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Add CPIO initial ramdisk with build infrastructure and parser (AI)

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Build system changes:
- scripts/gen_initrd.sh packs all files from apps/ into a newc-format
  CPIO archive at build/isodir/boot/initrd.cpio.
- CMakeLists.txt adds 'initrd' target as ISO dependency. GRUB loads the
  archive as a Multiboot2 module via 'module2 /boot/initrd.cpio'.
- apps/README added as placeholder file for initial ramdisk content.

Kernel changes:
- kernel.c scans Multiboot2 tags for MULTIBOOT_TAG_TYPE_MODULE to find
  the initrd's physical address range, then passes it to cpio_init().
- cpio.c/h implements a parser for the SVR4/newc CPIO format:
  - cpio_init(): lists archive contents on startup
  - cpio_find(): look up a file by name (handles ./ prefix)
  - cpio_next(): iterate through all entries
  - cpio_count(): count files in archive
- The initrd lives in identity-mapped physical memory, so no additional
  mapping is needed to access it.

Verified in QEMU: GRUB loads the module at 0x0014A000, CPIO parser
finds the README file (38 bytes). All existing functionality (Ring 3
processes, syscalls) continues to work.
This commit is contained in:
AI
2026-02-23 12:16:24 +00:00
parent 71e2ae482a
commit 3d5fb4c267
9 changed files with 412 additions and 4 deletions
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README.md
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@@ -50,7 +50,7 @@ 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.
- [x] 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.
- [ ] 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.