Add VFS subsystem and initrd filesystem driver (AI)

VFS subsystem (vfs.c/h):
- Mount table with up to 16 mount points, longest-prefix path matching.
- File descriptor table (256 entries, fds 0-2 reserved for std streams).
- Path resolution walks mount table then delegates to filesystem's
  finddir() for each path component.
- Operations: open, close, read, write, seek, readdir, stat.
- Each filesystem driver provides a vfs_fs_ops_t with callbacks.

Initrd filesystem driver (initrd_fs.c/h):
- Read-only VFS driver backed by the CPIO ramdisk.
- Mounted at '/initrd' during boot.
- Zero-copy reads: file data points directly into the CPIO archive
  memory, no allocation or copying needed.
- Supports readdir (flat iteration) and finddir (name lookup).

Bug fix: resolve_path was overwriting file-specific fs_data (set by
finddir, e.g. pointer to CPIO file data) with the mount's fs_data
(NULL). Fixed to preserve fs_data from finddir.

Verified in QEMU: kernel reads /initrd/README via VFS and prints its
contents. Ring 3 user process continues to work.

CMakeLists.txt

@@ -19,15 +19,26 @@ add_subdirectory(src)
 file(MAKE_DIRECTORY ${CMAKE_SOURCE_DIR}/release)
 file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/isodir/boot/grub)
 
-# Create grub.cfg for ISO
-file(WRITE ${CMAKE_BINARY_DIR}/isodir/boot/grub/grub.cfg "set timeout=0\nset default=0\nsearch --set=root --file /boot/kernel.bin\nmenuentry \"ClaudeOS\" { multiboot2 /boot/kernel.bin }")
+# Generate CPIO initial ramdisk from apps directory.
+# All files in apps/ are packed into a newc-format CPIO archive.
+set(INITRD_FILE ${CMAKE_BINARY_DIR}/isodir/boot/initrd.cpio)
+add_custom_command(
+    OUTPUT ${INITRD_FILE}
+    COMMAND ${CMAKE_SOURCE_DIR}/scripts/gen_initrd.sh ${CMAKE_SOURCE_DIR}/apps ${INITRD_FILE}
+    DEPENDS ${CMAKE_SOURCE_DIR}/apps
+    COMMENT "Generating CPIO initial ramdisk"
+)
+add_custom_target(initrd DEPENDS ${INITRD_FILE})
+
+# Create grub.cfg for ISO - includes module2 for the initrd
+file(WRITE ${CMAKE_BINARY_DIR}/isodir/boot/grub/grub.cfg "set timeout=0\nset default=0\nsearch --set=root --file /boot/kernel.bin\nmenuentry \"ClaudeOS\" { multiboot2 /boot/kernel.bin\n  module2 /boot/initrd.cpio }")
 
 
 # ISO Generation
 add_custom_target(iso ALL
     COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/kernel ${CMAKE_BINARY_DIR}/isodir/boot/kernel.bin
     COMMAND grub-mkrescue -o ${CMAKE_SOURCE_DIR}/release/claude-os.iso ${CMAKE_BINARY_DIR}/isodir
-    DEPENDS kernel
+    DEPENDS kernel initrd
     COMMENT "Generating bootable ISO image"
 )
 

README.md

@@ -50,9 +50,9 @@ Once a task is completed, it should be checked off.
 - [x] Create an initial driver architecture, allowing different drivers included in the kernel to test whether they should load or not.
 - [x] Create a VGA driver. On startup, some memory statistics should be displayed, as well as boot progress.
 - [x] Create subsystem for loading new processes in Ring 3.
-- [ ] Update the build script to generate a ramdisk containing any applications to run. This initial ramdisk is in CPIO format.
-- [ ] Write a VFS subsystem.
-- [ ] Write a VFS driver that provides the contents of the CPIO initial ramdisk to the VFS layer.
+- [x] Update the build script to generate a ramdisk containing any applications to run. This initial ramdisk is in CPIO format.
+- [x] Write a VFS subsystem.
+- [x] Write a VFS driver that provides the contents of the CPIO initial ramdisk to the VFS layer.
 - [ ] Create a `hello-world` app. It should print `Hello, World` to its own stdout. The kernel should route this to Qemu and to the VGA dispaly. Ensure this work.
 - [ ] Implement the fork system call.
 - [ ] Implement environment variables. Apps should be able to modify this, and it should be copied to new forks of an app.

apps/README

@@ -0,0 +1 @@
+This is the ClaudeOS initial ramdisk.

