Re: How to debug kernel before there is no printk mechanism?

[linux-os]

On Wed, 30 Mar 2005, Vicente Feito wrote:

> Video memory is at b800:0000, for humans 0x0000b800, not at 0x000b8000

Wrong. "real-mode" can use a segment address of b800, that doesn't
work in protected mode. A segment address of b800:0000 is never
under any conditions 0000b800.

FYI, a real-mode segment is a 16-byte entity, therefore there are
many segment:offset combinations that can get you to 0x000b8000.

> On Wednesday 30 March 2005 04:47 pm, linux-os wrote:
> > On Wed, 30 Mar 2005, krishna wrote:
> > > Hi all,
> > >
> > > How can one debug kernel before there is no printk mechanism in kernel.
> > >
> > > Regards,
> > > Krishna Chaitanya
> >
> > Write directly to screen memory at 0x000b8000, or write to the
> > RS-232C UART while polling the TX buf empty bit, or just write
> > bits that mean something to you out the printer port.
> >
> > Screen - memory is 16-bit words with the high-word being
> > an attibute byte. FYI 0x07 is a good B&W byte. You can
> > initialize a pointer to it as:
> >
> > unsigned short *screen = 0xc00b8000; Since low memory
> > is always mapped, the above cheat will work. The 0xc0000000
> > is PAGE_OFFSET.
> >
> > An early '486 was brought up into a 32-bit protected-mode
> > (non linux) operating system using these debugging methods.
> > The first time I got to see some symbol written to the
> > screen in protected-mode marked the start of a week-end-
> > long party. Have fun!
> >
> > Cheers,
> > Dick Johnson
> > Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
> >   Notice : All mail here is now cached for review by Dictator Bush.
> >                   98.36% of all statistics are fiction.
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
 Notice : All mail here is now cached for review by Dictator Bush.
                 98.36% of all statistics are fiction.
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