1 files changed, 32 insertions, 0 deletions
diff --git a/swtch.S b/swtch.S
new file mode 100644
index 0000000..786e9ac
--- /dev/null
+++ b/swtch.S
@@ -0,0 +1,32 @@
+#   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
+
+  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)
+
+  # 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
+
+  ret