I did some work on this 2 years ago (back in June of 2024). I am letting myself get distracted from working on a Zynq network driver by continuing this investigation.
In a nutshell, the NAND chip on my Antminer S9 board, as I received them, contains U-boot and a linux system what can be booted when the jumpers are set for NAND boot. I want to dig into the details, both for NAND and for the SD card boot.
Tom's Software pages / tom@mmto.org