--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/swtch.S	Fri May 26 23:11:05 2017 +0900
@@ -0,0 +1,26 @@
+# Context switch
+#
+#   void swtch(struct context **old, struct context *new);
+#
+# Save current register context in old
+# and then load register context from new.
+# The stack is as r4_svc-r12_svc, lr_svc, sp_usr, lr_usr, and pc_usr
+.global swtch
+
+swtch:
+    STMFD   r13!, {r4-r12, lr}  // push svc r4-r12, lr to the stack
+
+    # switch the stack
+    STR     r13, [r0]           // save current sp to the old PCB (**old)
+    MOV     r13, r1             // load the next stack
+
+    # load the new registers. pc_usr is not restored here because
+    # LDMFD^ will switch mode if pc_usr is loaded. We just simply
+    # pop it out as pc_usr is saved on the stack, and will be loaded
+    # when we return from kernel to user space (swi or interrupt return)
+
+    LDMFD   r13!, {r4-r12, lr}  // pop svc r4-r12, lr
+
+    # return to the caller
+    bx      lr
+