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140 lines
4.9 KiB
Plaintext
140 lines
4.9 KiB
Plaintext
<!doctype linuxdoc system>
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<article>
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<title>cc65 internals
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<author><url url="mailto:bbbradsmith@users.noreply.github.com" name="Brad Smith">
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<abstract>
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Internal details of cc65 code generation,
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such as calling assembly functions from C.
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</abstract>
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<!-- Table of contents -->
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<toc>
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<!-- Begin the document -->
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<sect>Calling assembly functions from C<p>
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<sect1>Calling conventions<p>
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There are two calling conventions used in cc65:
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<itemize>
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<item><tt/cdecl/ - passes all parameters on the C-stack.
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<p>
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<item><tt/fastcall/ - passes the rightmost parameter in
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registers <tt>A/X/sreg</tt> and all others on the C-stack.
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<p>
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</itemize>
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The default convention is <tt/fastcall/, but this can be changed with
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the <tt/--all-cdecl/ command line option. If a convention is specified in
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the function's declaration, that convention will be used instead.
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Variadic functions will always use <tt/cdecl/ convention.
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If the <tt/--standard/ command line option is used,
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the <tt/cdecl/ and <tt/fastcall/ keywords will not be available.
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The standard compliant variations <tt/__cdecl__/ and <tt/__fastcall__/ are always available.
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If a function has a prototype, parameters are pushed to the C-stack as their respective types
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(i.e. a <tt/char/ parameter will push 1 byte), but if a function has no prototype, default
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promotions will apply. This means that with no prototype, <tt/char/ will be promoted
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to <tt/int/ and be pushed as 2 bytes. K & R style function prototypes may be used,
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but they will function the same as if no prototype was used.
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<sect1>Prologue, before the function call<p>
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If the function is declared as fastcall, the rightmost argument will be loaded into
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the <tt>A/X/sreg</tt> registers:
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<itemize>
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<item><tt/A/ - 8-bit parameter, or low byte of larger types<p>
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<item><tt/X/ - 16-bit high byte, or second byte of 32-bits<p>
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<item><tt/sreg/ - Zeropage pseudo-register including high 2 bytes of 32-bit parameter<p>
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</itemize>
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All other parameters will be pushed to the C-stack from left to right.
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The rightmost parameter will have the lowest address on the stack,
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and multi-byte parameters will have their least significant byte at the lower address.
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The <tt/sp/ pseudo-register is a zeropage pointer to the base of the C-stack.
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If the function has no prototype or is variadic
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the <tt/Y/ register will contain the number of bytes pushed to the stack for this function.
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Example:
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<tscreen><verb>
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// C prototype
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void cdecl foo(unsigned bar, unsigned char baz);
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; C-stack layout within the function:
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;
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; +------------------+
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; | High byte of bar |
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; Offset 2 ->+------------------+
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; | Low byte of bar |
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; Offset 1 ->+------------------+
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; | baz |
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; Offset 0 ->+------------------+
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; Example code for accessing bar. The variable is in A/X after this code snippet:
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;
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ldy #2 ; Offset of high byte of bar
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lda (sp),y ; High byte now in A
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tax ; High byte now in X
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dey ; Offset of low byte of bar
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lda (sp),y ; Low byte now in A
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</verb></tscreen>
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<sect1>Epilogue, after the function call<p>
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<sect2>Return requirements<p>
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If the function has a return value, it will appear in the <tt>A/X/sreg</tt> registers.
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Functions with an 8-bit return value (<tt/char/ or <tt/unsigned char/) are expected
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to promote this value to a 16-bit integer on return, and store the high byte in <tt/X/.
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The compiler will depend on the promoted value in some cases (e.g. implicit conversion to <tt/int/),
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and failure to return the high byte in <tt/X/ will cause unexpected errors.
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This problem does not apply to the <tt/sreg/ pseudo-register, which is only
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used if the return type is 32-bit.
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If the function has a void return type, the compiler will not depend on the result
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of <tt>A/X/sreg</tt>, so these may be clobbered by the function.
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The C-stack pointer <tt/sp/ must be restored by the function to its value before the
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function call prologue. It may pop all of its parameters from the C-stack
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(e.g. using the <tt/runtime/ function <tt/popa/),
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or it could adjust <tt/sp/ directly.
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If the function has no prototype, or is variadic the <tt/Y/ register contains the
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number of bytes pushed to the stack on entry, which may be added to <tt/sp/ to restore its original state.
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The internal pseudo-register <tt/regbank/ must not be changed by the function.
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<sect2>Clobbered state<p>
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The <tt/Y/ register may be clobbered by the function.
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The compiler will not depend on its state after a function call.
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The <tt>A/X/sreg</tt> registers may be clobbered if any of them
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are not used by the return value (see above).
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Many of the internal pseudo-registers used by cc65 are available for
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free use by any function called by C, and do not need to be preserved.
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Note that if another C function is called from your assembly function,
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it may clobber any of these itself:
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<itemize>
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<item><tt>tmp1 .. tmp4</tt><p>
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<item><tt>ptr1 .. ptr4</tt><p>
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<item><tt>regsave</tt><p>
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<item><tt>sreg</tt> (if unused by return)<p>
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</itemize>
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</article>
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