Emphasize importance of corner cases.

[pintos-anon] / doc / userprog.texi

diff --git a/doc/userprog.texi b/doc/userprog.texi
index 894c2f3cd99d63dbfe1813f6894793b3ef9c9395..78ef78d8bfc8d9ba3e41a5b67407f7a1390e01e7 100644 (file)
--- a/doc/userprog.texi
+++ b/doc/userprog.texi
@@ -466,7 +466,7 @@ the @file{filesys} directory, and release it afterward.  Don't forget
 that @func{process_execute} also accesses files.  @strong{For now, we
 recommend against modifying code in the @file{filesys} directory.}
 
-We have provided you a function for each system call in
+We have provided you a user-level function for each system call in
 @file{lib/user/syscall.c}.  These provide a way for user processes to
 invoke each system call from a C program.  Each of them calls an
 assembly language routine in @file{lib/user/syscall-stub.S}, which in
@@ -474,8 +474,11 @@ turn invokes the system call interrupt and returns.
 
 When you're done with this part, and forevermore, Pintos should be
 bulletproof.  Nothing that a user program can do should ever cause the
-OS to crash, halt, assert fail, or otherwise stop running.  The sole
-exception is a call to the @code{halt} system call.
+OS to crash, halt, assert fail, or otherwise stop running.  It is
+important to emphasize this point: our tests will try to break your
+system calls in many, many ways.  You need to think of all the corner
+cases and handle them.  The sole way a user program should be able to
+cause the OS to halt is by invoking the @code{halt} system call.
 
 If a system call is passed an invalid argument, acceptable options
 include returning an error value (for those calls that return a
@@ -971,3 +974,9 @@ In this example, the caller's stack pointer would be at
 The 80@var{x}86 convention for function return values is to place them
 in the @samp{EAX} register.  System calls that return a value can do
 so by modifying the @samp{eax} member of @struct{intr_frame}.
+
+You should try to avoid writing large amounts of repetitive code for
+implementing system calls.  Each system call argument, whether an
+integer or a pointer, takes up 4 bytes on the stack.  You should be able
+to take advantage of this to avoid writing much near-identical code for
+retrieving each system call's arguments from the stack.