Outline

1
Outline

• Basic memory management
• Swapping
• Virtual memory
• Page replacement algorithms
• Modeling page replacement algorithms
• Design issues for paging systems
• Implementation issues
• Segmentation

2
When OS Involves Paging?

• Process creation
• Process execution
• Page faults
• Process termination

3
Paging in Process Creation

• Determine (initial) size of program and data
• Assign appropriate page frames
• Create page table
• Process is running ? page table in memory
• Create swap area
• For the pages swapped out
• Record info about page table and swap area in
  process table

4
Paging During Process Running

• When a process is scheduled for execution
• Reset MMU
• Flush TLB (translation Lookaside Buffer)
• Copy or point to new process page table
• Bring some or all of new process pages into
  memory
• Reduce the number of page faults

5
Paging When Page Faults

• Read registers to identify virtual address
  causing the page fault
• Locate the page needed on disk
• Find available page frame
• Evict an old page if necessary
• Read in the page
• Execute the faulting instruction again

6
Paging When Process Exits

• Release page table
• Release pages and swap area on disk
• If some pages are shared by other processes, keep
  them.

7
Swap Area

• Swap area reserved for processes pages
• A chunk of pages reserved on disk, same size of
  of pages of the process
• Each process has its own swap area, recorded in
  process table
• Initialize swap area before a process runs
• Copy entire process image, or
• Load the process in memory and let it be paged
  out as needed

8
Swap Area for Growing Processes

• Process may grow after starting
• Data area and stack may grow
• Reserve separate swap areas for text (program),
  data, and stack
• Reserve nothing in advance
• Allocate disk space when pages swapped out
• De-allocate when page swapped in again
• Have to keep track of pages on disk
• A disk address per page, costly!

9
Comparison of Two Methods
Paging to a static swap area
Backing up pages dynamically
10
Outline

• Basic memory management
• Swapping
• Virtual memory
• Page replacement algorithms
• Modeling page replacement algorithms
• Design issues for paging systems
• Implementation issues
• Segmentation

11
A Motivating Example

• Many tables when compiling a program
• Symbol table names and attributes of variables
• Constant table integer/floating-point constants
• Parse tree syntactic analysis
• Stack for procedure calls within the compiler
• Each table needs contiguous chunks of virtual
  address space. But tables grow/shrink as
  compilation proceeds.
• How to manage space for these tables?

12
With One-Dimensional Address
Virtual address space

• Take space from tables with an excess of room
• Tedious work
• Free programmers from managing expanding and
  contracting tables
• Segments many completely independent address
  spaces

13
Segments

• Segment a two dimensional memory
• Each segment has a linear sequence of address (0
  to some maximum)
• Different segments may have different lengths
• Segment lengths may change during execution
• Different segments can grow/shrink independently
• Address segment number address within the
  segment (offset)

14
Multiple Segments in A Process

• Segments are logical entities
• Programmers are aware of them.
• A segment may contain a procedure, or an array,
  but not a mixture of different types.
• Facilitate separate protection.

15
Paging Vs. Segmentation
16
Implementing Pure Segmentation

• External fragmentation

Time ?
17
Segmentation With Paging MULTICS

• For large segments, only the working set should
  be kept in memory
• Paging segments segment has own page table
• Each program has a segment table
• One entry (descriptor) per segment
• Segment table is itself a segment
• If (part of) a segment is in memory, its page
  table must be in memory
• Address segment virtual page offset
• Segment ? page table
• Page table virtual page ? page frame address
• Page frame address offset ? physical address

18
The MULTICS virtual memory
Virtual Address
Page frame
Page table for segment 2
Segment table
Segment descriptor contains the memory address of
the page table.
Page frame
Page table for segment 0
19
Summary

• Fixed partitions
• Multiple queues Vs. single queue
• Degree of multiprogramming
• Relocation and protection
• Swapping
• Virtual memory Vs. physical memory
• Bitmap and linked list
• Holes

20
Summary (Cont.)

• Virtual memory
• Pages Vs. page frames
• Page tables
• Page replacement algorithms (aging and WSClock)
• Modeling paging systems
• Stack algorithms
• Predict page faults using distance string

21
Summary (Cont.)

• Design issues
• Local Vs. global allocations
• Load control to reduce thrashing
• Shared pages
• Implementation issues
• Page fault handling and swap area
• Segmentation
• Each segment has its own address space
• Advantages
• Pure segmentation and segmentation with paging

22
CMPT 300 Operating System

• Chapter 5Input/Output

23
Outline

• Principles of I/O hardware
• Principles of I/O software
• I/O software layers
• Disks
• Clocks

24
Block and Character Devices

• Block devices store info in fixed-size blocks
• Examples disks
• Each block has its own address
• Each block can be read/written independently
• Data are read/write in the units of block
• Character devices no block structure,
  deliver/accept a stream of characters
• Examples keyboard, printers, network, mice
• Not addressable, no seek operation
• Devices not in the classification clock

