Root file system on Flash mounted R/O

[Vasant Kanchan <v_kanchan%yahoo.com@localhost>]

Hi,

I have a minimal file system  currently on a ZIP disk which I can boot 
from.
I would like to move this to a CF card which I would prefer to mount 
read only.
Unfortunately there are writes to /tmp /var and /root done by the apps.
One possiblity to mout ramdisks at /tmp, /var /root.
Another possibilty is to union mount the ramdisk on top of  root .
I couldn't  get this to work.
Any suggestions to handle this.
Thanks for your answers.

Kanchan