first steps into assembler

Based on last daily, I did some more testing.
Your link to Calling_convention gave a first help.
In my best understanding reg0 to reg3 are used for getting data.
By testing this, I could get 4 values back, but where is the 5th ?

var adder = E.asm("int(int,int,int,int,int)",
                  "mov r0,rn", where n is 1 to 3
                  "bx  lr");

Another problem is JsVar, how could this be interpreted ?
I would like to play around with Uint-Arrays.
This one works, but now I want to change a value in the array.
My understanding of C is too poor to get this out of the sources.

var x,i;
x = Uint8Array(10);
for(i = 0; i < 10; i++) x[i] = i;
var adder = E.asm("JsVar(JsVar)",
                  "bx  lr");
adder(x)