25
Huge Range in Speeds

• Many orders of magnitude in data rates
• Keyboard 10 bytes/sec
• Mouse 100 bytes/sec
• Laser printer 100Kb/sec
• IDE disk 5Mb/sec
• PCI bus 528Mb/sec
• Challenge how to design a general structure to
  control various I/O devices?
• Multi-bus system

26
Structure of I/O Units

• A mechanical component the device itself
• Disk plates, heads, motors, arm, etc.
• Monitor tube, screen, etc.
• An electronic component device controller,
  adaptor
• Disk issuing commands to mechanical components,
  assembling, checking and transferring data
• Monitor read characters to be displayed and
  generate electrical signals to modulate the CRT
  beam

27
Mechanical / Electronic Components
Mechanical components
Floppy disk
Hard disk
Monitor
Keyboard
Floppy disk controller
Video controller
Keyboard controller
Hard disk controller
CPU
Memory
Electronic components
Bus
28
Device Controller

• Registers in I/O controllers
• CPU writes commands into registers
• CPU reads states of devices from registers
• Data buffer for transferring data
• How CPU distinguishes different registers and
  data buffers?
• I/O port number
• Memory-mapped I/O

29
I/O Port Number

• Each control register is assigned a unique I/O
  port number (8-/16-bit integer)
• Instruction IN and OUT
• IN REG, PORT
• OUT PORT, REG
• Separated address spaces
• IN R0, 4 and MOV R0, 4are completely different.

Memory
I/O ports
30
Memory-Mapped I/O

• Map all control registers into memory space
• Usually at the top of the address space
• Each register is assigned a unique memory
  address, e.g., 0xFF10
• Hybrid scheme (used in Pentium)

I/O buffered data
I/O ports
Memory-mapped
Hybrid
Memory
31
Pros Cons of Memory-Mapped I/O

• Advantages
• Easier for programming
• Easier to protect and share I/O devices
• Save time of accessing control registers
• Disadvantages
• Caching a control register is disastrous
• Disable caching for selected pages
• I/O devices cannot see the memory addresses with
  separate buses for memory and I/O devices (see
  next slide)

32
Single-bus
All addresses go here
CPU reads/writes of memory go over this
high-bandwidth bus
Dual-bus
33
Interrupts
1. Device finishes a work
Interrupt controller
3. CPU acks interrupt
Disk
Keyboard
CPU
Clock
2. Controller issues interrupt
Printer
Bus
34
Interrupt Processing

• I/O devices raise interrupt by asserting a signal
  on a bus line assigned
• Multiple interrupts ? the one with high priority
  goes first
• Interrupt controller interrupts CPU
• Put device on address lines
• Device ? check interrupt vector table for
  interrupt handler (a program)
• Enable interrupts shortly after the handler starts

35
Direct Memory Access (DMA)

• Request data from I/O without DMA
• Device controller reads data from device
• It interrupts CPU when a byte/block of data
  available
• CPU reads controllers buffer into main memory
• Too many interruptions, expensive
• DMA direct memory access
• A DMA controller with registers read/written by
  CPU
• CPU programs the DMA what to transfer where
• Source, destination and size
• DMA interrupts CPU only after all the data are
  transferred.

36
Operations of DMA
DMA controller
Disk controller
Main memory
Drive
1. CPU programs the DMA and controller
Buffer
CPU
Address
Count
4. Ack
Control
2. DMA requires transfer to memory
3. Data transferred
Interrupt when done
Bus
37
Transfer Modes

• Word-at-a-time (cycle stealing)
• DMA controller acquires the bus, transfer one
  word, and releases the bus
• CPU waits for bus if data is transferring
• Cycle stealing steal an occasional bus cycle
  from CPU once in a while
• Burst mode
• DMA holds the bus until a series of transfers
  complete
• More efficient since acquiring bus takes time
• Block the CPU from using bus for a substantial
  time

38
Outline

• Principles of I/O hardware
• Principles of I/O software
• I/O software layers
• Disks
• Clocks

39
Issues of The I/O Software

• Device independence
• Uniform naming
• Error handling
• Buffering
• Others

40
How to Perform I/O?

• Programmed I/O (Polling/Busy Waiting)
• Acquire I/O device (e.g., printer)
• Blocked if it is being used by another process
• Copy the buffer from use space to kernel space
• Step 1 keep checking the state until it is
  ready.
• Step 2 send one character to printer. Go to Step
  1.
• Release the I/O.
• The CPU do all the work
• Simple
• Waste a lot of CPU time on busy waiting

41
An Example
Q Why does OS copy the buffer from user space to
kernel space?
42
Interrupt-Driven I/O
Print system call copy_from_user(buffer, p,
count) enable_interrupts() while
(printer_status_reg!READY) printer_data_regist
erp0 scheduler()
Interrupt service procedure if (count0)
unblock_user() else printer_data_regis
terpI count-- i
acknowledge_interrupt() return_from_interrupt()
Interrupt occurs on every character!
43
I/O Using DMA

