1. Chapter 6
External Memory

2. Types of External Memory
Magnetic Disk
RAID (Redundant Array of Independent Disks)
Removable
Optical
CD-ROM
CD-Recordable (CD-R)
CD-R/W
DVD
DVD-R
DVD-RW
Magnetic Tape

3. Magnetic Disk
Disk substrate coated with magnetizable material (iron oxide…rust)
Substrate originally was aluminium - Is now glass
Improved surface uniformity
Increases reliability
Reduction in surface defects
Reduced read/write errors
Lower flight heights (head rides on air gap)
Better stiffness
Better shock/damage resistance

4. Disk Data Layout - Platter

5. Tracks and Cylinders

6. Multiple Platters

7. Disk Layout Methods Diagram

8. Physical Characteristics of Disk Systems

9. Inductive Write MR Read

10. Typical Hard Disk Drive Parameters

11. Formating
Why?
Must be able to identify position of data: start of track and sector
Marks tracks and sectors

12. Winchester Disk Format
30 fixed-length sectors per track

13. Speed Parameters Seek time Latency (Rotational) Access time
Time to position head at track
Latency (Rotational)
Time for head to rotate to beginning of sector
Access time
- Seek time + Latency time
Transfer rate
- The rate at which data can be transferred after access
T = b / N * 1/r
Transfer time = bytes transferred / bytes/track * sec/revolution (track)
Note: How does organization on disk (e.g. random vs sequential) effect total time?

14. Timing of Disk I/O Transfer

15. RAID – Goals: Speed, Reliability, Standardization
Redundant Array of Independent Disks
(or Redundant Array of Inexpensive Disks ?)
Set of physical disks viewed as single logical drive by O/S
Data distributed across physical drives
Can use redundant capacity to store parity information
7 “levels” of RAID organization (not a hierarchy)
0 not really a RAID organization (no redundancy)
1, 3 used for high transfer rate
5, 6 used for high transaction rate
2, 4 not commercially available
Requirements for high transfer rate:
High transfer rate along entire path between host memory to disk drives
Application must make I/O requests that drive disks efficiently

16. Redundant Array of Independent Disks
Not RAID
Not used
Not used

17. RAID 0, 1, 2

18. RAID 0 Description: Characteristics: No redundancy (Not “really” RAID)
Data striped across all disks
Round Robin striping
Characteristics:
Increase speed
Multiple data requests probably not on same disk
Disks seek in parallel
A set of data is likely to be striped across multiple disks

19. RAID 1 Description: Characteristics: Mirrored Disks
Data is striped across disks
2 copies of each stripe on separate disks
Read from either
Write to both
Characteristics:
Recovery is simple
Swap faulty disk & re-mirror
No down time
Expensive

20. RAID 2 (not used) Description: Characteristics: Disks are synchronized
Very small stripes
Often single byte/word
Error correction calculated across corresponding bits on disks
Multiple parity disks store Hamming code error correction in corresponding positions
Characteristics:
Lots of redundancy
Very Expensive
Not used commercially

21. RAID 3 & 4

22. RAID 3 Description: Characteristics: Similar to RAID 2
Only one “redundant” disk, no matter how large the array
Simple parity bit for each set of corresponding bits
Characteristics:
Data on failed drive can be reconstructed from surviving data and parity info
Very high transfer rates
Not very expensive or complex

23. RAID 4 (not used) Description: Characteristics:
Each disk operates independently
Good for high I/O request rate
Large stripes
Bit by bit parity calculated across stripes on each disk
Parity stored on parity disk
Characteristics:
Good for high request rates rather than high transfer rates
Every write impacts the parity disk so it becomes a bottleneck.
Not used commercially

24. RAID 5 & 6

25. Characteristics: RAID 5 Description: Very similar to RAID 4
Parity striped across all disks
Round robin allocation for parity stripe
Characteristics:
Avoids RAID 4 bottleneck at parity disk
Commonly used in network servers

26. Three disks need to fail for data loss
RAID 6
Description:
Two parity calculations
Stored in separate blocks on different disks
User requirement of N disks needs N+2 disks
Characteristics:
High data availability
Three disks need to fail for data loss
Significant write penalty (two parity calculations)

27. RAID Comparison (1)
Not RAID
Not used

28. Raid Comparison (2)
Not used

29. Types of External Memory
Magnetic Disk
RAID (Redundant Array of Independent Disks)
Removable
Optical
CD-ROM
CD-Recordable (CD-R)
CD-R/W
DVD
DVD-R
DVD-RW
Magnetic Tape

30. Optical Products

31. Optical Storage CD-ROM
Originally for audio
650Mbytes giving over 70 minutes audio
Polycarbonate coated with highly reflective coat, usually aluminium
Data stored as pits
Read by reflecting laser
Constant packing density
Constant linear velocity

32. CD Construction

33. CD Layout

34. CD reader

35. Other speeds are quoted as multiples e.g. 24x
CD-ROM Drive Speeds
Audio is single speed
Constant linear velocity
1.2 m/sec
Track (spiral) is 5.27km long
Gives 4391 seconds = 73.2 minutes
Other speeds are quoted as multiples
e.g. 24x
Quoted figure is maximum drive can achieve
Note: CD-ROM has option of error correction (not on CD)

36. CD-ROM Format Mode 0 = blank data field
Mode 1 = 2048 byte data + error correction
Mode 2 = 2336 byte data

37. Random Access on CD-ROM & CD-R
Difficult
Process:
Move head to rough position
Set correct speed
Read address
Adjust to required location