docs/cpio.md

@@ -0,0 +1,81 @@
+# CPIO Initial Ramdisk
+
+## Overview
+
+The initial ramdisk (initrd) provides files to the kernel at boot time before
+any filesystem drivers are available. It is a CPIO archive in SVR4/newc format,
+loaded by GRUB as a Multiboot2 module.
+
+## Build Process
+
+During the build, the script `scripts/gen_initrd.sh` packs all files from the
+`apps/` directory into a CPIO archive:
+
+```
+apps/
+├── README          (placeholder)
+├── hello-world     (future: flat binary)
+└── sh              (future: shell binary)
+```
+
+The archive is placed at `build/isodir/boot/initrd.cpio` and included in the
+ISO image. GRUB loads it as a module via:
+
+```
+module2 /boot/initrd.cpio
+```
+
+## CPIO Format
+
+The newc (SVR4) CPIO format uses 110-byte headers with hex ASCII fields:
+
+```
+Offset  Size  Field
+0       6     Magic ("070701")
+6       8     Inode
+14      8     Mode
+22      8     UID
+30      8     GID
+38      8     Nlink
+46      8     Mtime
+54      8     Filesize
+62      8     Devmajor
+70      8     Devminor
+78      8     Rdevmajor
+86      8     Rdevminor
+94      8     Namesize
+102     8     Check
+```
+
+After the header: filename (namesize bytes, padded to 4-byte boundary),
+then file data (filesize bytes, padded to 4-byte boundary). The archive
+ends with a `TRAILER!!!` entry.
+
+## Kernel Interface
+
+The kernel finds the initrd by scanning Multiboot2 boot information for
+`MULTIBOOT_TAG_TYPE_MODULE` (type 3). The module's physical memory range
+is identity-mapped, so it can be read directly.
+
+```c
+#include "cpio.h"
+
+// Find a file
+cpio_entry_t entry;
+if (cpio_find("hello-world", &entry) == 0) {
+    // entry.data = pointer to file contents
+    // entry.datasize = file size
+}
+
+// Iterate all files
+uint32_t offset = 0;
+while (cpio_next(&offset, &entry) == 0) {
+    // entry.name, entry.data, entry.datasize
+}
+```
+
+## Files
+
+- `scripts/gen_initrd.sh` — Build script to generate CPIO archive
+- `src/cpio.h` / `src/cpio.c` — CPIO parser (find, iterate, count)
+- `CMakeLists.txt` — `initrd` target and `module2` in grub.cfg

docs/vfs.md

@@ -0,0 +1,78 @@
+# Virtual Filesystem (VFS)
+
+## Overview
+
+The VFS provides a unified interface for file and directory operations across
+different filesystem implementations. Filesystem drivers register ops structs
+and are mounted at specific paths. Path resolution finds the longest-matching
+mount point and delegates to that filesystem.
+
+## Architecture
+
+```
+ User/Kernel Code
+      │
+      ▼
+  vfs_open("/initrd/hello-world", 0)
+      │
+      ▼
+  VFS: find_mount("/initrd/hello-world")
+      │  → mount "/initrd", rel_path = "hello-world"
+      ▼
+  resolve_path → initrd_finddir("hello-world")
+      │
+      ▼
+  vfs_read(fd, buf, size)
+      │  → initrd_read(node, offset, size, buf)
+      ▼
+  Returns file data from CPIO archive
+```
+
+## Mount Points
+
+Filesystems are mounted at absolute paths. The VFS supports up to 16
+simultaneous mounts. Path resolution uses longest-prefix matching:
+
+```
+Mount: "/initrd"  → handles /initrd/*
+Mount: "/sys"     → handles /sys/*
+Mount: "/dev"     → handles /dev/*
+```
+
+## File Operations
+
+| Function | Description |
+|----------|-------------|
+| `vfs_open(path, flags)` | Open a file, returns fd |
+| `vfs_close(fd)` | Close a file descriptor |
+| `vfs_read(fd, buf, size)` | Read bytes, advances offset |
+| `vfs_write(fd, buf, size)` | Write bytes, advances offset |
+| `vfs_seek(fd, offset, whence)` | Seek within file |
+| `vfs_readdir(path, idx, out)` | Read directory entry |
+| `vfs_stat(path, out)` | Get file info |
+
+## Filesystem Driver Interface
+
+Each filesystem provides a `vfs_fs_ops_t` struct:
+
+```c
+typedef struct vfs_fs_ops {
+    int (*open)(vfs_node_t *node, uint32_t flags);
+    void (*close)(vfs_node_t *node);
+    int32_t (*read)(vfs_node_t *node, uint32_t offset, uint32_t size, void *buf);
+    int32_t (*write)(vfs_node_t *node, uint32_t offset, uint32_t size, const void *buf);
+    int (*readdir)(vfs_node_t *dir, uint32_t idx, vfs_dirent_t *out);
+    int (*finddir)(vfs_node_t *dir, const char *name, vfs_node_t *out);
+} vfs_fs_ops_t;
+```
+
+## Initrd Filesystem Driver
+
+The initrd filesystem (`initrd_fs.c`) provides read-only access to the CPIO
+ramdisk. It is automatically mounted at `/initrd` during boot. Files are
+accessed via zero-copy reads directly from the CPIO archive in memory.
+
+## Files
+
+- `src/vfs.h` / `src/vfs.c` — VFS core: mount table, fd table, path resolution
+- `src/initrd_fs.h` / `src/initrd_fs.c` — CPIO ramdisk VFS driver

scripts/gen_initrd.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# Generate CPIO initial ramdisk from apps directory
+# Usage: gen_initrd.sh <apps_dir> <output_file>
+set -e
+
+APPS_DIR="$1"
+OUTPUT="$2"
+
+# Ensure output directory exists
+mkdir -p "$(dirname "$OUTPUT")"
+
+cd "$APPS_DIR"
+find . -not -name '.' | cpio -o -H newc > "$OUTPUT" 2>/dev/null
+echo "Generated initrd: $(wc -c < "$OUTPUT") bytes"

src/CMakeLists.txt

@@ -16,6 +16,9 @@ add_executable(kernel
     tss.c
     process.c
     syscall.c
+    cpio.c
+    vfs.c
+    initrd_fs.c
     interrupts.S
     kernel.c
 )