• Too many interrupts in interrupt-driven I/O
• DMA reduces of interrupts from 1/char to
  1/buffer printed

Print system call copy_from_user(buffer, p,
count) set_up_DMA_controller() scheduler()
Interrupt service procedure acknowledge_interrup
t() unblock_user() return_from_interrupt()
44
Outline

• Principles of I/O hardware
• Principles of I/O software
• I/O software layers
• Disks
• Clocks

45
Layers Overview
46
Interrupt Handlers

• Hide I/O interrupts deep in OS
• Device driver starts I/O and blocks (e.g., down a
  mutex)
• Interrupt wakes up driver
• Process an interrupt
• Save registers ( which to where?)
• Set up context (TLB, MMU, page table)
• Run the handler (usually the handler will be
  blocked)
• Choose a process to run next
• Load the context for the newly selected process
• Run the process
• Take considerable number of CPU instructions

47
Device Drivers

• Device-specific code for controlling I/O devices
• Written by manufacture, delivered along with
  device
• One driver for one (class) device(s)
• Position part of OS kernel, below the rest of OS
• Interfaces for rest of OS
• Block device and character device have different
  interfaces

48
Logical Position of Device Drivers
User program
User space
Rest of the OS
Kernel space
Printer driver
Printer controller
Hardware
printer
Devices
49
How to Install a Driver?

• Re-compile and re-link the kernel
• Drivers and OS are in a single binary program
• UNIX systems
• Often run by computer centers, devices rarely
  change
• Dynamically loaded during OS initialization
• Windows system
• Devices often change
• Difficult to obtain source code
• Users dont know how to compile OS

50
Functions of Device Drivers

• Accept abstract read/write requests
• Error checking, parameter converting
• Check status, initialize device, if necessary
• Issue a sequence of commands
• May block and wait for interrupt
• Check error, return data
• Other issues re-entrant, up-call, etc.

51
Device-Independent I/O Software

• Why device-independent I/O software?
• Perform I/O functions common to all devices
• Provide a uniform interface to user-level software

• Functions in device-independent software
• Uniform interfacing for devices drivers
• Buffering
• Error reporting
• Allocating and releasing dedicated devices
• Providing a device-independent block size

52
Uniform Interfacing for Device Drivers

• New device ? modify OS, no good
• Provide the same interface for all drivers
• Easy to plug a new driver
• In reality, not absolutely identical, but most
  functions are common
• Name I/O devices in a uniform way
• Mapping symbolic device names onto the proper
  driver
• Treat device name as file name in UNIX
• E.g., hard disk /dev/disk0 is a special file. Its
  i-node contains the major device number, which is
  used to locate the appropriate driver, and minor
  device number.

53
Buffering for Input

• Motivation consider a process that wants to read
  data from a modem
• User process handles one character at a time.
• It blocks if a character is not available
• Each arriving character causes an interrupt
• User process is unblocked and reads the
  character.
• Try to read another character and block again.
• Many short runs in a process inefficient!
• Overhead of context switching

54
Buffering in User Space

• Set a buffer in user process space
• User process is waked up only if the buffer is
  filled up by interrupt service procedure. More
  efficient.
• Can the buffer be paged out?
• Yes where to put the next character?
• No the pool of available pages shrink

Buffering in user space
55
Buffering in Kernel

• Two buffers one in kernel and one in user
• Interrupt handler puts characters into the buffer
  in kernel space
• This buffer is locked
• When full, copy the kernel buffer to user buffer
• Bring in the user buffer if necessary
• Where to store the new arrived characters during
  page loading?

Buffering in kernel
56
Double Buffering in Kernel

• Two kernel buffers
• When the first one fills up, but before it has
  been emptied, the second one is used.
• Buffers are used in turn while one is being
  copied to user space, the other is accumulating
  new input

Double buffering
57
Downside of Data Buffering

• Many sequential buffering slow down transmission

Process A
Process B
User space
5
1
Kernel space
2
4
Network controller
3
Whats it for?
Network
58
Handling I/O Errors

• Programming errors ask for something impossible
• E.g. writing a keyboard, reading a printer
• Invalid parameters, like buffer address
• Report an error code to caller
• Actual I/O error
• E.g. write a damaged disk block
• Handled by device driver and/or
  device-independent software
• System error
• E.g. root directory or free block list is
  destroyed
• display message, terminate system

59
Allocating Dedicated Devices

• Before using a device, make the system call open
• When the device is unavailable
• The call fails, or
• The caller is blocked and put on a queue
• Release the device by making the close system call

60
User-Space I/O Software

• Libraries interface between user programs and
  system calls
• E.g., write(fd, buffer, nbytes), printf/scanf
• Spooling
• User-level, controlled by a daemon
• Eliminated unnecessarily waiting/deadlock

61
Summary I/O Software
I/O reply
I/O request