38. CD-ROM CD-R for & against
Large capacity (?)
Easy to mass produce
Removable
Robust
Expensive for small runs
Slow
Read only

39. CD-RW Erasable Getting cheaper Mostly CD-ROM drive compatible
Phase change
Material has two different reflectivities in different phase states

40. DVD - technology Multi-layer Very high capacity (4.7G per layer)
Full length movie on single disk
Using MPEG compression

41. CD vs DVD

42. Two objectives had to be resolved to make the DVDs viable.
The linear velocity of a DVD must be held constant and be able to reproduce a vertical frame rate of frames/second
Every DVD player had to have absolute tracking accuracy to insure the extremely narrow laser beam would scan exactly in the middle of the track where the data was recorded. 
The solution:
The disk is pressed with the track grooves accurately pre-cut and encoded with a constant bit rate frequency. Thus a blank DVD disk isn't really blank at all.

43. Types of External Memory
Magnetic Disk
RAID (Redundant Array of Independent Disks)
Removable
Optical
CD-ROM
CD-Recordable (CD-R)
CD-R/W
DVD
DVD-R
DVD-RW
Magnetic Tape

44. Magnetic Tape Serial access Slow Very cheap
Used for backup and archive

45. Chapter 7
Input/Output

46. Input/Output Challenges
Must support a wide variety of peripherals
Delivering different amounts of data
At different speeds
In different formats
All slower than CPU and RAM
Need standardized I/O Interfaces (modules) or channels

47. Generic Model of I/O Device Interface (Module)

48. I/O Interface Function
Support single or multiple devices
Hide or reveal device properties
Provide
Control & Timing
CPU Communication
Device Communication
Data Buffering
Perhaps Error Detection

49. I/O Device Interface Diagram

50. Input/Output Alternatives
Programmed I/O Interface
Interrupt driven I/O Interface
Direct Memory Access (DMA) I/O Interface

51. Programmed I/O Programmer has direct control over I/O
Sensing status
Read/write commands
Transferring data
CPU (program) must wait for I/O module to complete operation
Usually not a good use of CPU time

52. Programmed I/O - details
Program initiates I/O operation
Requests Write, sends data, and waits for done
Requests Read and waits for data ready
I/O module performs operation
I/O module sets status bits (in Status Register) to confirm operation is done
Program is continuously “Polling the I/O status register” checking status bits for I/O Ready/Done
Actually program may wait or come back later
Program completes I/O operation (processes data)
Reads data or Writes next data
Program continues
In Programmed I/O, the program must wait for the I/O operation to be completed before moving on

53. Interrupt driven I/O Programmer Issues I/O command to I/O interface
Similar to step one in programmed I/O
Programmer goes on to other work, i.e. does not have to “poll the device” and wait for I/O completion
Interface Requests an Interrupt when I/O is ready
Interface Requests “Service Routine (program) be executed”
CPU switches context and executes the I/O service routine.
At the end of the service routine, the context is switched back and the original program continues.
Interrupt driven I/O relieves the need for polling I/O devices and programming the I/O service

54. Interrupt Physical Model
CPU
General Purpose Registers
Program counter (PC)
Stack Pointer (SP)
User stack Pointer Storage
Supervisor Stack Pointer Storage
Program Status Word (PSW) – Includes
State – user/supervisor, priority, etc.
Program Priority
Condition Codes (CC)
Hardware to communicate over the BUS
Address, Data, and Data Control
Bus status and control
Memory
User program
Interrupt Service Routine Program
Operating System
Interrupt Vector Table
Includes an entry that points to the Interrupt Service Routine (Interrupt vector #)
Device
Status/Control Register(s) – Includes:
Interrupt Enable bit
Interrupt bit (sometimes called ready or done)
Priority Level for Interrupt Service Routine (In hardware or firmware)
Interrupt vector number (In hardware or firmware)
Hardware to communicate with CPU over the BUS

55. Interface Registers Keyboard Status Register (16 bit) Keyboard Device:
Bit 15 Done Bit
Bit 14 Interrupt Enable Bit
Bits 0-2 Priority
Keyboard Data Register (16 bit)
Contains character entered
Keyboard Interrupt Vector (16 bit)
Contains the “address” in the Interrupt Vector Table
Display Device:
Display Status Register (16 bit)
Bit 15 Ready Bit
Display Data Register (16 Bit)
Contains character to be displayed
Display Interrupt vector (16 bit)

56. Interrupt Sequence What does the programmer do?
What does the computer do?

57. Interrupt Sequence
Programmer Action:
Enable Interrupts by setting “intr enable” bit in Device Status Reg
Enabling Mechanism for device:
When device wants service, and its enable bit is set
(i.e, the I/O device has the right to request service), and
the device priority is higher than the priority of the presently running program, and
execution of an instruction is complete, then
Process to Service the Interrupt:
The Processor saves the “state” of the program (must be able to return to program)
The Processor goes into Privileged (or Supervisor) Mode
The Priority level is set (established by the interrupting device)
The context is switched
The user SP is saved and the Supervisor SP loaded
The (PC) and the (PSR) are PUSHED onto the Supervisor Stack
The contents of the other registers are not saved. Why?
The CC’s are cleared. Why?
The Processor Loads the PC from the Interrupt Vector Table
The device provides the Vector Table entry number
Interrupt Service Routine is executed
The routine ends with an “RTI” instruction
The context is switched back
The stored user: PSR (POP into PSR), PC (POP into PC), USP loaded (POP into SP)
The Processor goes into User mode
8) And the next instruction in the original program is fetched