src/cpio.c

@@ -0,0 +1,179 @@
+/**
+ * @file cpio.c
+ * @brief CPIO newc archive parser implementation.
+ *
+ * Parses CPIO archives in the SVR4/newc format. The archive is expected
+ * to be loaded into memory by GRUB as a Multiboot2 module.
+ */
+
+#include "cpio.h"
+#include <string.h>
+
+/* Debug print helpers defined in kernel.c */
+extern void offset_print(const char *str);
+extern void print_hex(uint32_t val);
+
+/** Pointer to the CPIO archive in memory. */
+static const uint8_t *archive = NULL;
+
+/** Size of the archive (0 if unknown). */
+static uint32_t archive_len = 0;
+
+/**
+ * Parse an N-character hexadecimal ASCII string to uint32_t.
+ *
+ * @param s Pointer to hex string.
+ * @param n Number of characters to parse.
+ * @return Parsed value.
+ */
+static uint32_t parse_hex(const char *s, int n) {
+    uint32_t val = 0;
+    for (int i = 0; i < n; i++) {
+        char c = s[i];
+        uint32_t digit;
+        if (c >= '0' && c <= '9') {
+            digit = (uint32_t)(c - '0');
+        } else if (c >= 'a' && c <= 'f') {
+            digit = (uint32_t)(c - 'a' + 10);
+        } else if (c >= 'A' && c <= 'F') {
+            digit = (uint32_t)(c - 'A' + 10);
+        } else {
+            break;
+        }
+        val = (val << 4) | digit;
+    }
+    return val;
+}
+
+/**
+ * Round up to 4-byte boundary.
+ */
+static inline uint32_t align4(uint32_t v) {
+    return (v + 3) & ~3u;
+}
+
+/**
+ * Parse a CPIO entry at the given offset.
+ *
+ * @param offset  Byte offset into the archive.
+ * @param entry   Output entry information.
+ * @return Offset of the next entry, or 0 on error/end.
+ */
+static uint32_t parse_entry(uint32_t offset, cpio_entry_t *entry) {
+    if (!archive) return 0;
+
+    const cpio_newc_header_t *hdr = (const cpio_newc_header_t *)(archive + offset);
+
+    /* Verify magic */
+    if (memcmp(hdr->magic, CPIO_MAGIC, 6) != 0) {
+        return 0;
+    }
+
+    uint32_t namesize = parse_hex(hdr->namesize, 8);
+    uint32_t filesize = parse_hex(hdr->filesize, 8);
+    uint32_t mode = parse_hex(hdr->mode, 8);
+
+    /* Filename starts right after the header */
+    const char *name = (const char *)(archive + offset + CPIO_HEADER_SIZE);
+
+    /* Data starts after header + name, aligned to 4 bytes */
+    uint32_t data_offset = align4(offset + CPIO_HEADER_SIZE + namesize);
+    const void *data = archive + data_offset;
+
+    /* Next entry starts after data, aligned to 4 bytes */
+    uint32_t next_offset = align4(data_offset + filesize);
+
+    entry->name = name;
+    entry->namesize = namesize;
+    entry->data = data;
+    entry->datasize = filesize;
+    entry->mode = mode;
+
+    return next_offset;
+}
+
+void cpio_init(const void *archive_start, uint32_t archive_size) {
+    archive = (const uint8_t *)archive_start;
+    archive_len = archive_size;
+
+    offset_print("  CPIO: archive at ");
+    print_hex((uint32_t)archive_start);
+    offset_print("  CPIO: size = ");
+    print_hex(archive_size);
+
+    /* Count and list entries */
+    uint32_t count = 0;
+    uint32_t off = 0;
+    cpio_entry_t entry;
+    while (1) {
+        uint32_t next = parse_entry(off, &entry);
+        if (next == 0) break;
+        if (strcmp(entry.name, CPIO_TRAILER) == 0) break;
+
+        offset_print("  CPIO: [");
+        offset_print(entry.name);
+        offset_print("] size=");
+        print_hex(entry.datasize);
+
+        count++;
+        off = next;
+    }
+
+    offset_print("  CPIO: ");
+    print_hex(count);
+    offset_print("  CPIO: files found\n");
+}
+
+int cpio_find(const char *name, cpio_entry_t *entry) {
+    if (!archive) return -1;
+
+    uint32_t off = 0;
+    while (1) {
+        uint32_t next = parse_entry(off, entry);
+        if (next == 0) return -1;
+        if (strcmp(entry->name, CPIO_TRAILER) == 0) return -1;
+
+        /* Match by name. CPIO entries often have "./" prefix, try both. */
+        if (strcmp(entry->name, name) == 0) return 0;
+
+        /* Try matching without "./" prefix */
+        if (entry->name[0] == '.' && entry->name[1] == '/' &&
+            strcmp(entry->name + 2, name) == 0) {
+            return 0;
+        }
+
+        /* Try matching with "./" prefix */
+        if (name[0] != '.' && entry->namesize > 2) {
+            /* Already handled above */
+        }
+
+        off = next;
+    }
+}
+
+int cpio_next(uint32_t *offset, cpio_entry_t *entry) {
+    if (!archive) return -1;
+
+    uint32_t next = parse_entry(*offset, entry);
+    if (next == 0) return -1;
+    if (strcmp(entry->name, CPIO_TRAILER) == 0) return -1;
+
+    *offset = next;
+    return 0;
+}
+
+uint32_t cpio_count(void) {
+    if (!archive) return 0;
+
+    uint32_t count = 0;
+    uint32_t off = 0;
+    cpio_entry_t entry;
+    while (1) {
+        uint32_t next = parse_entry(off, &entry);
+        if (next == 0) break;
+        if (strcmp(entry.name, CPIO_TRAILER) == 0) break;
+        count++;
+        off = next;
+    }
+    return count;
+}

