to install an old orthography Malayalam Unicode font which is required to read the posts below]
In Malayalam, following are the functions of Visible Virama:
- Quarter maatra(മാത്ര) vowel sounding similar to ഉ(U), അ(a) or ഇ(I); called /samvruthokaaram/. Examples: അവന് (meaning 'for him'), which is a different lexeme compared to അവൻ(meaning 'he'). That is, graphic function of representing quarter ഇ(I) vowel and phonetic representation of replacing default vowel with the quarter ഇ(I) vowel to the previous letter.
- Indicates the preceding consonant C1 is forming a conjunct with succeeding consonant C2. Example: ഉണ്ട which is same as ഉണ്ട. Here Visible Virama does not produce any sound what so ever - zero മാത്ര(maatra). That is, phonetic function of vowel remover and graphical function of joiner.
- To represent the component boundary in a composite word. Example: ദേശ്രാഗം (meaning 'raga - desh') which is different from ദേശ്രാഗം (meaningless). Another example would be ‘ദൃക്സാക്ഷി’ which should not be rendered as ‘ദൃക്സാക്ഷി’. That is graphical function of boundary seperator(non-joiner) and the phonetic function of removing default vowel from previous letter.
Unicode recognizes functionality-2 alone with visible virama.
Reference: കേരളപാണിനീയം, പീഠിക - A. R. Raja Raja Varma