1. Computer Memory System
PART 3: (2/2)
Computer Memory System
CHAPTER 5:
Internal Memory
and
CHAPTER 6:
External Memory

2. CHAPTER 5: Internal Memory

3. Semiconductor Memory Types
Category
Erasure
Write Mechanism
Volatility
Random-access memory (RAM)
Read-write memory
Electrically, byte-level
Electrically
Volatile
Read-only memory (ROM)
Read-only memory
Not possible
Masks
Nonvolatile
Programmable ROM (PROM)
Erasable PROM (EPROM)
Read-mostly memory
UV light, chip-level
Electrically Erasable PROM (EEPROM)
Flash memory
Electrically, block-level

4. Semiconductor Memory RAM
Misnamed as all semiconductor memory is random access
Read/Write
Volatile
Temporary storage
Static or dynamic

5. Memory Cell Operation

6. Dynamic RAM Bits stored as charge in capacitors Charges leak
Need refreshing even when powered
Simpler construction
Smaller per bit
Less expensive
Need refresh circuits
Slower
Main memory
Essentially analogue
Level of charge determines value

7. Dynamic RAM Structure

8. DRAM Operation Address line active when bit read or written Write Read
Transistor switch closed (current flows)
Write
Voltage to bit line
High for 1 low for 0
Then signal address line
Transfers charge to capacitor
Read
Address line selected
transistor turns on
Charge from capacitor fed via bit line to sense amplifier
Compares with reference value to determine 0 or 1
Capacitor charge must be restored

9. Static RAM Bits stored as on/off switches No charges to leak
No refreshing needed when powered
More complex construction
Larger per bit
More expensive
Does not need refresh circuits
Faster
Cache
Digital
Uses flip-flops

10. Stating RAM Structure

11. Static RAM Operation Transistor arrangement gives stable logic state
C1 high, C2 low
T1 T4 off, T2 T3 on
State 0
C2 high, C1 low
T2 T3 off, T1 T4 on
Address line transistors T5 T6 is switch
Write – apply value to B & compliment to B
Read – value is on line B

12. SRAM v DRAM(1/2) Both volatile Dynamic cell Static
Power needed to preserve data
Dynamic cell
Simpler to build, smaller
More dense
Less expensive
Needs refresh
Larger memory units
Static
Faster
Cache

13. SRAM v DRAM(2/2)

14. Read Only Memory (ROM) Permanent storage Nonvolatile Microprogramming
Library subroutines
Systems programs (BIOS)
Function tables

15. Types of ROM Written during manufacture Programmable (once)
Very expensive for small runs
Programmable (once)
PROM
Needs special equipment to program
Read “mostly”
Erasable Programmable (EPROM)
Erased by UV
Electrically Erasable (EEPROM)
Takes much longer to write than read
Flash memory
Erase whole memory electrically

16. Their Usages………

17. Organisation in detail
A 16Mbit chip can be organised as 1M of 16 bit words
A bit per chip system has 16 lots of 1Mbit chip with bit 1 of each word in chip 1 and so on
A 16Mbit chip can be organised as a 2048 x 2048 x 4bit array
Reduces number of address pins
Multiplex row address and column address
11 pins to address (211=2048)
Adding one more pin doubles range of values so x4 capacity

18. Refreshing Refresh circuit included on chip Disable chip
Count through rows
Read & Write back
Takes time
Slows down apparent performance

19. Typical 16 Mb DRAM (4M x 4)

20. Packaging

21. 256kByte Module Organisation

22. 1MByte Module Organisation

23. Interleaved Memory Collection of DRAM chips Grouped into memory bank
Banks independently service read or write requests
K banks can service k requests simultaneously

24. Error Correction Hard Failure Soft Error
Permanent defect
Soft Error
Random, non-destructive
No permanent damage to memory
Detected using Hamming error correcting code

25. Error Correcting Code Function

26. Hamming Code 6

27. P P P P

28. P1 P P P

29. Advanced DRAM Organization
Basic DRAM same since first RAM chips
Enhanced DRAM
Contains small SRAM as well
SRAM holds last line read (c.f. Cache!)
Cache DRAM
Larger SRAM component
Use as cache or serial buffer

30. Synchronous DRAM (SDRAM)
Access is synchronized with an external clock
Address is presented to RAM
RAM finds data (CPU waits in conventional DRAM)
Since SDRAM moves data in time with system clock, CPU knows when data will be ready
CPU does not have to wait, it can do something else
Burst mode allows SDRAM to set up stream of data and fire it out in block
DDR-SDRAM sends data twice per clock cycle (leading & trailing edge)

