booting from usb pen drive

Donald Becker becker at
Mon Nov 24 17:41:04 EST 2003

On Mon, 24 Nov 2003, Mark Hahn wrote:

> > > > Depends if your mobo has a boot from usb option.
> > 
> > It's slightly more complex than that: only some (many, but not all) USB
> > memory devices are usable as boot media.
> I've seen that advertised, but it was unclear to me whether it was a 
> purely marketing feature or not.
> what does the device need to do to support booting?

It surprised me that Intel needed to list which USB memory devices were
usable as boot devices.

> > We've been doing this for years with Scyld BeoBoot: use the BIOS to load
> > both the kernel and an ramdisk '/'.  The now-standard Linux approach is
> right, but this is actually two-step, no?  that is, the bios only loads
> the bootsector and jumps to it.  your code in the bootsector (or just 
> the generic code in the kernel's boot.S) is then responsible for making 
> further bios calls for reading more than that 512B.  so if the bios 
> doesn't provide a floppy-like block driver, it wouldn't work.

Correct.  The bootloader
  - is in 16 bit mode,
  - may only use the basic BIOS entry points for reading blocks,
  - must follow rules such as periodically calling the keyboard-read loop

The key is that your bootloader must load everything the final system
might need before exiting 16 bit mode, 'cause there ain't no goin' back.

> I guess what I'm wondering is whether a bios that provides USB-booting
> does actually provide a block driver.

Yes, but it's a BIOS block driver -- it's not suitable for general
purpose use.  The functionality might be divided between polling
hardware with interrupts disabled and doing things within the
keyboard-read calls.  It might re-program the timer and PIC chips, or
use the SIM mode of the processor.

Donald Becker				becker at
Scyld Computing Corporation
914 Bay Ridge Road, Suite 220		Scyld Beowulf cluster system
Annapolis MD 21403			410-990-9993

Beowulf mailing list, Beowulf at
To change your subscription (digest mode or unsubscribe) visit

More information about the Beowulf mailing list