755CX, Linux/XFree86 800x600 mode ??

[fateh@watson.ibm.com (Fateh A. Tipu)]

Problem:
--------
I am trying to find out how to get 800x600 res on a 755CX (SVGA)
running Linux and XFree86 3.1.2.

Now the Details:
----------------
I have recently installed Linux (Slackware 3.0.0) with XFree86 3.1.2
on my Thinkpad 755CX (SVGA model). I use OS/2 Boot Manager to boot
either to OS/2 or to Linux.

X is running properly in 640x480 mode. I have note been able to get
800x600 mode on the LCD. 800x600 mode and also 1024x768 mode work fine
on an external display. This machine has WD 90C24 video chip. 

I have tried changing the XF86Config but it did not help. All I get 
on the LCD is horizontal lines on the top 3/4 of the display and a 
verticle line about an inch from the right side.

I read in a HOWTO that a command "PS2 MODE3X ON" has to be executed
in DOS to enable video mode 3 extensions and then boot Linux using
LOADLIN.EXE, to be able to use 800x600 resolution. I don't have this
setup so couldn't try it.

Any help is appreciated.

-- Tipu