Even if you write your own VC4 boot code, like a lot of these processors (including the Allwinner!) there's an onboard boot ROM which is the first thing which e

Even if you write your own VC4 boot code, like a lot of these processors (including the Allwinner!) there's an onboard boot ROM which is the first thing which executes. On the Pi the boot ROM, which runs on the VC4, has enough intelligence to mount the SD card and load a VC4 executable from there into the internal SRAM. My understanding is that this initialises the SDRAM, and then loads another VC4 executable, which contains the RTOS which runs alongside Linux. Once that's up and running it initialises the ARM processor, loads the ARM kernel, and starts that going. But you'll never get away from proprietary blobs.