kernel config sio with flag 0x80

[Cheng Jin]

Hi, all

I am having problems trying to config my 5.0-Release kernel for remote 
debugging.

from the handbook below,

http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/kernelconfig-config.html

it seems to be as simple as adding a line like

"device          sio0     at isa? port IO_COM1 flags 0x80 irq 4"

and  "options DDB" to the kernel config file, except sio0 line doesnt 
actually work.

when i tried config -g, it complains about

"devices with zero units are not likely to be correct"

I tried different combos such as leaving out device numbers with 
only flags.  I also looked around the web and FreeBSD mailinglist but 
didnt find anything helpful.  Is there a different way of enabling serial 
console for debugging?  

It also seems that the documentation on kernel config has gotten much 
worse since the 2.* days.  Where was the helpful LINT file that explained 
all the options correctly?

Thanks,

Cheng