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- ahci.rs: polling AHCI on PCI class 1/6/1, bus mastering + ABAR map,
port 0 up, IDENTIFY + READ/WRITE DMA EXT through one 4KiB DMA buffer
- fat.rs: BPB parse, whole-FAT RAM cache with write-back to both copies,
LFN read/write, dirs, multi-cluster files, FSInfo free-count sync,
space-padded dot entries ('..' = cluster 0 under root), fsck-clean
- fs.rs: /disk mount node + 24-slot ephemeral node pool; ls/cat/hexdump/
echo/mkdir/touch/rm/edit all work on disk paths unchanged
- QEMU: explicit bus assignment (disk=ide.0, ESP=ide.1); the implicit
fat:rw:esp drive was stealing AHCI port 0
- Debugged against QEMU source: header DW0 is CFL in bits 4:0 / PRDTL in
bits 31:16 (Linux libahci layout), FIS count at bytes 12-13, device at
byte 7; QEMU rejects H2D FIS flags bits 6:4, so direction is the
header W bit only
- Verified: full shell round-trip on /disk, reboot persistence (poweroff
then mdir/mtype and a fresh boot), fsck.fat clean
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