From: Ben Pfaff <blp@cs.stanford.edu>
Date: Mon, 20 Sep 2004 19:06:58 +0000 (+0000)
Subject: Comments.
X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?p=pintos-anon;a=commitdiff_plain;h=70902182c386c446255ba0bb84e08d0b53d541de

Comments.
---

diff --git a/src/threads/mmu.h b/src/threads/mmu.h
index 2af85bc..4d4aa6a 100644
--- a/src/threads/mmu.h
+++ b/src/threads/mmu.h
@@ -93,6 +93,16 @@ static inline void *pg_round_down (const void *va) {
   return (void *) ((uintptr_t) va & ~PGMASK);
 }
 
+/* Base address of the 1:1 physical-to-virtual mapping.  Physical
+   memory is mapped starting at this virtual address.  Thus,
+   physical address 0 is accessible at PHYS_BASE, physical
+   address address 0x1234 at (uint8_t *) PHYS_BASE + 0x1234, and
+   so on.
+
+   This address also marks the end of user programs' address
+   space.  Up to this point in memory, user programs are allowed
+   to map whatever they like.  At this point and above, the
+   virtual address space belongs to the kernel. */
 #define	PHYS_BASE ((void *) LOADER_PHYS_BASE)
 
 /* Returns kernel virtual address at which physical address PADDR
diff --git a/src/userprog/exception.c b/src/userprog/exception.c
index dead766..4f3e4f9 100644
--- a/src/userprog/exception.c
+++ b/src/userprog/exception.c
@@ -101,7 +101,8 @@ kill (struct intr_frame *f)
 #define PF_U 0x4    /* 0: kernel, 1: user process. */
 
 /* Page fault handler.  This is a skeleton that must be filled in
-   to implement virtual memory.
+   to implement virtual memory.  Some solutions to project 2 may
+   also require modifying this code.
 
    At entry, the address that faulted is in CR2 (Control Register
    2) and information about the fault, formatted as described in