+ /* Suppose the existence of some function that saved all
+ * registers, called call_into_lisp, then restored GP registers and
+ * returned. It would look something like this:
+
+ push ebp
+ mov ebp esp
+ pushfl
+ pushal
+ push $0
+ push $0
+ pushl {address of function to call}
+ call 0x8058db0 <call_into_lisp>
+ addl $12,%esp
+ popal
+ popfl
+ leave
+ ret
+
+ * What we do here is set up the stack that call_into_lisp would
+ * expect to see if it had been called by this code, and frob the
+ * signal context so that signal return goes directly to call_into_lisp,
+ * and when that function (and the lisp function it invoked) returns,
+ * it returns to the second half of this imaginary function which
+ * restores all registers and returns to C
+
+ * For this to work, the latter part of the imaginary function
+ * must obviously exist in reality. That would be post_signal_tramp
+ */
+
+ u32 *sp=(u32 *)*os_context_register_addr(context,reg_ESP);
+
+ /* return address for call_into_lisp: */
+ *(sp-15) = (u32)post_signal_tramp;
+ *(sp-14) = function; /* args for call_into_lisp : function*/
+ *(sp-13) = 0; /* arg array */
+ *(sp-12) = 0; /* no. args */
+ /* this order matches that used in POPAD */
+ *(sp-11)=*os_context_register_addr(context,reg_EDI);
+ *(sp-10)=*os_context_register_addr(context,reg_ESI);
+
+ *(sp-9)=*os_context_register_addr(context,reg_ESP)-8;
+ /* POPAD ignores the value of ESP: */
+ *(sp-8)=0;
+ *(sp-7)=*os_context_register_addr(context,reg_EBX);
+
+ *(sp-6)=*os_context_register_addr(context,reg_EDX);
+ *(sp-5)=*os_context_register_addr(context,reg_ECX);
+ *(sp-4)=*os_context_register_addr(context,reg_EAX);
+ *(sp-3)=*context_eflags_addr(context);
+ *(sp-2)=*os_context_register_addr(context,reg_EBP);
+ *(sp-1)=*os_context_pc_addr(context);
+
+#elif defined(LISP_FEATURE_X86_64)
+ u64 *sp=(u64 *)*os_context_register_addr(context,reg_RSP);
+ /* return address for call_into_lisp: */
+ *(sp-18) = (u64)post_signal_tramp;
+
+ *(sp-17)=*os_context_register_addr(context,reg_R15);
+ *(sp-16)=*os_context_register_addr(context,reg_R14);
+ *(sp-15)=*os_context_register_addr(context,reg_R13);
+ *(sp-14)=*os_context_register_addr(context,reg_R12);
+ *(sp-13)=*os_context_register_addr(context,reg_R11);
+ *(sp-12)=*os_context_register_addr(context,reg_R10);
+ *(sp-11)=*os_context_register_addr(context,reg_R9);
+ *(sp-10)=*os_context_register_addr(context,reg_R8);
+ *(sp-9)=*os_context_register_addr(context,reg_RDI);
+ *(sp-8)=*os_context_register_addr(context,reg_RSI);
+ /* skip RBP and RSP */
+ *(sp-7)=*os_context_register_addr(context,reg_RBX);
+ *(sp-6)=*os_context_register_addr(context,reg_RDX);
+ *(sp-5)=*os_context_register_addr(context,reg_RCX);
+ *(sp-4)=*os_context_register_addr(context,reg_RAX);
+ *(sp-3)=*context_eflags_addr(context);
+ *(sp-2)=*os_context_register_addr(context,reg_RBP);
+ *(sp-1)=*os_context_pc_addr(context);
+
+ *os_context_register_addr(context,reg_RDI) =
+ (os_context_register_t)function; /* function */
+ *os_context_register_addr(context,reg_RSI) = 0; /* arg. array */
+ *os_context_register_addr(context,reg_RDX) = 0; /* no. args */