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 Post subject: Mix C code and assembly code
PostPosted: Wed Oct 30, 2013 5:50 pm 
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Joined: Fri Mar 04, 2011 9:36 pm
Posts: 500
First, you could read Chapter 8 “Application Binary Interface (ABI)” in programming guide to get related information.

A C source code can be compiled to an assembly by gcc.
For example, there is a simple C code “simple.c”.
In cygwin console, I use gcc command:
$ nds32le-elf-gcc -O2 -S simple.c
Then it generate a simple.s file.
You can use gcc to view the assembly code.
(see attached example2.zip)

a. Assembly call an C function (see attached example2.zip)

Arguments will be passed in general-purpose registers (r0-r5).
For 4-byte primitive type, the return value is returned in r0.

In C code, we write:
int a=2,b=3,c=4, sum,number;
sum=simple(a,b,c);
number=sum+1;

In Assembly, we write:
movi $r2, 4 ! Third argument:c
movi $r1, 3 ! Second argument: b
movi $r0, 2 ! First argument: a
bal simple ! bal is equal to jal, call function simple
addi $r1, $r0, 1 ! return value sum is put on $r0


b. C call a assembly function (see attached example3.zip)
In C code, we write:
int simple(int x, int y, int z)
{
int result;
result=x+y+z;
return result;
}

In Assembly, we write:
simple:
add $r0, $r1, $r0
add $r0, $r0, $r2
ret

These three arguments are passing in $r0, $1, $r2.
Return value is passing in $r0.


Download example codes:
Attachment:
example2.zip [24.44 KiB]
Downloaded 271 times

Attachment:
example3.zip [33.52 KiB]
Downloaded 282 times


Note:
The following files (main.c and simple.c) in these example are excluded from build.
Attachment:
5.gif
5.gif [ 5.67 KiB | Viewed 3538 times ]


These two files (main.c and simple.c) are just for reference.

Here are the method to exclude some files:
viewtopic.php?f=23&t=746&p=877


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