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

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

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

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)

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

Queueing Theory

Queuing Theory

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