09 - IO Subsystem

Question 1

State three classes of IO devices

Answer 1

Very wide range of devices that interact with the computer via input/output (IO):

(1) Human readable: graphical displays, keyboard, mouse, printers
(2) Machine readable: disks,tapes, CD, sensors
(3) Communications: modems, network interfaces,radios

Question 2

Differences between IO devices

Answer 2

(1) Data rate : keyboard vs network
(2) Control complexity : printer vs disk
(3) Transfer unit and direction : blocks vs characters vs frame stores (???)
(4) Data representation (???)
(5) Error handling (???)

Question 3

Why is the IO subsystem the “messiest” part of the OS.

Answer 3

(1) Variety of devices / applications
(2) Dimensions of variation
=> Character-stream or block
=> Sequential or random-access
=> Synchronous or asynchronous
=> Shareable or dedicated
=> Speed of operation
=> Read-write,read-only, or write-only

Question 4

Why does OS need device classes.

Answer 4

5 or 6 dimensions of variation between IO devices : character stream or block …

Thus, completely homogenising device API is not possible so OS generally splits
devices into four classes

Question 5

State the four types of device classes

Answer 5

1. Block devices
2. Character devices
3. Network devices
4. Misc devices

Question 6

Examples : block devices

Answer 6

• disk drives, CD
• Commands : read, write, seek
• Access either
  a (raw),
  b via filesystem (“cooked”),
  c or memory mapped.

Question 7

Examples : character devices

Answer 7

e. g. keyboards, mice, serial
- Commands: get, put
- Layer libraries on top for line editing etc…

Question 8

Examples : network devices

Answer 8

• Vary enough from block & character devices to get their own interface.
• Unix, Windows NT use Berkeley Socket interface… API for internet sockets, Unix domain sockets, used for IPC.

Question 9

Examples : misc devices

Answer 9

• Current time, elapsed time, timers clocks.

UNIX : ioctl system call covers other odd aspects of IO

Question 10

Examples of virtual devices

Answer 10

Terminal => terminal stream
Frame buffer => window 
Raw mouse => event stream
Disk block => files 
Parallel port => print spooler 
Raw ethernet frames => transport protocols

Question 11

Explain how the OS creates an interface between user programs and raw hardware

Answer 11

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Question 12

Describe how polled mode IO works

Answer 12

Consider simple device with three registers : status, data, command (diagram).
HOST can read / write these registers via bus.
Polled mode operation :
1. H repeatedly reads device_busy until clear
2. H sets e.g. (write) bit in command register, puts data into data register.
3. H sets command-ready and sets device_busy.
4. D performs write operation (slow)
5. D clears command_ready and then clears device_busy.

Question 13

Problem with polled mode IO

Answer 13

?????

- Time wasted while D is writing the data / performing its IO operation.

Question 14

Purpose of interrupt driven IO

Answer 14

• CPU speed&raquo_space;» IO device speed

- CPU provides interrupt mechanism to handle mismatch between CPU and device speeds.

Question 15

Describe : interrupt driven IO

Answer 15

1. End of each instruction => CPU check interrupt lines for pending interrupt.
   => Need not precisely occur at definite point in instruction stream.
2. If line asserted, CPU:
   2a Save PC, processor state.
   2b Change processor mode => kernel mode
   2c Index interrupt vector => jump to service routine.

3 Finish interrupt handling => use rti instruction to resume.

4. More complex CPUs :
   - Multiple priority levels of interrupt
   - hardware vectoring of interrupt
   - mode dependent registers

Question 16

Handling interrupt

Answer 16

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Question 17

Explain BLOCKING IO system calls

Answer 17

BLOCKING :

• Process suspended until IO completed.
• Easy to use and understand
• Insufficient for some needs (???? what ???)

Question 18

Explain NONBLOCKING IO

Answer 18

• IO call returns as much as available. (??)
• Returns almost immediately with count of bytes read or written (possibly 0)
• Can be used by e.g. user interface code.
• Essentially application level “polled IO”.

Question 19

Explain ASYNCHRONOUS IO

Answer 19

• Process runs concurrently as IO executes.
• IO subsystem explicitly signals to the process when its IO request is complete.
• Most flexible (and potentially efficient … why)
• Also most complex to use (why ….)

Question 20

Why is IO buffering needed

Answer 20

1. Cope with impedance mismatches between devices (speed, transfer size)
2. OS may buffer data in memory.

Question 21

Buffering strategies

Answer 21

• Single buffering : OS assigns a system buffer to the user request
• Double buffering : process consumes from one buffer, while system fills the next.
• Circular buffering : most useful for IO which occurs in bursts.

Question 22

Explain single buffering

Answer 22

• OS assigns single buffer to user request ,
• OS performs transfer, moves buffer to user space when complete (remap or copy ????)
• Request new buffer for more IO, then reschedule app to consume (READAHEAD or ANTICIPATED INPUT ???)
• OS must track buffers
• Also affects swap logic - if IO is to same disk as swap device : doesn’t make sense to swap process out as it will be behind the now queued IO request ???

Question 23

Make comparison between no buffering and single buffering

Answer 23

• t := time to input block,
• c : computation time between blocks
• With single buffering, time (what time ???) is max(c, t) + m, where m is the time to move data from buffer to user memory.
• Without buffering, execution time between blocks is t + c.
  ????
• For a terminal: is the buffer a line or a char, depends on user response required.

Question 24

Double buffering

Answer 24

• e.g. used in video rendering
• Rough performance comparison : takes max(c, t) .: possible to keep device at full speed if c < t, but if c > t process will not have to wait for IO. ???
• Prevents need to suspend user process between IO operations
  … also explains why two buffers is better than one buffer, twice as big.
• Need to manage buffers and processes to ensure process doesn’t start consuming from an only partially filled buffer.

Question 25

Circular buffering

Answer 25

1. Allow consumption from buffer at fixed rate, potentially lower than burst rate of arriving data
2. Typically use circular linked list = FIFO buffer with queue length ???

Question 26

BURST RATE ???

Answer 26

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