I am looking for a detailed manual explaining the headers inside of assembly so I can build a function in assembly and call it from C. I saw following link (http://www.keil.com/support/docs/50.htm) but I want to understand what the headers are and choose them on purpose. Any help would be appreciated.
Did you read the Interfacing C to Assembler section in the Manual - as cited in that document?
Yes I did, I am not just looking to make this work, I am also looking to understand what the options are specifically in the head of the assembly file. I was told that there are a lot of options and customizations available in the assembly file head.
Isn't that precisely what the Manual tells you?
Exactly what is it that you want to know?
What "options" do you mean? What do you mean by the "head" of the assembly file?
If you want to know about assembler, see the Assembler Manual:
http://www.keil.com/support/man/docs/a51/
In addition to the Interfacing C to Assembler section http://www.keil.com/support/man/docs/c51/c51_ap_ctoasm.htm
You will also need to read:
Segment Naming Conventions http://www.keil.com/support/man/docs/c51/c51_ap_segname.htm
Data Storage Formats http://www.keil.com/support/man/docs/c51/c51_ap_datastorage.htm
You might also take a look at the Optimiser: http://www.keil.com/support/man/docs/c51/c51_ap_optimizer.htm