Raspberry Pi 400 will not boot RISC OS

[JackZielke]

I installed RISC OS but I got a black screen with a blinking cursor when I tried to boot it. I replaced 3 files in the fat16 boot drive and the OS booted right up.

Installing RISC OS on the Raspberry Pi 4

I replaced riscos.img from BCM2835Dev.5.29.zip (Click the Beta RPi ROM download link). I replaced fixup4.dat and start4.elf from the October 2020 firmware. I did not need to download the RISC OS Pi image. The image installed from PINN worked as-is once the boot files were patched up.