Introduction Processes Inter Process Communication Deadlocks Memory Management

File Systems Protection and Security I/O Systems

1. Overview

   One of the important tasks of the operating system is to control all of the I/O devices, such as issuing commands concerning data transfer or status polling, catching and processing interrupts as well as handling different kind of errors. Two conflicting trends:

   increasing standardization of software and hardware interfaces
   inreasing broad variety of I/O devices
   

2. I/O Hardware
   Special I/O ports -- special I/O instructions and ports for communication
   memory-mapped I/O -- device-control registers are mapped into the address space of the processor; accessing I/O is like accessing a memory location

   

   An I/O port usually consists of four registers

   • status register -- indicates current states such as whether current command has completed, whether any data for read ...
   • control register -- used to change the mode of the device
   • data-in register -- read by host to get input
   • data-out register -- written by host to send output
   handshake -- interaction between the host and a controller

   Polling
   busy waiting

   

   Interrupts

   Two kinds:

   • nonmaskable interrupt ( NMI ) -- cannot be masked off, reserved for serious errors
   • maskable interrupt -- can be turned off by the CPU before the execution of critical instruction sequences that must not be interrupted

   

   An example of interrupt table

   Direct Memory Access (DMA)

   • Overview DMA is an operational transfer mode which allows data transfer within memory or between memory and I/O device without processor's intervention. A special DMA controller manages that data transfer.

     

3. Terminals

   character devices

   Two kinds:

   

4. Clocks and Timers

   

   Maintaining the time of day;
   Preventing processes from running longer than they are allowed to;
   Accounting for CPU usage;
   Handling the ALARM system call made by user processes;
   Providing watch dog timers for parts of the system itself;
   Doing profiling, monitoring and gathering statistics.

5. Disks

   provide the bulk of secondary storage

   Disk Structure:

   addressed as large one-dimensional logical blocks ~ 512 bytes

   Constant linear veloctiy ( CLV ) -- the density of bits per track is uniform; outer tracks can hold more sectors ( e.g. CD-ROM, DVD-ROM drives )

   Constant angular velocity ( CAV ) -- the disk rotation speed stay constant; the density of bits decreases from inner tracks to outer tracks to keep the data rate constant

   Disk Scheduling:

   requests are queued

   FCFS ( first-come-first-served ) scheduling

   SSTF ( Shortest-seek-time-first ) scheduling

   

   SCAN scheduling

   the disk arm starts at one end of the disk, and moves toward the other end, servicing requests as it reaches each cylinder; when it reaches the other end, the direction of head movement is reversed,

   e.g. queue = 98, 183, 37, 122, 14, 124, 65, 67
   current head position at 53, moving towards track 0
   service sequence: 37, 14, 65, 67, 98, 122, 124, 183

   C-SCAN ( circular scan ) scheduling

   when it reaches the other end, it immediately return to the beginning

   service sequence of above example: 65, 67, 98, 122, 124, 183, 14, 37

   RAID ( redundant arrays of inexpensive disks )

   RAID 0: non-redundant block-level striping ( improves speed but no fault tolerance )

   RAID 1: mirrored disks ( fault tolerance but no improvement in speed )

   RAID 2: memory-style error correcting ( ECC ), not used in practice because level 3 is better

   RAID 3: bit-level stripping with parity ( both speed and fault tolerance )

   RAID 4: block-level striping with parity ( both speed and fault toerance )

6. Blocking and Nonblocking I/O
   blocking -- execution of application is suspended

   nonblocking -- continue application execution while waiting for I/O

7. Performance

   I/O is a major factor in system performance