UMUC CS 412, Sp 97, Input and Output

• Hardware
• Software
• Disks
• Clocks
• Terminals

Hardware

I/O Devices

• Block devices
• Typically 128 to 1024 Bytes
• Random access, get any block
• Disks, CDROM
• Tapes can be treated this way, but not in the spirit
• Character devices
• Stream of characters, no seek operation
• Terminals, printers, tapes, punch cards, network interfaces, mice (rats?)
• Robotic controls: sensors and actuators
• Other types:
• Clocks
• Memory mapped screens

Device Controllers

• Mechanical component  - device itself
• Electronic component - device controller or adapter - printed circuit card
• May control more than one device
• Typically standard: ISO, ANSI, IEEE, etc.
• Architecture:
• Bus - micro and mini computers
• I/O channels - mainframes
• Disks
• Looks like serial bit stream at a low level
• Preamble include cylinder and sector numbers, sector size, and similar data
• Disk controller S/W (firmware) to turn bit stream into blocks of data
• Must also to checksums and error detection, perhaps correction
• Terminal
• Put characters on screen
• Horizontal and vertical retrace signals
• Controller I/O
• Memory mapped - rather large sections of memory space are devoted to various devices or ports
• IBM PC - I/O addresses and Interrupt vectors

Direct Memory Access (DMA)

• Typically an option with block oriented devices
• Data transferred to/from memory directly from/to controller with CPU involvement
• CPU starts operation, device interrupts upon completion
• Provides address and length of data to transfer
• Data actually transferred to an internal buffer on controller card
• Because controller must act in real time, sychronized to disk rotation
• If data not available, controller will terminate operation with an error
• Interleaving sectors on disk (see T, page 210, Figure 5-4)
• Allow time between transfer of sectors for system recovery & error checking
• Striping between cylinders and between tracks

Software

Goals of I/O software

• Device independence
• Try to program all devices the same way - from the user's and programmer's point of view
• UNIX solution - look at everything as a character stream file
• Uniform naming
• Identify all devices using similar terminology
• Error handling
• Get the lower layers to handle errors - do not bug the users or programmers if possible
• Synchronous vs asynchronous transfers
• Blocking - issue read or write and wait for operation to complete
• Nonblocking - issue operation and continue, requires interrupts and interrupt handlers, perhaps polling
• Sharable vs dedicated devices
• In a multiuser computer, essentially everything is sharable
• Viewed as layers:
• Interrupt handlers
• Device drivers
• Device independent operating system software
• User level software

Interrupt handlers

• Hide from users and programmers
• Process can block itself:
• DOWN on a semaphore
• WAIT on a condition
• RECEIVE on a message
• O/S then sends message to process starting it

Device drivers

• Each driver handles a class of similar devices
• Knowledge about the following should stop at device driver level:
• Number of sectors per track
• Number of registers and their meaning
• Arm motion, interleave factors, motor drives, head settling times, etc.
• Interface level: get/put block N
• Queue device operation requests if this makes sense
• May block or poll
• Check for errors and try to recover

Device-independent I/O software

• Functions (See T, page 214, Figure 5-5)
• Uniform interfacing for device drivers
• Device naming
• Device protection
• Device independent block size
• Buffering
• Storage allocation on devices
• Allocating and releasing dedicated devices
• Error reporting

User-space I/O software

• Library procedures
• formatting
• putting I/O into lines
• Spooling - sharing device by taking turns
• Spooling daemon - only process allowed to write to physical device
• Spooling directory
• Applications include printers and network I/O

Disks

• Key benefits: 
• large storage capacity
• low price per bit
• save information when powered off

Hardware

• heads, cylinders, tracks, sectors
• Typically, 8 to 32 sectors per track
• Normally same throughout disk
• Overlapped seeks are possible if there is more than one disk drive

