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

["Randy.Dunlap" <rddunlap@osdl.org>]

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

Here's an implementation by Keith Owens:
http://kernelnewbies.org/documents/videochar.txt

The patch there is to Linux 2.4.18, but should be workable
on many versions.

> 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!


-- 
~Randy