Arabu-Thamizh or Arwi

The spread of Islam to different parts of the world led to attempts in using the Arabic script for writing of vernacular languages. This led to the development of ingenious orthographic styles based on the Arabic script in many parts of the world, some of which survive to this day and some of which are extinct. There is a long list of languages from Europe to south-east Asia written using such scripts. Farsi, Urdu, Kurdish, Pashto and Uighur are some of the most important non-Semitic languages of today that use the Arabic letters.

Arabu-Thamizh or Arwi is Tamil, written using the Arabic script and is also distinguished by the use of many Arabic words instead of their Tamil equivalents. Arwi found use amongst the Tamil Muslims of Tamil Nadu and Kerala and the Moors of Sri Lanka. The use of Arwi among the Kerala Muslims has almost disappeared nowadays, however it is still used in Tamil Nadu and Sri Lanka. There has been a shift from Arabu-Thamizhu to Arabi-Malayalam among the Tamil Muslims of Kerala who nowadays, use the latter exclusively for imparting religious education. 

The writing system of Arwi  is based on the abjad style as any other Arabic influenced writing system and is written from right to left in cursive. Almost all the languages written using the Arabic script is based on the Perso-Arabic script with the notable exceptions of  Jawi influenced scripts in the South-east and Arwi. 

In most of Perso-Arabic based languages, the voiceless bilabial stop /p/ is represented by making modifications to the Arabic letter 'ب'  . In Urdu, Farsi, Arabi- Malayalam etc. the letter 'پ' represents /p/.

However in Arwi and Jawi,  /p/ is indicated by  ڣ, by adding a diacrirtc below the letter 'ف'  of the Arabic script ( /f/ -voiceless labiodental fricative). It can be seen that in Hebrew which is a Semitic language, a similar practice is followed, where the letter פּ (/p/ ) without the diacritic (dot) represents /f/. This may be an indication that that Arwi developed independently without influences from the Perso-Arabic script.

Letters unique to Arwi (Wikipedia)