarOS includes a native FAT12 driver, which supports both read and write operations.

Implementation Details:

  • BPB (BIOS Parameter Block): Parsed from the first sector to get cluster size and FAT location.
  • Cluster Chains: Follows the FAT table to read files spanning multiple non-contiguous clusters.
  • 8.3 Filenames: Supports standard short filenames (e.g., README.TXT).
  • Directory Management: Supports mkdir, rmdir, and readdir operations.

4. Buffer Cache

Source Files: buffer.c, buffer.h

The Buffer Cache (buffer_t) reduces physical I/O by keeping recently accessed sectors in memory. If a sector is "dirty" (modified), it is only written back to disk when buffer_flush() is called or the system shuts down.

5. Storage Initialization

The kernel initializes available storage devices at boot.

  1. PCI Scan: The kernel scans the PCI bus to find IDE/ATA controllers.
  2. ATA Discovery: For each controller found, the kernel probes for connected Master/Slave drives.
  3. Device Registration: Disks are registered in the device registry (e.g., ata0, ata1).
  4. VFS Mount: The boot partition (usually on ata0) is mounted at the root (/) of the VFS.

6. Test/Example: Reading a file from VFS

This code demonstrates how to use the high-level VFS API within the kernel:

void demo_filesystem() {
    int fd = vfs_open("/BIN/HELLO.TXT", 0);
    if (fd >= 0) {
        char buffer[64];
        int bytes = vfs_read(fd, buffer, 63);
        buffer[bytes] = '\0';
        printf("File Content: %s\n", buffer);
        vfs_close(fd);
    }
}

References