Developer uses Claude to emulate a QIC-117 tape drive in 86Box

A retrocomputing developer used Claude to reverse-engineer tape drive firmware and add QIC-117 tape backup support to the 86Box PC emulator.

AI-drafted from 2 sources. This story was written with AI from the outside reporting listed under Sources, and checked by automated tools before it went up. The originals have the full detail.

The PC emulator 86Box can now pretend to be a QIC tape drive, the sort of thing that used to sit inside a beige tower and back up your hard disk onto a cartridge the size of a bar of soap. A developer who writes at dmitrybrant.com built the feature by getting the AI model Claude to do two jobs at once: implement an old hardware protocol, and reverse-engineer a chip nobody had bothered to document.

86Box emulates DOS and early Windows era PCs at a low level, running actual BIOS code rather than faking the results. According to the developer's write-up, no emulator had previously bothered with QIC tape drives such as the Colorado 250 or the Travan family, despite a full public specification for the protocol they use, called QIC-117. That protocol runs over the same floppy disk controller that talks to your floppy drive, or in some cases over the parallel port. The developer, who says they regularly recover data from old QIC tapes for preservation purposes, wanted a way to feed a binary tape image back into original DOS and Windows backup software running inside the emulator, and have it believe it was talking to a real drive.

Why the QIC-117 spec was not enough on its own

Getting Claude Opus 5 to implement the bare QIC-117 specification took, in the developer's words, “just three or four prompts”. The trouble started immediately afterwards. The spec leaves room for manufacturer-specific “diagnostic” commands, particularly during the phase where backup software tries to work out exactly which drive it is talking to. Every vendor uses these differently, and none of it is documented outside the silicon itself. With only the standard spec implemented, a single MS-DOS utility, HP's Colorado Backup 7, would detect and read the emulated drive. Microsoft Backup for Windows 95 refused, because it tried to push the drive into a vendor-specific diagnostic mode that nothing in the public spec describes.

Disassembling a chip instead of guessing

Rather than wire up a real Colorado drive and sniff the floppy bus, the developer took the more direct route: unsoldering the ROM chip from a physical drive, dumping it, and handing the raw bytes and a photo of the circuit board to Claude with the instruction to “see what you can do with this”. The model disassembled the firmware, running on an Intel 8051 microcontroller, and worked out the full set of custom commands along with the ways the drive deviates from the published spec on ordinary ones. The developer says Claude also wrote disassemblers for the 8051 and for the Zilog Z8, used in a couple of other drives in the collection, “as an effortless prerequisite for its work”. The results, including a FINDINGS.md document detailing every command uncovered, sit in a public repository linked from the write-up, alongside notes on tape recovery techniques the analysis makes newly possible.

What this means for tape preservation

With the diagnostic commands filled in, more DOS and Windows backup tools now detect and read the emulated drive correctly inside 86Box, and the developer went on to add support for a parallel port variant, emulating an Iomega Ditto drive that is a QIC-117 device underneath with an extra chip bolted on for EPP communication over the LPT port. The developer is careful to note this is still not the gold standard for tape archiving: a properly rigorous preservation would capture the raw flux transitions, the actual magnetic patterns on the tape, rather than the decoded bytes a drive's controller hands over. That, and the possibility of using a tool like a Greaseweazle to read those transitions directly with the newly documented commands, is flagged as future work. For now, what exists is a working emulated drive that lets decades-old backup software read tape images it was never going to see again by any other means, and a fairly unusual demonstration of an AI model being pointed at unlabelled silicon and asked to explain itself.

Sources

Image: dmitrybrant.com

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