Arm scheduling algorithms

• Parameters related to performance:
• Seek time - moving the arm
• Rotational delay
• Data transfer time (rate)
• If many disk access requests are outstanding, they can be ordered
• significantly better performance
• Algorithms:
• FCFS - first come first served
• SSF - shortest seek first
• elevator algorithm - remember which way heads are going
• switching heads faster than moving heads, so priority to requests in same cylinder
• RAID - redundant array of inexpensive disks
• For example, 38 bits at a time, very fast transfer rate; seek time still a problem
• Use error correction coding

Error handling

• Programming error
• Transient checksum error - try again
• Permanent checksum error - e.g., bad disk block
• mark block bad, 
• mark file bad
• reconstruct information through backups or previous versions
• bad block maps
• Seek error - arm went to wrong track
• reset, recalibrate
• pull arm back to beginning and try again
• arm tells where it is by using a stepper motor - basic operation is: move head one track (cylinder) in or out
• Controller error - card will not accept commands
• Try reset
• Try CPU directed reset
• Try reboot
• Try new card

Track at a time caching

• Not available at the device independent layer

RAM disks

• Use some RAM as a disk drive
• Convenient for some software operations

Clocks

• Time of day
• CPU sharing

Hardware

• Counters
• line frequency - 50 or 60 Hz
• independent clock chip, powered by battery; similar to wrist watch
• CPU clock, bus clock, vertical drive, horizontal drive, etc.
• Operation
• load with target count
• count down
• interrupt when reaches 0
• modes: reset to original value and continue or stop
• interrupt may enabled or disabled
• Periodic interrupts are called "clock ticks"
• Maximum period determined by clock being counted and register size

Software

• Clock driver
• Duties - vary, but typically:
• maintain time of day
• time slice processes
• time limit processes
• account for CPU usage
• ALARM system call from users
• watchdog timers
• profiling, monitoring and statistics gathering
• Maintaining time of day 
• problem is: how big a register?
• 64 bits, seconds since 1/1/70, good until 2038
• 32 bit time of day in seconds, 32 bit number of ticks in current second
• 32 bit counter ticks since boot, 32 bit system boot time in seconds
• Many interrupts on a single clock - 
• sort and queue interrupts in a linked list, 
• keep track of who requested interrupt
• Watchdog timers - give some hardware devices time to perform requested actions

Terminals

Hardware

• RS-232 interfaces
• Hardcopy - on paper (decwriter)
• screen - 24x80 characters
• Intelligent - larger character set, reacts to more control signals
• Blit - memory buffer
• Memory mapped interface
• Character oriented
• bit/pixel oriented
• vector oriented

Memory-mapped terminals

• Video RAM
• switch/load during vertical refresh (v-drive)
• Video controller
• takes bytes and generates video control signals
• 200 to 1200 points per line (pixels)
• 200 to 1200 lines per screen, sometimes more
• colors - red/green/blue
• LUT's, look up tables
• refresh rate: 30 to 70 times per second
• Progressive vs interlaced
• Character
• character size (e.g., 9x14 pixels)
• attributes - blink, reverse, color
• Keyboards
• code for each key
• code for any modifiers (control, shift, caps lock, alt, num lock, etc.)
• interrupt on depress and another on release

Input software

• keystroke mode (character oriented, raw mode)
• all keys actions directly sent to program
• program must do line editing
• line mode (line oriented, cooked mode)
• driver provides programs with a full line of input
• input not available until user types <cr>
• type-ahead buffering
• echoing - 
• local or remote
• passwords are a problem
• program writing while user typing?
• <cr> <lf> problem
• special characters (to O/S), see T, page 233, Figure 5-18 for example from UNIX
• escape character to surpress meaning of special characters (e.g., \)
• I/O control
• CTRL-S and CTRL-Q
• X-ON and X-OFF
• BREAK

Output software

• RS-232 terminals
• buffer for each terminal
• Memory mapped (character)
• write character directly into memory
• Memory mapped (pixel)
• go through a drawing package
• determine and map through font
• generate glyph in correct location
• many attributes possible
• type managers for cross-platform and cross-device operations
• WSYIWYG - what you see is what you get ?!?!?!
• Screen control:
• scrolling
• cursor position
• bell 
• Graphical screen control systems:
• Mac
• Windows 3.1
• Windows 95/NT
• X
• Others