![I reverse-engineered a Garmin GPS firmware on a Sunday morning at 15 — cracked the undocumented .GCD format, confirmed GarminOS/OS20/ARM, extracted bootloader assets nobody had seen before [OC]](https://external-preview.redd.it/LQ3J7v_B6KGyIHcyX1XCiwLiZy1IywVowu8wW8xfW8M.png?width=1080&crop=smart&auto=webp&s=f0a75cc3e52f789d2cd3bf2f3de0c0d7002ea758)
I reverse-engineered a Garmin GPS firmware on a Sunday morning at 15 — cracked the undocumented .GCD format, confirmed GarminOS/OS20/ARM, extracted bootloader assets nobody had seen before [OC]
Started this morning with a Garmin nüvi 2455 collecting
dust in a drawer. No documentation existed for the .GCD
firmware format. Here's what I found.
**The OS:** GarminOS built on OS20 RTOS (STMicroelectronics)
on ARM. Confirmed via leaked source paths in the binary:
gpk\gps_st\garminos\os20\gps_st_os20_task.c
**The format:** 9-byte record header:
[01 00 01 00] [TYPE] [FieldA: 2B] [FieldB: 2B] [Payload]
**What's inside GUPDATE.GCD (48MB):**
- 19 records mapped
- 20 PNG bootloader assets extracted (SOFTWARE, SYSTEM,
LOADER, MISSING, LOADING... + UI icons)
- ARM Thumb code blobs confirmed via entropy analysis
- 6 compressed streams with unknown compression method
**Workflow:** Claude + Claude Code + Codex + Mistral Vibe
**Toolkit:** github.com/theodorebeaupre-prog/garmin-gcd-toolkit
I'm 15, Quebec City. Happy to answer questions.