A surface scanner, FAT12 consistency checker, file-recovery utility, defragmenter and raw-FDC head diagnostic for Windows 9x (95/98/Me). Licensed under the GNU GPL v3, see LICENSE. Report bugs to k@iczelia.net.

1. Standard - read-only verification. Reads every sector and reports anything that does not come back cleanly.
2. Thorough - destructive per-sector surface test: read the original, then write-and-verify the original, its inverse, a PRNG block and that block's inverse, and finally restore the original.
3. Diagnostic - probes the drive. Uses BIOS INT 13h via VWIN32; with fdchk.vxd loaded it switches to raw FDC commands and surfaces the full ST0/ST1/ST2/C/H/R/N result phase.
4. Check filesystem - walks FAT12 read-only for cross-linked clusters, lost chains, size mismatches, chain cycles and bad FAT entries.

• Recover - after a surface scan, walks the directory tree, copies out every file with zero-filled holes where bad sectors fell, and writes fdchk-recovery.log. With auto-fix on, new bad clusters are marked 0xFF7 in both FATs.
• Format - quick or full FAT12 format of a 1.44 MB disk.
• Defrag - repacks files and subdirectories into contiguous low clusters (depth-first, alphabetical at each level) and zeroes every free cluster.
• Batch mode - after each scan, prompts to insert the next disk.
• Logs - an in-app viewer for the log files in <exe-dir>\logs.

In Diagnostic mode the grid becomes a cyl x head map (160 cells):

Requires i686-w64-mingw32-gcc and nasm; mtools is needed only for make floppy.

make           fdchk.exe + fdchk.vxd
make floppy    fdchk.img - a 1.44 MB FAT12 image carrying the EXE
make debug     with-CRT build carrying symbols
make clean