cleaner swtch.S

1 files changed, 15 insertions, 21 deletions

diff --git a/swtch.S b/swtch.S
index 786e9ac..ada98f3 100644
--- a/swtch.S
+++ b/swtch.S
@@ -1,32 +1,26 @@
-#   void swtch(struct context *old, struct context *new);
+#   void swtch(struct context **old, struct context **new);
 #  
 # Save current register context in old
 # and then load register context from new.
 
 .globl swtch
 swtch:
-  # Save old registers
   movl 4(%esp), %eax
+  movl 8(%esp), %edx
 
-  popl 0(%eax)  # %eip
-  movl %esp, 4(%eax)
-  movl %ebx, 8(%eax)
-  movl %ecx, 12(%eax)
-  movl %edx, 16(%eax)
-  movl %esi, 20(%eax)
-  movl %edi, 24(%eax)
-  movl %ebp, 28(%eax)
+  # Save old callee-save registers
+  pushl %ebp
+  pushl %ebx
+  pushl %esi
+  pushl %edi
 
-  # Load new registers
-  movl 4(%esp), %eax  # not 8(%esp) - popped return address above
-
-  movl 28(%eax), %ebp
-  movl 24(%eax), %edi
-  movl 20(%eax), %esi
-  movl 16(%eax), %edx
-  movl 12(%eax), %ecx
-  movl 8(%eax), %ebx
-  movl 4(%eax), %esp
-  pushl 0(%eax)  # %eip
+  # Switch stacks
+  movl %esp, (%eax)
+  movl (%edx), %esp
 
+  # Load new callee-save registers
+  popl %edi
+  popl %esi
+  popl %ebx
+  popl %ebp
   ret