From fd708d30ecda1ab46a83cb3af4d0915f88948c5d Mon Sep 17 00:00:00 2001 From: Brad Smith Date: Fri, 26 Feb 2016 17:11:11 -0500 Subject: [PATCH] draft of cc65-intern document --- doc/cc65-intern.sgml | 138 +++++++++++++++++++++++++++++++++++++++++++ doc/index.sgml | 3 + 2 files changed, 141 insertions(+) create mode 100644 doc/cc65-intern.sgml diff --git a/doc/cc65-intern.sgml b/doc/cc65-intern.sgml new file mode 100644 index 000000000..f3aef939a --- /dev/null +++ b/doc/cc65-intern.sgml @@ -0,0 +1,138 @@ + + +
+cc65 internals +<author><url url="mailto:brad@rainwarrior.ca" name="Brad Smith"> +<date>2016-02-27 + +<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> an 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. + +K & R style function prototypes may be used, but they do not alter the calling conventions in any way. + +<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 tyes<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/Y/ register will contain the number of bytes pushed to the stack for this function, +and the <tt/sp/ pseudo-register is a zeropage pointer to the base of the C-stack. + +Example: +<tscreen><verb> +// C prototype +void 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> + +Variadic functions push all parameters exactly as other <tt/cdecl/ convention functions, +but the value of <tt/Y/ should be used to determine how many bytes of parameters +were placed onto the stack. + +<sect1>Epilogue, after the functiona 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. +On entry to the function the <tt/Y/ register contains the number of bytes +pushed to the stack, 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> + diff --git a/doc/index.sgml b/doc/index.sgml index b6ef06ef9..5b36db6e9 100644 --- a/doc/index.sgml +++ b/doc/index.sgml @@ -58,6 +58,9 @@ <tag><htmlurl url="coding.html" name="coding.html"></tag> Contains hints on creating the most effective code with cc65. + + <tag><htmlurl url="cc65-intern.html" name="cc65-intern.html"></tag> + Describes internal details of cc65, such as calling conventions. <tag><htmlurl url="using-make.html" name="using-make.html"></tag> Build programs, using the GNU Make utility. -- 2.39.5