Storing data in my Logitech MX Vertical DPI register because why not
tldr; You can store 2 bytes in the DPI register of Logitech MX Vertical
The plan was to use the mouse's built-in flash memory as a small, constant storage device. The mouse moves between computers all the time, so it could technically carry data with it. Well, technically lol
I spent a while trying to find what looked like a nonvolatile storage feature (0x1c00) that had six slots and read/write flags. It seemed like it could be useful, but it's not very practical because macOS's IOHIDManager blocks the long HID++ reports you need to actually talk to it. I guess there are other ways to avoid the IOHIDManager, but yea...
What actually worked was the DPI register. The mouse saves your DPI setting in flash, and it's happy to take any u16 values without any quantisation. I can write any 2-byte value I want, and it'll still be there when I unplug, power cycle, move it to a different computer, and so on
I know 2 bytes are nothing but was still a funny thing to do