booting from usb pen drive
becker at scyld.com
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.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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