TSCII (Tamil Standard Code for Information Interchange)
TSCII is the first initiative to type English and Tamil using a single font. In TSCII English is displayed as English while Tamil will occupy the upper slots of the ASCII (I mean it from the 128-255).
So we need a software to interpret Tamil (otherwise we can't type directly you may press Alt+xxx) .This is a standard established through online discussion.
There are many software's exist among them I prefer nalinam from http://www.nalinam.com.
External Links
TSCII article in English wikipedia the free encyclopedia.
TSCII article in Tamil wikipedia the free encyclopedia.