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diff --git a/src/threads/switch.S b/src/threads/switch.S
index fe06d77..374da9e 100644
--- a/src/threads/switch.S
+++ b/src/threads/switch.S
@@ -1,9 +1,25 @@
-#include "switch.h"
+#include "threads/switch.h"
+
+#### struct thread *switch_threads (struct thread *cur, struct thread *next);
+####
+#### Switches from CUR, which must be the running thread, to NEXT,
+#### which must also be running switch_threads(), returning CUR in
+#### NEXT's context.
+####
+#### This function works by assuming that the thread we're switching
+#### into is also running switch_threads().  Thus, all it has to do is
+#### preserve a few registers on the stack, then switch stacks and
+#### restore the registers.  As part of switching stacks we record the
+#### current stack pointer in CUR's thread structure.
 
 .globl switch_threads 
 switch_threads:
 	# Save caller's register state.
-	# Note that the SVR4 ABI allows us to destroy %eax, %ecx, %edx.
+	#
+	# Note that the SVR4 ABI allows us to destroy %eax, %ecx, %edx,
+	# but requires us to preserve %ebx, %ebp, %esi, %edi.  See
+	# [SysV-ABI-386] pages 3-11 and 3-12 for details.
+	#
 	# This stack frame must match the one set up by thread_create().
 	pushl %ebx
 	pushl %ebp