Pi 4 boot from USB

Just for fun, I tried to boot RC16 from one of the Pi 4 on-board USB 3.0 ports ….
…. and it worked.

Having updated the EEPROM to July 16 2020, I took the SD card out of its usual socket and put it in an adapter plugged into the RPi USB 3.0. (The Pi 4 SD card socket is empty.)

The boot took a little longer but worked.

I have the filesystem configured as SCSI Drive 4 with !Boot etc on an mSATA drive connected to a hub on the USB-C port ….
…. but how did the kernel read the CMOS file on the Pi USB 3.0 to know what my configuration is?

The Pi bootloader is responsible for loading CMOS.

Interesting, Thank You.

I’m confused…so do the onboard USB 3.0 ports work now ? In the RC16 there’s still the note that they wouldn’t ? After boot, can you use the ports with usual USB sticks ?

so do the onboard USB 3.0 ports work now ?

In RISCOS, No.

After boot, can you use the ports with usual USB sticks ?

No.

But before RISCOS boots, the latest Pi bootloader (16th July) can use them.

I tried this and it just falls to Supervisor prompt. I changed boot FS to SCSI but still just lands on Supervisor.

I am attempting on a Pi400, any pointers to a solution would be nice :)

Use the Risc OS 5.28 image, not RC16.

I tried this and it just falls to Supervisor prompt. I changed boot FS to SCSI but still just lands on Supervisor.

By default CMOS is only saved to SDFS, a card in the slot that is. If the firmware is on a boot USB device then CMOS needs to be saved explicitly as described here.

Perhaps we should have a CMOS-USB file included in the standard distro with a note in the readme (aimed at new users) that it should be copied over the original CMOS file for machines booting from USB. Or (dare I say it) for RISC OS to boot from USB if the OS was loaded from there and from SDcard if the OS was loaded from there but only if the boot file system defined in CMOS is not present. Can it offer the user the option ‘boot from SDcard failed, would you like to boot from USB’ as another alternative?

Didn’t know this was already covered, thanks for the help.