src/cpio.h

@@ -0,0 +1,92 @@
+/**
+ * @file cpio.h
+ * @brief CPIO newc archive parser.
+ *
+ * Parses CPIO archives in the SVR4/newc format (magic "070701").
+ * Used to read files from the initial ramdisk loaded by GRUB.
+ */
+
+#ifndef CPIO_H
+#define CPIO_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+/**
+ * CPIO newc header (110 bytes).
+ * All fields are 8-character hexadecimal ASCII strings.
+ */
+typedef struct cpio_newc_header {
+    char magic[6];       /**< Must be "070701". */
+    char ino[8];
+    char mode[8];
+    char uid[8];
+    char gid[8];
+    char nlink[8];
+    char mtime[8];
+    char filesize[8];
+    char devmajor[8];
+    char devminor[8];
+    char rdevmajor[8];
+    char rdevminor[8];
+    char namesize[8];
+    char check[8];
+} cpio_newc_header_t;
+
+/** Size of the CPIO newc header in bytes. */
+#define CPIO_HEADER_SIZE 110
+
+/** CPIO newc magic string. */
+#define CPIO_MAGIC "070701"
+
+/** Trailer entry name that marks end of archive. */
+#define CPIO_TRAILER "TRAILER!!!"
+
+/**
+ * CPIO file entry (result of iteration or lookup).
+ */
+typedef struct cpio_entry {
+    const char *name;    /**< Filename (pointer into archive). */
+    uint32_t namesize;   /**< Length of filename including NUL. */
+    const void *data;    /**< Pointer to file data within archive. */
+    uint32_t datasize;   /**< Size of file data in bytes. */
+    uint32_t mode;       /**< File mode/permissions. */
+} cpio_entry_t;
+
+/**
+ * Initialize the CPIO parser with the archive location.
+ *
+ * @param archive_start Pointer to the start of the CPIO archive in memory.
+ * @param archive_size  Size of the archive in bytes (0 if unknown).
+ */
+void cpio_init(const void *archive_start, uint32_t archive_size);
+
+/**
+ * Find a file in the CPIO archive by name.
+ *
+ * @param name    Filename to search for (without leading "./").
+ * @param entry   Output: filled with file information if found.
+ * @return 0 on success, -1 if not found.
+ */
+int cpio_find(const char *name, cpio_entry_t *entry);
+
+/**
+ * Iterate through all entries in the CPIO archive.
+ *
+ * Call with *offset = 0 to start. Returns 0 on success, -1 when no
+ * more entries exist or the TRAILER is reached.
+ *
+ * @param offset  In/out: current position in the archive.
+ * @param entry   Output: filled with the next entry's information.
+ * @return 0 on success, -1 at end of archive.
+ */
+int cpio_next(uint32_t *offset, cpio_entry_t *entry);
+
+/**
+ * Get the number of files in the CPIO archive (excluding TRAILER).
+ *
+ * @return Number of files.
+ */
+uint32_t cpio_count(void);
+
+#endif /* CPIO_H */

