Initial Upload

swtch.S

@@ -0,0 +1,29 @@
+# Context switch
+#
+#   void swtch(struct context **old, struct context *new);
+# 
+# Save the current registers on the stack, creating
+# a struct context, and save its address in *old.
+# Switch stacks to new and pop previously-saved registers.
+
+.globl swtch
+swtch:
+    movl 4(%esp), %eax
+    movl 8(%esp), %edx
+
+    # Save old callee-saved registers
+    pushl %ebp
+    pushl %ebx
+    pushl %esi
+    pushl %edi
+
+    # Switch stacks
+    movl %esp, (%eax)
+    movl %edx, %esp
+
+    # Load new callee-saved registers
+    popl %edi
+    popl %esi
+    popl %ebx
+    popl %ebp
+    ret