fix asm_switch_context() for the x86 and x86_64 archs

[bertos.git] / bertos / emul / switch_x86_64.s

diff --git a/bertos/emul/switch_x86_64.s b/bertos/emul/switch_x86_64.s
index de0f8f65ad4315cb77974ddf67718233fed2c8de..df12bed17dffe6f3e1fc3d3a9e5ebc7ab9d7181f 100644 (file)
--- a/bertos/emul/switch_x86_64.s
+++ b/bertos/emul/switch_x86_64.s
@@ -33,7 +33,20 @@
  * \version $Id$
  * \author Bernie Innocenti <bernie@codewiz.org>
  *
- * \brief i386 context switch
+ * \brief x86_64 context switch
+ *
+ * x86_64 function call convention:
+ * --------------------------------
+ *  arguments           |  callee-saved      | extra caller-saved | return
+ * [callee-clobbered]   |                    | [callee-clobbered] |
+ * -------------------------------------------------------------------------
+ * rdi rsi rdx rcx r8-9 | rbx rbp [*] r12-15 | r10-11             | rax, rdx
+ *
+ *  [*]  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.
  */
 
 /* void asm_switch_context(void **new_sp [%rdi], void **save_sp [%rsi]) */
@@ -42,7 +55,7 @@ asm_switch_context:
        pushq   %rbp
        pushq   %rdi
        pushq   %rsi
-       pushq   %rbx
+       pushq   %rcx
        pushq   %r8
        pushq   %r9
        pushq   %r10
@@ -53,7 +66,7 @@ asm_switch_context:
        popq    %r10
        popq    %r9
        popq    %r8
-       popq    %rbx
+       popq    %rcx
        popq    %rsi
        popq    %rdi
        popq    %rbp