src/initrd_fs.c

@@ -0,0 +1,123 @@
+/**
+ * @file initrd_fs.c
+ * @brief CPIO initial ramdisk VFS driver implementation.
+ *
+ * Provides a read-only VFS interface to the CPIO archive loaded at boot.
+ * Files are accessed directly from the archive memory (zero-copy reads).
+ */
+
+#include "initrd_fs.h"
+#include "vfs.h"
+#include "cpio.h"
+#include <string.h>
+
+/* Debug print helpers defined in kernel.c */
+extern void offset_print(const char *str);
+extern void print_hex(uint32_t val);
+
+/**
+ * Read from a file in the initrd.
+ * Data is read directly from the CPIO archive (memory-mapped).
+ */
+static int32_t initrd_read(vfs_node_t *node, uint32_t offset,
+                           uint32_t size, void *buf) {
+    if (!node || !node->fs_data || !buf) return -1;
+
+    /* fs_data points to the file's data within the CPIO archive */
+    const uint8_t *data = (const uint8_t *)node->fs_data;
+    uint32_t file_size = node->size;
+
+    if (offset >= file_size) return 0;
+
+    uint32_t remaining = file_size - offset;
+    if (size > remaining) size = remaining;
+
+    memcpy(buf, data + offset, size);
+    return (int32_t)size;
+}
+
+/**
+ * Read a directory entry from the initrd root.
+ * The initrd is a flat archive — all files are at the root level.
+ */
+static int initrd_readdir(vfs_node_t *dir, uint32_t idx, vfs_dirent_t *out) {
+    (void)dir;
+
+    uint32_t off = 0;
+    uint32_t current = 0;
+    cpio_entry_t entry;
+
+    while (cpio_next(&off, &entry) == 0) {
+        /* Skip the "." directory entry if present */
+        if (entry.name[0] == '.' && entry.name[1] == '\0') continue;
+
+        /* Strip "./" prefix */
+        const char *name = entry.name;
+        if (name[0] == '.' && name[1] == '/') name += 2;
+
+        /* Skip empty names */
+        if (*name == '\0') continue;
+
+        if (current == idx) {
+            memset(out, 0, sizeof(vfs_dirent_t));
+            strncpy(out->name, name, VFS_MAX_NAME - 1);
+            out->inode = current;
+            out->type = VFS_FILE;
+            return 0;
+        }
+        current++;
+    }
+
+    return -1; /* No more entries */
+}
+
+/**
+ * Find a file by name within the initrd.
+ */
+static int initrd_finddir(vfs_node_t *dir, const char *name, vfs_node_t *out) {
+    (void)dir;
+
+    cpio_entry_t entry;
+    if (cpio_find(name, &entry) != 0) {
+        return -1;
+    }
+
+    memset(out, 0, sizeof(vfs_node_t));
+    /* Strip "./" prefix for the node name */
+    const char *display_name = entry.name;
+    if (display_name[0] == '.' && display_name[1] == '/') {
+        display_name += 2;
+    }
+    strncpy(out->name, display_name, VFS_MAX_NAME - 1);
+    out->type = VFS_FILE;
+    out->size = entry.datasize;
+    out->mode = entry.mode;
+    out->fs_data = (void *)entry.data; /* Direct pointer into CPIO archive */
+
+    return 0;
+}
+
+/** Filesystem operations for the initrd. */
+static vfs_fs_ops_t initrd_ops = {
+    .open    = NULL,  /* No special open needed */
+    .close   = NULL,  /* No special close needed */
+    .read    = initrd_read,
+    .write   = NULL,  /* Read-only */
+    .readdir = initrd_readdir,
+    .finddir = initrd_finddir,
+};
+
+int init_initrd_fs(void) {
+    if (cpio_count() == 0) {
+        offset_print("  INITRD_FS: no files in ramdisk\n");
+    }
+
+    int ret = vfs_mount("/initrd", &initrd_ops, NULL);
+    if (ret != 0) {
+        offset_print("  INITRD_FS: failed to mount\n");
+        return -1;
+    }
+
+    offset_print("  INITRD_FS: mounted at /initrd\n");
+    return 0;
+}

src/initrd_fs.h

@@ -0,0 +1,20 @@
+/**
+ * @file initrd_fs.h
+ * @brief CPIO initial ramdisk VFS driver.
+ *
+ * Provides a read-only filesystem backed by the CPIO initial ramdisk.
+ * Mounted at "/initrd" to expose the contents of the ramdisk via the VFS.
+ */
+
+#ifndef INITRD_FS_H
+#define INITRD_FS_H
+
+/**
+ * Initialize the initrd filesystem driver and mount it at "/initrd".
+ * Must be called after init_vfs() and cpio_init().
+ *
+ * @return 0 on success, -1 on failure.
+ */
+int init_initrd_fs(void);
+
+#endif /* INITRD_FS_H */

src/kernel.c

@@ -13,6 +13,9 @@
 #include "tss.h"
 #include "syscall.h"
 #include "process.h"
+#include "cpio.h"
+#include "vfs.h"
+#include "initrd_fs.h"
 
 void offset_print(const char *str)
 {
@@ -54,6 +57,26 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     init_pmm(addr);
     offset_print("PMM initialized\n");
 
+    /* Scan Multiboot2 tags for the initrd module */
+    uint32_t initrd_start = 0, initrd_end = 0;
+    {
+        struct multiboot_tag *tag;
+        for (tag = (struct multiboot_tag *)(addr + 8);
+             tag->type != MULTIBOOT_TAG_TYPE_END;
+             tag = (struct multiboot_tag *)((uint8_t *)tag + ((tag->size + 7) & ~7u))) {
+            if (tag->type == MULTIBOOT_TAG_TYPE_MODULE) {
+                struct multiboot_tag_module *mod = (struct multiboot_tag_module *)tag;
+                initrd_start = mod->mod_start;
+                initrd_end = mod->mod_end;
+                offset_print("Initrd module at ");
+                print_hex(initrd_start);
+                offset_print(" to ");
+                print_hex(initrd_end);
+                break; /* Use first module */
+            }
+        }
+    }
+
     init_paging();
     offset_print("Paging initialized\n");
 
@@ -72,6 +95,35 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     init_kmalloc();
     offset_print("Memory allocator initialized\n");
 
+    /* Initialize CPIO ramdisk if module was loaded */
+    if (initrd_start != 0) {
+        cpio_init((const void *)initrd_start, initrd_end - initrd_start);
+        offset_print("CPIO ramdisk initialized\n");
+    } else {
+        offset_print("No initrd module found\n");
+    }
+
+    init_vfs();
+    offset_print("VFS initialized\n");
+
+    if (initrd_start != 0) {
+        init_initrd_fs();
+        offset_print("Initrd filesystem mounted\n");
+
+        /* Test VFS: read a file from the initrd */
+        int fd = vfs_open("/initrd/README", 0);
+        if (fd >= 0) {
+            char buf[64];
+            int32_t n = vfs_read(fd, buf, sizeof(buf) - 1);
+            if (n > 0) {
+                buf[n] = '\0';
+                offset_print("VFS read /initrd/README: ");
+                offset_print(buf);
+            }
+            vfs_close(fd);
+        }
+    }
+
     init_tss();
     offset_print("TSS initialized\n");
 

