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