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<article>
<title>cc65 internals
<author><url url="mailto:bbbradsmith@users.noreply.github.com" name="Brad Smith">
<date>2016-02-27

<abstract>
Internal details of cc65 code generation,
such as calling assembly functions from C.
</abstract>

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<sect>Calling assembly functions from C<p>

<sect1>Calling conventions<p>

There are two calling conventions used in cc65:

<itemize>
  <item><tt/cdecl/ - passes all parameters on the C-stack.
  <p>
  <item><tt/fastcall/ - passes the rightmost parameter in
  registers <tt>A/X/sreg</tt> and all others on the C-stack.
  <p>
</itemize>

The default convention is <tt/fastcall/, but this can be changed with
the <tt/--all-cdecl/ command line option. If a convention is specified in
the function's declaration, that convention will be used instead.
Variadic functions will always use <tt/cdecl/ convention.

If the <tt/--standard/ command line option is used,
the <tt/cdecl/ and <tt/fastcall/ keywords will not be available.
The standard compliant variations <tt/__cdecl__/ and <tt/__fastcall__/ are always available.

If a function has a prototype, parameters are pushed to the C-stack as their respective types
(i.e. a <tt/char/ parameter will push 1 byte), but if a function has no prototype, default
promotions will apply. This means that with no prototype, <tt/char/ will be promoted
to <tt/int/ and be pushed as 2 bytes. K &amp; R style function prototypes may be used,
but they will function the same as if no prototype was used.

<sect1>Prologue, before the function call<p>

If the function is declared as fastcall, the rightmost argument will be loaded into
the <tt>A/X/sreg</tt> registers:

<itemize>
  <item><tt/A/ - 8-bit parameter, or low byte of larger types<p>
  <item><tt/X/ - 16-bit high byte, or second byte of 32-bits<p>
  <item><tt/sreg/ - Zeropage pseudo-register including high 2 bytes of 32-bit parameter<p>
</itemize>

All other parameters will be pushed to the C-stack from left to right.
The rightmost parameter will have the lowest address on the stack,
and multi-byte parameters will have their least significant byte at the lower address.

The <tt/sp/ pseudo-register is a zeropage pointer to the base of the C-stack.
If the function has no prototype or is variadic
the <tt/Y/ register will contain the number of bytes pushed to the stack for this function.

Example:
<tscreen><verb>
// C prototype
void cdecl foo(unsigned bar, unsigned char baz);

; C-stack layout within the function:
;
;            +------------------+
;            | High byte of bar |
; Offset 2 ->+------------------+
;            | Low byte of bar  |
; Offset 1 ->+------------------+
;            | baz              |
; Offset 0 ->+------------------+

; Example code for accessing bar. The variable is in A/X after this code snippet:
;
    ldy     #2      ; Offset of high byte of bar
    lda     (sp),y  ; High byte now in A
    tax             ; High byte now in X
    dey             ; Offset of low byte of bar
    lda     (sp),y  ; Low byte now in A
</verb></tscreen>

<sect1>Epilogue, after the function call<p>

<sect2>Return requirements<p>

If the function has a return value, it will appear in the <tt>A/X/sreg</tt> registers.

Functions with an 8-bit return value (<tt/char/ or <tt/unsigned char/) are expected
to promote this value to a 16-bit integer on return, and store the high byte in <tt/X/.
The compiler will depend on the promoted value in some cases (e.g. implicit conversion to <tt/int/),
and failure to return the high byte in <tt/X/ will cause unexpected errors.
This problem does not apply to the <tt/sreg/ pseudo-register, which is only
used if the return type is 32-bit.

If the function has a void return type, the compiler will not depend on the result
of <tt>A/X/sreg</tt>, so these may be clobbered by the function.

The C-stack pointer <tt/sp/ must be restored by the function to its value before the
function call prologue. It may pop all of its parameters from the C-stack
(e.g. using the <tt/runtime/ function <tt/popa/),
or it could adjust <tt/sp/ directly.
If the function has no prototype, or is variadic the <tt/Y/ register contains the
number of bytes pushed to the stack on entry, which may be added to <tt/sp/ to restore its original state.

The internal pseudo-register <tt/regbank/ must not be changed by the function.

<sect2>Clobbered state<p>

The <tt/Y/ register may be clobbered by the function.
The compiler will not depend on its state after a function call.

The <tt>A/X/sreg</tt> registers may be clobbered if any of them
are not used by the return value (see above).

Many of the internal pseudo-registers used by cc65 are available for
free use by any function called by C, and do not need to be preserved.
Note that if another C function is called from your assembly function,
it may clobber any of these itself:

<itemize>
  <item><tt>tmp1 .. tmp4</tt><p>
  <item><tt>ptr1 .. ptr4</tt><p>
  <item><tt>regsave</tt><p>
  <item><tt>sreg</tt> (if unused by return)<p>
</itemize>



</article>

