1. Device Drivers, FAT, Queuing Theory, Memory Mapped Files
Section 11
Device Drivers, FAT, Queuing Theory, Memory Mapped Files
April 7th, 2017
Taught by Joshua Don

2. FAT FAT stands for File Allocation Table
It is one type of file system Recall…
Files live on disk
Files are broken up into logical blocks (ie. 4KB)
These blocks may not be sequential on disk Questions:
Which blocks correspond to which file?
What is the right order of blocks for the file?
How to manage directories/folders?
The kernel needs some metadata to know the right order of the blocks for a file
The kernel needs some way of mapping entries in a directory to subdirectories or files

3. FAT cont.
Files are accessed by their path (ie. /users/Josh/Desktop/test.txt)
We map a path to a number
The number associated with a file’s path is used as an index into a file allocation table
The index is the index of the file’s first header block
The i’th header block corresponds to the i’th disk data block
The header blocks contains a pointer to the next header block for the file
Linked list data structure

4. FAT cont. Free list is a linked list of the free blocks
Some consequences of FAT:
Fragmentation on disk is likely (free list is not necessarily sequential blocks)
Reading at the end of a file requires traversing through all of the prior header blocks
Simple, so widely supported by different OS
4GB file size limit in FAT32 (uses 32 bits for file size)

5. FAT cont.
Two questions…. How do we deal with directories, and how do we map file paths to file numbers?
A directory is a file too!
It contains entries for each item stored in the directory
Each entry contains the name of the item, as well as the file number for the item
The item can be a file or another directory

6. Queueing Theory

7. Queuing Theory

8. Queuing Theory, Continued
Some formulas (listed in the discussion sheet):
Tq = Tser x ½(1+C) x u/(1 – u))