Writing asm library and run from C

Hello!
I have a some trouble.
I'm writing some library on ASM. This library contane some function takes as a parameter two doubel or float (in Cx51, this types identicly) variable. If i call this function from ASM i manualy place this variable, example, in R0 R1 R2 R3 - first var. and R4 R5 R6 R7 - second var,
but if i definate this function in C

double func(double var1, double var2)

, this mean next: on call this function first variable by compiler will be placed in R4 - R7 registers, but second variable will be placed in a place determined by the compiler. See http://www.keil.com/support/docs/2314.htm.
How i can write similar function in my library, which can be called from C source(with out more operation with vars).
Example :

double var3 = my_func(double var1,double var 2)

.
Thank for help!.