1. Lecture Topics: 11/20 HW 7 What happens on a memory reference Traps
File systems

2. HW 7 Comments virtual address -> VPN
If you can’t get a miss rate below 1% then find the lowest possible miss rate (i.e. the miss rate with maximum memory)
all of the misses are compulsory misses
Add 1% to whatever this miss rate is and find the minimum memory size that yields this miss rate
For example, if the smallest possible miss rate is 2.3%, then find the minimum amount of memory that yields a miss rate of 3.3%.
Your answers don’t have to be identical to the sample solutions, but they should be close

3. Virtual Address -> VPN

4. Working Set
The working set of a process is the set of pages that it is actually using
usually much smaller than the amount of memory that is allocated
As long as a process's working set fits in memory it won't thrash
Formally: the set of pages a job has referenced in the last T seconds
How do we pick T?
too big => could run more programs
too small => thrashing

5. What happens on a memory reference?
An instruction refers to memory location X:
Is X's VPN in the TLB?
Yes: get data from cache or memory. Done.
(Often don't look in TLB if data is in the L1 cache)
Trap to OS to load X's VPN into the cache
OS: Is X's VP located in physical memory?
Yes: replace TLB entry with X's VPN. Return control to CPU, which restarts the instruction. Done.
Must load X's VP from disk
pick a page to replace, write it back to disk if dirty
load X's VP from disk into physical memory
Replace the TLB entry with X's VPN. Return control to CPU, which restarts the instruction.

6. Traps A trap is an exception generated by the CPU Traps are caused by
errors (divide by 0, invalid memory access)
system call (i.e. a user request for the OS to do something)
When a trap occurs, the CPU transfers control to the operating system
A vector stores which OS service routine to use for each trap
one routine for system calls
one routine for handling page faults
Trap number chooses the vector entry to use

7. What happens on a Trap?
CPU switches protection mode from user to kernel (aka privileged or supervisor mode)
Jumps to the specific trap handler
like an interrupt service routine
The trap handler
saves the CPU state (registers etc.)
sees what it needs to do
does what it needs to do
restores user state
returns the the user program (switches back to user mode)

8. Files
A user-level abstraction for “a collection of bytes in (non-volatile) storage”
Files have:
A name
Possibly a type
A location - which device, where on that device
Size (and possibly maximum size)
Protection - who may read and write this?
Time, date, and user identification

9. File operations
What operations should a file system support?

10. File Operations File creation Writing a file make room for the file
enter the new file into the directory
Writing a file
specify the file and the data to write to the file
OS keeps track of your location in the file
successive writes are placed one after the other in the file

11. More File Operations Reading a file Repositioning within a file
specify the file and the buffer into which the data should be read
OS keeps track of your location in the file
Location pointer is often shared between read and write operations
Repositioning within a file
Changes the location pointer
Often called “seeking”
No actual I/O

12. Yet More File Operations
Deleting a file
Find the directory entry and delete it
Mark the space used by the file as free
Don’t actually “erase” the file
Truncating a file
Throw away all the data in the file
Keep the attribute information

13. Opening and Closing Files
The above six operations are enough
But for convenience, we have the notion of the open file
The open system call tells the OS that the specified file will be used by several operations
user need not specify name each time
OS need not search directories each time
Location pointers, etc. need only be maintained for open files

14. Directories The directory lists all of the files in the volume
A volume is a logical disk - there may be more than one volume per physical disk (or more than one physical disk per volume)
We put the directory at the beginning so we always know where to start
Directory
Files
Volume

15. Single-Level Directories
In a single-level directory structure, the directory lists all files and their offsets
Like a table of contents
notes
Spring
todo
ideas
notes410
Spring00
todo
ideas

16. Two-Level Directories
Single-level directories suffer from name collision
If you and I both name a file “prog1.c” then one file will overwrite the other
Split up the space into top-level directories for each user
Keep a directory for each user’s files, and a directory of the user directories

17. Tree-Structured Directories
Let directories contain subdirectories
Arrange files in a tree
To name a file, specify a list of directories from the top down, plus the name of the file itself
This is called a path name
A path beginning at the root is an absolute path; if part of the path is implied, it’s a relative path

18. The Current Directory
Set the current directory with setcwd() system call
All future open() calls interpret path names relative to the current directory
Saves on directory lookups
Initial current directory is often set at login time, to the user’s home directory

19. Disk Block Allocation The basic unit of storage on a disk is a block
Block sizes vary, but think 512 bytes
Each file is stored in one or more blocks
For simplicity, blocks are not split between files; leftover space at the end of a block is wasted
When creating or enlarging a file, which disk block(s) should be allocated to the file?

20. Contiguous Allocation
In contiguous allocation, a file gets blocks b, b+1, b+2, ...
Direntry stores starting location, length
Two blocks with sequential numbers are very likely to be in the same track, so no head movement is required
What’s the problem?

21. Linked allocation
In linked allocation, a file gets a linked list of disk blocks
Direntry stores starting location
Each block contains data and a pointer to the next block
Advantages?
Disadvantages?

22. Indexed allocation
In indexed allocation, the file gets a list of disk blocks
An index block contains the block list
Advantages
Disadvantages
How do we deal with very large files?

23. other file information
Unix Inodes
data
other file information
data
data
direct blocks
data
data
data
1 indirect
data
data
2 indirects
data
3 indirects
data

24. Free Space How do you find free disk blocks?
Bitmap: One long string of bits represents the disk, one bit per block
Linked list: each free block points to the next one (slow!)
Grouping: list free blocks in the first free block
Counting: keep a list of streaks of free blocks and their lengths