src/vfs.c

@@ -0,0 +1,295 @@
+/**
+ * @file vfs.c
+ * @brief Virtual Filesystem (VFS) subsystem implementation.
+ *
+ * Manages mount points and routes file operations to the appropriate
+ * filesystem driver. Path resolution walks the mount table to find the
+ * longest-matching mount point, then delegates to that fs's operations.
+ */
+
+#include "vfs.h"
+#include <string.h>
+
+/* Debug print helpers defined in kernel.c */
+extern void offset_print(const char *str);
+extern void print_hex(uint32_t val);
+
+/** Mount table. */
+static vfs_mount_t mounts[VFS_MAX_MOUNTS];
+
+/** Open file descriptor table. */
+static vfs_fd_t fd_table[VFS_MAX_OPEN_FILES];
+
+/**
+ * Find the mount point that best matches a given path.
+ * Returns the index of the longest matching mount, or -1 if none.
+ *
+ * @param path     The absolute path to resolve.
+ * @param rel_path Output: pointer within `path` past the mount prefix.
+ * @return Mount index, or -1.
+ */
+static int find_mount(const char *path, const char **rel_path) {
+    int best = -1;
+    uint32_t best_len = 0;
+
+    for (int i = 0; i < VFS_MAX_MOUNTS; i++) {
+        if (!mounts[i].active) continue;
+
+        uint32_t mlen = strlen(mounts[i].path);
+
+        /* Check if path starts with this mount point */
+        if (strncmp(path, mounts[i].path, mlen) != 0) continue;
+
+        /* Must match at a directory boundary */
+        if (mlen > 1 && path[mlen] != '\0' && path[mlen] != '/') continue;
+
+        if (mlen > best_len) {
+            best = i;
+            best_len = mlen;
+        }
+    }
+
+    if (best >= 0 && rel_path) {
+        const char *r = path + best_len;
+        /* Skip leading slash in relative path */
+        while (*r == '/') r++;
+        *rel_path = r;
+    }
+
+    return best;
+}
+
+/**
+ * Resolve a path to a VFS node by finding the appropriate mount
+ * and asking the filesystem driver.
+ *
+ * @param path Full absolute path.
+ * @param out  Output node.
+ * @return 0 on success, -1 on failure.
+ */
+static int resolve_path(const char *path, vfs_node_t *out) {
+    const char *rel_path = NULL;
+    int mount_idx = find_mount(path, &rel_path);
+
+    if (mount_idx < 0) {
+        return -1;
+    }
+
+    vfs_mount_t *mnt = &mounts[mount_idx];
+
+    /* If rel_path is empty, we're looking at the mount root */
+    if (*rel_path == '\0') {
+        /* Return a directory node for the mount root */
+        memset(out, 0, sizeof(vfs_node_t));
+        strncpy(out->name, mnt->path, VFS_MAX_NAME - 1);
+        out->type = VFS_DIRECTORY;
+        out->ops = mnt->ops;
+        out->fs_data = mnt->fs_data;
+        out->mount_idx = mount_idx;
+        return 0;
+    }
+
+    /* Walk path components through the filesystem */
+    vfs_node_t current;
+    memset(&current, 0, sizeof(vfs_node_t));
+    current.type = VFS_DIRECTORY;
+    current.ops = mnt->ops;
+    current.fs_data = mnt->fs_data;
+    current.mount_idx = mount_idx;
+
+    /* Parse path components */
+    char component[VFS_MAX_NAME];
+    const char *p = rel_path;
+
+    while (*p) {
+        /* Skip slashes */
+        while (*p == '/') p++;
+        if (*p == '\0') break;
+
+        /* Extract component */
+        const char *end = p;
+        while (*end && *end != '/') end++;
+
+        uint32_t clen = (uint32_t)(end - p);
+        if (clen >= VFS_MAX_NAME) clen = VFS_MAX_NAME - 1;
+        memcpy(component, p, clen);
+        component[clen] = '\0';
+
+        /* Look up this component in the current directory */
+        if (!current.ops || !current.ops->finddir) {
+            return -1;
+        }
+
+        vfs_node_t child;
+        if (current.ops->finddir(&current, component, &child) != 0) {
+            return -1;
+        }
+
+        child.ops = mnt->ops;
+        /* Preserve fs_data set by finddir; only use mount fs_data if not set */
+        if (!child.fs_data) {
+            child.fs_data = mnt->fs_data;
+        }
+        child.mount_idx = mount_idx;
