[christina_b]

This isn't the same, the document in question relates to the QPU ISA. The VPU ISA hasn't been officially documented but there were many projects that involved reverse engineering it. The VC4 ISA is documented here:

https://github.com/hermanhermitage/videocoreiv

The VPU is basically a general purpose RISC processor with some fancy vector instructions on top. In fact, most of the firmware that runs on it is written in C.

[TD-Linux]

Ah okay, this explains a lot! I somehow thought that the initial boot happened on the QPU.