For the past few months, I have been using my spare time reverse engineering a set top box that will not be named. I am trying to run a media server or use it as a debian desktop. It is rocking a Broadcom BCM7410, which has little to no documentation as it was custom made for the company that builds the device. Right now I have found the serial header and have been communicating with it, it has CFE installed on it, but I can not abort the startup due to the integrated watchdog. Recently, via a datasheet for a similar chip in the same family,I was able to follow the traces on the bottom of the board, and find a very well hidden EJTAG solder points in a 14 pin formation (mentioned in the Linux MIPS wiki: https://www.linux-mips.org/wiki/JTAG). I found a piece of software that is compatible with a JTAG programmer I picked up designed to de-brick routers. It does seem to support the BCM7401, again which is in the same family, but the software will reject any chip that is not defined in the software, I was looking to add my device, but it requires something known as a "mpi register base". I could only think of it as meaning the Message Passing Interface base register, but I can't find any information on it. My question is, what does it mean and where/how can I find it? Thanks!
Link to required information in code: https://github.com/zoobab/zjtag/blob/master/zjtag.c#L285
Link to device list: https://github.com/zoobab/zjtag/blob/master/zjtag.c#L304