+        current = child;
+        p = end;
+    }
+
+    *out = current;
+    return 0;
+}
+
+/**
+ * Find a free file descriptor slot.
+ * @return Index (>= 0), or -1 if all slots are used.
+ */
+static int alloc_fd(void) {
+    /* Skip fds 0,1,2 (stdin, stdout, stderr) for user processes */
+    for (int i = 3; i < VFS_MAX_OPEN_FILES; i++) {
+        if (!fd_table[i].active) {
+            return i;
+        }
+    }
+    return -1;
+}
+
+void init_vfs(void) {
+    memset(mounts, 0, sizeof(mounts));
+    memset(fd_table, 0, sizeof(fd_table));
+    offset_print("  VFS: initialized\n");
+}
+
+int vfs_mount(const char *path, vfs_fs_ops_t *ops, void *fs_data) {
+    /* Find a free mount slot */
+    int slot = -1;
+    for (int i = 0; i < VFS_MAX_MOUNTS; i++) {
+        if (!mounts[i].active) {
+            slot = i;
+            break;
+        }
+    }
+
+    if (slot < 0) {
+        offset_print("  VFS: no free mount slots\n");
+        return -1;
+    }
+
+    strncpy(mounts[slot].path, path, VFS_MAX_PATH - 1);
+    mounts[slot].ops = ops;
+    mounts[slot].fs_data = fs_data;
+    mounts[slot].active = 1;
+
+    offset_print("  VFS: mounted at ");
+    offset_print(path);
+    offset_print("\n");
+
+    return 0;
+}
+
+int vfs_open(const char *path, uint32_t flags) {
+    vfs_node_t node;
+    if (resolve_path(path, &node) != 0) {
+        return -1;
+    }
+
+    int fd = alloc_fd();
+    if (fd < 0) {
+        return -1;
+    }
+
+    /* Call filesystem's open if available */
+    if (node.ops && node.ops->open) {
+        if (node.ops->open(&node, flags) != 0) {
+            return -1;
+        }
+    }
+
+    fd_table[fd].node = node;
+    fd_table[fd].offset = 0;
+    fd_table[fd].flags = flags;
+    fd_table[fd].active = 1;
+
+    return fd;
+}
+
+void vfs_close(int fd) {
+    if (fd < 0 || fd >= VFS_MAX_OPEN_FILES) return;
+    if (!fd_table[fd].active) return;
+
+    vfs_fd_t *f = &fd_table[fd];
+    if (f->node.ops && f->node.ops->close) {
+        f->node.ops->close(&f->node);
+    }
+
+    f->active = 0;
+}
+
+int32_t vfs_read(int fd, void *buf, uint32_t size) {
+    if (fd < 0 || fd >= VFS_MAX_OPEN_FILES) return -1;
+    if (!fd_table[fd].active) return -1;
+
+    vfs_fd_t *f = &fd_table[fd];
+    if (!f->node.ops || !f->node.ops->read) return -1;
+
+    int32_t bytes = f->node.ops->read(&f->node, f->offset, size, buf);
+    if (bytes > 0) {
+        f->offset += (uint32_t)bytes;
+    }
+    return bytes;
+}
+
+int32_t vfs_write(int fd, const void *buf, uint32_t size) {
+    if (fd < 0 || fd >= VFS_MAX_OPEN_FILES) return -1;
+    if (!fd_table[fd].active) return -1;
+
+    vfs_fd_t *f = &fd_table[fd];
+    if (!f->node.ops || !f->node.ops->write) return -1;
+
+    int32_t bytes = f->node.ops->write(&f->node, f->offset, size, buf);
+    if (bytes > 0) {
+        f->offset += (uint32_t)bytes;
+    }
+    return bytes;
+}
+
+int32_t vfs_seek(int fd, int32_t offset, int whence) {
+    if (fd < 0 || fd >= VFS_MAX_OPEN_FILES) return -1;
+    if (!fd_table[fd].active) return -1;
+
+    vfs_fd_t *f = &fd_table[fd];
+    int32_t new_offset;
+
+    switch (whence) {
+    case VFS_SEEK_SET:
+        new_offset = offset;
+        break;
+    case VFS_SEEK_CUR:
+        new_offset = (int32_t)f->offset + offset;
+        break;
+    case VFS_SEEK_END:
+        new_offset = (int32_t)f->node.size + offset;
+        break;
+    default:
+        return -1;
+    }
+
+    if (new_offset < 0) return -1;
+    f->offset = (uint32_t)new_offset;
+    return new_offset;
+}
+
+int vfs_readdir(const char *path, uint32_t idx, vfs_dirent_t *out) {
+    vfs_node_t node;
+    if (resolve_path(path, &node) != 0) {
+        return -1;
+    }
+
+    if (node.type != VFS_DIRECTORY) return -1;
+    if (!node.ops || !node.ops->readdir) return -1;
+
+    return node.ops->readdir(&node, idx, out);
+}
+
+int vfs_stat(const char *path, vfs_node_t *out) {
+    return resolve_path(path, out);
+}

