WTF Mode ("What's The Firmware" Mode)[1] is a secondary DFU Mode-like interface used on IMG1 devices - the first-generation iPhone and iPod touch, and all Samsung-based iPods.
It appears the purpose of WTF Mode is to support uploading images, such as ramdisks, larger than are supported by the DFU Mode in VROM. The small WTF image is uploaded to the device from DFU Mode or Recovery Mode. Once WTF Mode boots, identified by its USB product identifier, the large payload can be uploaded.
WTF Mode was released in separate Recovery IPSW images for all iPods. These separate images were also sparsely released for iPhone and iPod touch for iPhone OS 1.1.3, iPhone OS 2.0 and 2.2, and iPhone OS 3.1.3. The separate recovery firmware images are required to restore all Samsung-based iPods, as these are not included in the regular IPSWs, while the purpose of the separate images for iPhone and iPod touch is unclear.
The earliest version of WTF Mode, for iPhone OS 1.0, is 9.1 KiB. Versions based on iPhone OS 2.x are around 57 KiB, and the final release, made for the final revision of iPod nano (7th generation), is 146 KiB.
Revisions
Version 1 doesn't appear to do anything significant. It calls the same functions in the VROM that the VROM calls to run the DFU. The only observable difference is that this WTF is slower. This may also allow large files to be uploaded.
Version 2 enumerates a new DFU with USB product identifier 0x1227
. This DFU does proper signature checking and allows IMG3 files to be uploaded.
- ^ According to internal binaries such as PurpleRestore