answers to the memory mapped files section of the proj2 design document

1 files changed, 18 insertions, 9 deletions

diff --git a/proj2.txt b/proj2.txt
index 8572b3f..8bbd521 100644
--- a/proj2.txt
+++ b/proj2.txt
@@ -92,7 +92,7 @@ So pages of the stack don't get tracked inside the page table at all!
 If we do not find a valid entry in our supplemental page-table it could be
 very well be a valid stack access. The maximum offset we consider a valid
 access is caused by the PUSHA instruction, which may try to access an address
-32 bytes below the current stack pointer. Everthing higher we consider
+32 bytes below the current stack pointer. Everything higher we consider
 an exception.
 
                    MEMORY MAPPED FILES
@@ -123,16 +123,23 @@ struct page_table_entry:
 >> C2: Describe how memory mapped files integrate into your virtual
 >> memory subsystem.
 
-for every page required by the memory mapped file we create a new page table
-entry
-TODO
+For every memory mapped file we create a new per process mmap table. Its entries contain
+the virtual address of the first memory mapped file, the file handler and the number
+of pages which are needed for the mapping. Additionally we store the lowest
+free index, the highest used index and the capacity of the mmap table. We then
+create for ever page required by the memory mapped file a new page table
+entry.
 
 >> C3: Explain how you determine whether a new file mapping overlaps
 >> any existing segment.
 
-in mmap() we simply check if the required pages are still free (no entry in
-the page table).
-TODO
+We avoid overlaps of file mappings by determining in the systemcall which tries
+to map a file if the required number of pages for the mapping are still free.
+To obtain this information we lookup if there are entries in the page table
+that imply that the memory space needed for the mapping is occupied by other
+pages. Furthermore we must check if any page not in the page table (for
+instance loaded pages on the stack) occupies the space we want to map our file
+to.
 
 ---- RATIONALE ----
 
@@ -143,8 +150,10 @@ TODO
 >> implementation either does or does not share much of the code for
 >> the two situations.
 
-we share the same page table entries
-TODO
+As our implementation of the supplemental page table does not distinguish
+pages that came from memory mapped files and pages from executables, both the
+supplemental page table as the memory mapped file table can share the page
+table routines.
 
                      SURVEY QUESTIONS
                      ================