Procedure Call Conventions

Initialization points to the first address of frame stack after the fp register is loaded.

The address will be increased when the call instruction is executed later. When the ret instruction is executed, the fp register points to the address and falls back.

Calling Process -

Use call to call function, and fp points to the new frame.

• The pc address returned by the function is placed in [fp-1].

• The address pointed to by fp before the function call is placed in [fp-2]

• Function parameter processing: the first four input parameters are placed in the four registers r0, r1, r2, and r3. If there are more than 4 parameters, start with the fifth input parameter and descend accordingly in [fp-3] , [fp-4] , ⋯

• Local variables inside the function start at [fp], and the fp address is stored incrementally. The return value is stored in r0. If the return value is not a domain element, it needs to be returned by a memory pointer that returns the data.

Function call example:

Function prototype: foo(field a, field b, field c, field d, field e)

Input parameters：a=0x1, b=0x2, c=0x3, d=0x4, e=0x5

Function definition：

fn foo(field a, field b, field c, field d, field e) -> field {
    field res = 0;
    res = a+b;
    res = res * c;
    res = res + d + e;
    return res;
}

The fp, pc and memory states before and after the function call are as follows, where yellow represents the memory address, red represents the instruction address, and blue represents the register.