src/vfs.h

@@ -0,0 +1,197 @@
+/**
+ * @file vfs.h
+ * @brief Virtual Filesystem (VFS) subsystem.
+ *
+ * Provides a unified interface for file and directory operations across
+ * different filesystem implementations. Filesystem drivers register
+ * themselves and can be mounted at specific paths.
+ */
+
+#ifndef VFS_H
+#define VFS_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+/** Maximum number of open files across all processes. */
+#define VFS_MAX_OPEN_FILES 256
+
+/** Maximum number of mounted filesystems. */
+#define VFS_MAX_MOUNTS 16
+
+/** Maximum path length. */
+#define VFS_MAX_PATH 256
+
+/** Maximum filename length. */
+#define VFS_MAX_NAME 128
+
+/** File types. */
+#define VFS_FILE      0x01
+#define VFS_DIRECTORY 0x02
+#define VFS_CHARDEV   0x03
+#define VFS_BLOCKDEV  0x04
+#define VFS_SYMLINK   0x06
+
+/** Seek origins. */
+#define VFS_SEEK_SET 0
+#define VFS_SEEK_CUR 1
+#define VFS_SEEK_END 2
+
+/** Forward declarations. */
+struct vfs_node;
+struct vfs_dirent;
+struct vfs_fs_ops;
+
+/**
+ * Directory entry, returned by readdir.
+ */
+typedef struct vfs_dirent {
+    char name[VFS_MAX_NAME];  /**< Entry name. */
+    uint32_t inode;           /**< Inode number (fs-specific). */
+    uint8_t type;             /**< VFS_FILE, VFS_DIRECTORY, etc. */
+} vfs_dirent_t;
+
+/**
+ * VFS node representing a file or directory.
+ */
+typedef struct vfs_node {
+    char name[VFS_MAX_NAME];       /**< Node name. */
+    uint8_t type;                  /**< VFS_FILE, VFS_DIRECTORY, etc. */
+    uint32_t size;                 /**< File size in bytes. */
+    uint32_t inode;                /**< Inode number (fs-specific). */
+    uint32_t mode;                 /**< Permissions/mode. */
+
+    /** Filesystem-specific operations. */
+    struct vfs_fs_ops *ops;
+
+    /** Opaque pointer for the filesystem driver. */
+    void *fs_data;
+
+    /** Mount index (which mount this node belongs to). */
+    int mount_idx;
+} vfs_node_t;
+
+/**
+ * Filesystem operations provided by each filesystem driver.
+ */
+typedef struct vfs_fs_ops {
+    /** Open a file. Returns 0 on success. */
+    int (*open)(vfs_node_t *node, uint32_t flags);
+
+    /** Close a file. */
+    void (*close)(vfs_node_t *node);
+
+    /** Read up to `size` bytes at `offset`. Returns bytes read, or -1. */
+    int32_t (*read)(vfs_node_t *node, uint32_t offset, uint32_t size, void *buf);
+
+    /** Write up to `size` bytes at `offset`. Returns bytes written, or -1. */
+    int32_t (*write)(vfs_node_t *node, uint32_t offset, uint32_t size, const void *buf);
+
+    /** Read directory entry at index `idx`. Returns 0 on success, -1 at end. */
+    int (*readdir)(vfs_node_t *dir, uint32_t idx, vfs_dirent_t *out);
+
+    /** Look up a child by name within a directory. Returns 0 on success. */
+    int (*finddir)(vfs_node_t *dir, const char *name, vfs_node_t *out);
+} vfs_fs_ops_t;
+
+/**
+ * Mount point entry.
+ */
+typedef struct vfs_mount {
+    char path[VFS_MAX_PATH];     /**< Mount path (e.g., "/initrd"). */
+    vfs_fs_ops_t *ops;           /**< Filesystem operations. */
+    void *fs_data;               /**< Filesystem-specific data. */
+    int active;                  /**< 1 if mounted, 0 if free. */
+} vfs_mount_t;
+
+/**
+ * Open file descriptor.
+ */
+typedef struct vfs_fd {
+    vfs_node_t node;   /**< The file node. */
+    uint32_t offset;   /**< Current read/write offset. */
+    uint32_t flags;    /**< Open flags. */
+    int active;        /**< 1 if in use, 0 if free. */
+} vfs_fd_t;
+
+/**
+ * Initialize the VFS subsystem.
+ */
+void init_vfs(void);
+
+/**
+ * Mount a filesystem at the given path.
+ *
+ * @param path    Mount point path (e.g., "/initrd").
+ * @param ops     Filesystem operations.
+ * @param fs_data Filesystem-specific data pointer.
+ * @return 0 on success, -1 on failure.
+ */
+int vfs_mount(const char *path, vfs_fs_ops_t *ops, void *fs_data);
+
+/**
+ * Open a file by path.
+ *
+ * @param path  Absolute path to the file.
+ * @param flags Open flags (currently unused).
+ * @return File descriptor (>= 0), or -1 on failure.
+ */
+int vfs_open(const char *path, uint32_t flags);
+
+/**
+ * Close an open file descriptor.
+ *
+ * @param fd File descriptor.
+ */
+void vfs_close(int fd);
+
+/**
+ * Read from an open file.
+ *
+ * @param fd   File descriptor.
+ * @param buf  Buffer to read into.
+ * @param size Maximum bytes to read.
+ * @return Bytes read, or -1 on error.
+ */
+int32_t vfs_read(int fd, void *buf, uint32_t size);
+
+/**
+ * Write to an open file.
+ *
+ * @param fd   File descriptor.
+ * @param buf  Buffer to write from.
+ * @param size Bytes to write.
+ * @return Bytes written, or -1 on error.
+ */
+int32_t vfs_write(int fd, const void *buf, uint32_t size);
+
+/**
+ * Seek within an open file.
+ *
+ * @param fd     File descriptor.
+ * @param offset Offset to seek to.
+ * @param whence VFS_SEEK_SET, VFS_SEEK_CUR, or VFS_SEEK_END.
+ * @return New offset, or -1 on error.
+ */
+int32_t vfs_seek(int fd, int32_t offset, int whence);
+
+/**
+ * Read a directory entry.
+ *
+ * @param path  Path to the directory.
+ * @param idx   Entry index (0-based).
+ * @param out   Output directory entry.
+ * @return 0 on success, -1 at end or on error.
+ */
+int vfs_readdir(const char *path, uint32_t idx, vfs_dirent_t *out);
+
+/**
+ * Stat a file (get its node info).
+ *
+ * @param path Path to the file.
+ * @param out  Output node.
+ * @return 0 on success, -1 on failure.
+ */
+int vfs_stat(const char *path, vfs_node_t *out);
+
+#endif /* VFS_H */