X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=doc%2Fvm.texi;h=840e506a129586d9c2857f22a86a4325cdfafc01;hb=8abbb333aea445641d967befd3ca477502ea770b;hp=8ea6ee46b8c37b6897df3ddd365ed9428c731d1e;hpb=269b497e5a50effc8230ac3e91cf6d8b862659a0;p=pintos-anon

diff --git a/doc/vm.texi b/doc/vm.texi
index 8ea6ee4..840e506 100644
--- a/doc/vm.texi
+++ b/doc/vm.texi
@@ -318,8 +318,9 @@ useful for this purpose (@pxref{Hash Table}).
 
 It is possible to do this translation without adding a new data
 structure, by modifying the code in @file{userprog/pagedir.c}.  However,
-if you do that you'll need to carefully study and understand section 3.7
-in @bibref{IA32-v3}, and in practice it is probably easier to add a new
+if you do that you'll need to carefully study and understand section
+3.7, ``Page Translation Using 32-Bit Physical Addressing,'' in
+@bibref{IA32-v3a}, and in practice it is probably easier to add a new
 data structure.
 
 @item
@@ -483,8 +484,7 @@ allocate additional pages as necessary.
 
 Allocate additional pages only if they ``appear'' to be stack accesses.
 Devise a heuristic that attempts to distinguish stack accesses from
-other accesses.  You can retrieve the user program's current stack
-pointer from the @struct{intr_frame}'s @code{esp} member.
+other accesses.
 
 User programs are buggy if they write to the stack below the stack
 pointer, because typical real OSes may interrupt a process at any time
@@ -500,9 +500,24 @@ not be restartable in a straightforward fashion.)  Similarly, the
 @code{PUSHA} instruction pushes 32 bytes at once, so it can fault 32
 bytes below the stack pointer.
 
+You will need to be able to obtain the current value of the user
+program's stack pointer.  Within a system call or a page fault generated
+by a user program, you can retrieve it from @code{esp} member of the
+@struct{intr_frame} passed to @func{syscall_handler} or
+@func{page_fault}, respectively.  If you verify user pointers before
+accessing them (@pxref{Accessing User Memory}), these are the only cases
+you need to handle.  On the other hand, if you depend on page faults to
+detect invalid memory access, you will need to handle another case,
+where a page fault occurs in the kernel.  Reading @code{esp} out of the
+@struct{intr_frame} passed to @func{page_fault} in that case will obtain
+the kernel stack pointer, not the user stack pointer.  You will need to
+arrange another way, e.g.@: by saving @code{esp} into @struct{thread} on
+the initial transition from user to kernel mode.
+
 You may impose some absolute limit on stack size, as do most OSes.
-(Some OSes make the limit user-adjustable, e.g.@: with the
-@command{ulimit} command on many Unix systems.)
+Some OSes make the limit user-adjustable, e.g.@: with the
+@command{ulimit} command on many Unix systems.  On many GNU/Linux systems,
+the default limit is 8 MB.
 
 The first stack page need not be allocated lazily.  You can initialize
 it with the command line arguments at load time, with no need to wait
@@ -611,7 +626,7 @@ modified by the reference solution.
  17 files changed, 1532 insertions(+), 104 deletions(-)
 @end verbatim
 
-@item Do we need a working HW 2 to implement HW 3?
+@item Do we need a working Project 2 to implement Project 3?
 
 Yes.