Dash as an i2c slave, internal register question [Solved]

I’m using i2c-tools on a small linux device to try to write data to a Dash i2c slave. On the slave, I am using the Wire.h library. My problem is that my write command requires 3 paramters: i2c bus (which i know), device address (which can be set with the begin() function ) and lastly the internal DATA REGISTER address, which leads to my question: Where can I find documentation for what this address number is?

I tried looking in the MK22FN1M0VLH12 Microprocessor datasheet and couldn’t find anything in there. Is there a function I can run on the Dash side of thing to find the address of the i2c rx_buffer?

Looks like I misunderstood how the Dash (and the wire library) dealt with i2c requests. The onReceive() handler receives the i2c request byte string, and THATS IT. Whatever you do with that byte string from there is up to you. So there really isn’t a set-in-stone i2c internal register address. Thank you Erik for the clarification.