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calling C code
calling c code
==============
Example showing how to call a simple C function.
Firstly, we look at the [vbcc ABI](http://www.ibaug.de/vbcc/doc/vbcc_3.html#ABI-6) to see how the arguments are passed:
> By default, all function arguments are passed on the stack.
So if we have a function with the prototype:
```
void
PokeBitplanePointers(unsigned short* copper,
unsigned char* bitplanes,
unsigned short interlace,
unsigned short numBitplanes,
unsigned short screenWidthBytes)
```
We need to push each argument onto the stack in the reverse order of the declaration:
```
move.l #SCREEN_WIDTH_BYTES,-(sp) ; arguments are pushed onto the stack...
move.l #SCREEN_BIT_DEPTH,-(sp) ; in reverse order.
move.l #0,-(sp) ;
pea bitplanes ; make sure you push the address...
pea copperList ; of pointers, not the value.
jsr _PokeBitplanePointers ; C adds an _ to all global symbols.
add.l #20,sp ; Need to pop the arguments from the stack.
````
And after we have made the call, make sure you unwind the arguments from the stack. As seen above.
If the function returned a scalar type up to 4 bytes, it would be returned in d0. If you want to return more complex types, it's best to make a test function call in C and check the non-optimised generated code to see how to process the return value.
The C version of PokeBitplanePointers is [here](c_code.c).
try it
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