how to write a letter for salary increment

Hi friends,

i am working in a software company for past half year.i was told during the HR interview that my salary will be revised after 6 months.Beside my normal day to day working i am handling some additional roles such as system administrator,network administrator and security engineer.I just want to write a formal letter to hr manager for my salary increment and remind him about his commitment.please help me by providing sample letter which will be appropriate in my case.

thanks
chayan