7.5.1. 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 were 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.
Refer to Stack Frame for a Function with Variable Arguments
Related Information
Stack Frame for a Function with Variable Arguments
on page 150
7.5.2. 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.
Example 5. Returned struct
/* b() computes a structure-type result and returns it */
STRUCT b(int i, int j)
{
...
return result;
}
void a(...)
{
...
value = b(i, j);
}
In the example above, if the result type is no larger than 8 bytes,
b()
returns its
result in
r2
and
r3
.
If the return type is larger than 8 bytes, the Nios II C/C++ compiler treats this
program as if
a()
had passed a pointer to
b()
. The example below shows how the
Nios II C/C++ compiler sees the code in the Returned Struct example above.
7. Application Binary Interface
NII-PRG | 2018.04.18
Nios II Processor Reference Guide
153
