mirror of
https://github.com/cc65/cc65.git
synced 2024-12-27 15:29:46 +00:00
a0db846a97
That lets them match old-style definitions. It avoids "Type conflict" error messages. It allows shorter function calls. Fixed the types of some variables in "test/ref/otccex.c". It avoids crashes on 64-bit Windows (32-bit Windows with 64-bit pointers).
140 lines
4.9 KiB
Plaintext
140 lines
4.9 KiB
Plaintext
<!doctype linuxdoc system>
|
|
|
|
<article>
|
|
<title>cc65 internals
|
|
<author><url url="mailto:bbbradsmith@users.noreply.github.com" name="Brad Smith">
|
|
|
|
<abstract>
|
|
Internal details of cc65 code generation,
|
|
such as calling assembly functions from C.
|
|
</abstract>
|
|
|
|
<!-- Table of contents -->
|
|
<toc>
|
|
|
|
<!-- Begin the document -->
|
|
|
|
|
|
|
|
<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 & R"-style forward declarations 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 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 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>
|