reliable debug output

[Christian Widmer <cwidmer@iiic.ethz.ch>]

when debugging drivers using printk i've the problem that most often 
the msg passed to printk don't show up on the console since the machine 
crashed before. i also what to use my own ASSERT macros which halt the 
machine immediatly on an error. i could write some functions that write 
the debug string to the serial port but this is impractical since if've 
a cluster of pc and i can't connect them all to some serial lines for 
debugging. are there some functions which write directly into the graphic 
cards ram assuming that thy operate in console mode, no x-server running?
or is ther any other way to for reliable debug prints.