booting from usb pen drive

[Donald Becker]

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.com
Scyld Computing Corporation		http://www.scyld.com
914 Bay Ridge Road, Suite 220		Scyld Beowulf cluster system
Annapolis MD 21403			410-990-9993

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