Calling convention when calling Intel 8086 Assembly routines from C?

[Fabrizio-Caruso]

I am not sure about the calling convention I am supposed to assume when calling Intel 8086 Assembly routines from C.

Is the compiler automatically preserving all or some registers except BP and DI?

Let us assume I have an Assembly routine that uses pretty much all registers.
Then which registers am I supposed to explicitly push into the stack at the beginning of my routine to restore them with a pop command at the end?

Which registers will hold the output value? AX if 16 bit? AL if 8-bit?