Update preset.

[bertos.git] / bertos / emul / switch_i386.S

diff --git a/bertos/emul/switch_i386.S b/bertos/emul/switch_i386.S
index b605c15edde69ee29af960b6f9ce9bdcf4f76fe2..f00d447c1467093ed74dedc8c5722099efe0566c 100644 (file)
--- a/bertos/emul/switch_i386.S
+++ b/bertos/emul/switch_i386.S
@@ -30,10 +30,22 @@
  * Copyright 1999, 2000, 2001 Bernie Innocenti <bernie@codewiz.org>
  * -->
  *
- * \version $Id$
  * \author Bernie Innocenti <bernie@codewiz.org>
  *
  * \brief i386 context switch
+ *
+ * x86 function calling convention:
+ * --------------------------------
+ *  arguments         | callee-saved        | extra caller-saved | return
+ * [callee-clobbered] |                     | [callee-clobbered] |
+ * -------------------------------------------------------------------------
+ * eax edx ecx        | ebx edi esi ebp [*] | <none>             | eax, edx
+ *
+ *  [*]  In the frame-pointers case rbp must hold a base address for the
+ *       current stack frame.
+ *
+ * asm_switch_context() can be considered as a normal function call, so we need
+ * to save all the callee-clobbered registers minus the return registers.
  */
 
 #ifdef __APPLE__
@@ -47,15 +59,11 @@
 .globl SWITCH_CONTEXT
 SWITCH_CONTEXT:
        pushl   %ebp
-       pushl   %edi
-       pushl   %esi
-       pushl   %ebx
-       movl    0x24(%esp),%ebp         /* ebp = save_sp */
+       pushl   %ecx
+       movl    0x10(%esp),%ebp         /* ebp = save_sp */
        movl    %esp,(%ebp)             /* *save_sp = esp */
-       movl    0x20(%esp),%ebp         /* ebp = new_sp */
+       movl    0x0c(%esp),%ebp         /* ebp = new_sp */
        movl    (%ebp),%esp             /* esp = *new_sp */
-       popl    %ebx
-       popl    %esi
-       popl    %edi
+       popl    %ecx
        popl    %ebp
        ret