Specifications

7–8 Altera Corporation
Nios II Processor Reference Handbook User Guide October 2007
Arguments and Return Values
Arguments and
Return Values
This section discusses the details of passing arguments to functions and
returning values from functions.
Arguments
The first 16-bytes to a function are passed in registers r4 through r7. The
arguments are passed as if a structure containing the types of the
arguments was constructed, and the first 16-bytes of the structure are
located in r4 through r7.
A simple example:
int function (int a, int b);
The equivalent structure representing the arguments is:
struct { int a; int b; };
The first 16-bytes of the struct are assigned to r4 through r7. Therefore
r4 is assigned the value of a and r5 the value of b.
The first 16-bytes to a function taking variable arguments are passed the
same way as a function not taking variable arguments. The called
function must clean-up the stack as necessary to support the variable
arguments. See “Stack Frame for a Function with Variable Arguments” on
page 7–5.
Return Values
Return values of types up to 8-bytes are returned in r2 and r3. For return
values greater than 8-bytes, the caller must allocate memory for the result
and must pass the address of the result memory as a hidden zero
argument.
The hidden zero argument is best explained through an example.