Date: Wed, 30 Mar 2005 11:47:18 -0500 (EST)
From: linux-os <linux-os@analogic.com>
Reply-To: linux-os@analogic.com
To: krishna <krishna.c@globaledgesoft.com>
cc: Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: How to debug kernel before there is no printk mechanism?
In-Reply-To: <424AD247.4080409@globaledgesoft.com>
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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).
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