Implement FAT12 Filesystem Reader: Parse Boot Sector and FAT

FAT12 Boot Sector Layout

FAT12 is the classic DOS floppy format: boot sector, FAT tables, root directory, then data region. The BIOS loads the boot sector; your reader parses BPB fields to locate FAT and root dir.

On-Disk Geometry

From boot sector:

• Bytes per sector: usually 512
• Sectors per cluster
• Reserved sectors, FAT count, root entry count

For example, a 1.44 MB floppy has 224 root directory slots and 9 sectors per FAT copy.

Why This Matters in Production

FAT12 is the exact filesystem GRUB's stage-1 bootloader and real 1.44MB floppy images use, and its FAT-chain design is the direct ancestor of the FAT32 still mandated for every UEFI system partition today. Get the 12-bit packing math wrong (two entries jammed into three bytes) and you silently follow a corrupted cluster chain into someone else's file data.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will parse the BPB, walk the FAT chain, and read a root directory entry. This exercise requires converting 8.3 filenames to a readable string and following 0xFFF end-of-chain markers.