31. SDRAM

32. SDRAM Read Timing

33. RAMBUS Adopted by Intel for Pentium & Itanium Main competitor to SDRAM
Vertical package – all pins on one side
Data exchange over 28 wires < cm long
Bus addresses up to 320 RDRAM chips at 1.6Gbps
Asynchronous block protocol
480ns access time
Then 1.6 Gbps

34. RAMBUS Diagram

35. DDR SDRAM SDRAM can only send data once per clock
Double-data-rate SDRAM can send data twice per clock cycle
Rising edge and falling edge

36. DDR SDRAM Read Timing

37. Simplified DRAM Read Timing

38. Cache DRAM
Mitsubishi
Integrates small SRAM cache (16 kb) onto generic DRAM chip
Used as true cache
64-bit lines
Effective for ordinary random access
To support serial access of block of data
E.g. refresh bit-mapped screen
CDRAM can Pre-fetch data from DRAM into SRAM buffer
Subsequent accesses solely to SRAM

39. CHAPTER 6: External Memory

40. Types of External Memory
Magnetic Disk
RAID
Removable
Optical Discs
CD-ROM
CD-Recordable (CD-R)
CD-R/W
DVD
Magnetic Tape

41. Magnetic Disk
Disk substrate coated with magnetisable material (iron oxide…rust)
Substrate used to be aluminium
Now glass
Improved surface uniformity
Increases reliability
Reduction in surface defects
Reduced read/write errors
Lower flight heights (See later)
Better stiffness
Better shock/damage resistance

42. Read and Write Mechanisms
Recording & retrieval via conductive coil called a head
May be single read/write head or separate ones
During read/write, head is stationary, platter rotates
Write
Current through coil produces magnetic field
Pulses sent to head
Magnetic pattern recorded on surface below
Read (traditional)
Magnetic field moving relative to coil produces current
Coil is the same for read and write
Read (contemporary)
Separate read head, close to write head
Partially shielded magneto resistive (MR) sensor
Electrical resistance depends on direction of magnetic field
High frequency operation
Higher storage density and speed

43. Inductive Write MR Read

44. Data Organization and Formatting
Concentric rings or tracks
Gaps between tracks
Reduce gap to increase capacity
Same number of bits per track (variable packing density)
Constant angular velocity
Tracks divided into sectors
Minimum block size is one sector
May have more than one sector per block

45. Disk Data Layout

46. Disk Velocity
Bit near centre of rotating disk passes fixed point slower than bit on outside of disk
Increase spacing between bits in different tracks
Rotate disk at constant angular velocity (CAV)
Gives pie shaped sectors and concentric tracks
Individual tracks and sectors addressable
Move head to given track and wait for given sector
Waste of space on outer tracks
Lower data density
Can use zones to increase capacity
Each zone has fixed bits per track
More complex circuitry

47. Disk Layout Methods Diagram

48. Finding Sectors Must be able to identify start of track and sector
Format disk
Additional information not available to user
Marks tracks and sectors

49. Winchester Disk Format Seagate ST506

50. Characteristics Fixed (rare) or movable head Removable or fixed
Single or double (usually) sided
Single or multiple platter
Head mechanism
Contact (Floppy)
Fixed gap
Flying (Winchester)

51. Fixed/Movable Head Disk
Fixed head
One read write head per track
Heads mounted on fixed ridged arm
Movable head
One read write head per side
Mounted on a movable arm

52. Removable or Not Removable disk Non removable disk
Can be removed from drive and replaced with another disk
Provides unlimited storage capacity
Easy data transfer between systems
Non removable disk
Permanently mounted in the drive

53. Multiple Platter One head per side Heads are joined and aligned
Aligned tracks on each platter form cylinders
Data is striped by cylinder
reduces head movement
Increases speed (transfer rate)

54. Multiple Platters

55. Tracks and Cylinders

56. Floppy Disk 8”, 5.25”, 3.5” Small capacity Slow Universal Cheap
Up to 1.44Mbyte (2.88M never popular)
Slow
Universal
Cheap
Obsolete?

57. Winchester Hard Disk (1/2)
Developed by IBM in Winchester (USA)
Sealed unit
One or more platters (disks)
Heads fly on boundary layer of air as disk spins
Very small head to disk gap
Getting more robust

58. Winchester Hard Disk (2/2)
Universal
Cheap
Fastest external storage
Getting larger all the time
250 Gigabyte now easily available

59. Speed Seek time (Rotational) latency Access time = Seek + Latency
Moving head to correct track
(Rotational) latency
Waiting for data to rotate under head
Access time = Seek + Latency
Transfer rate

60. Timing of Disk I/O Transfer

61. RAID Redundant Array of Independent Disks
Redundant Array of Inexpensive Disks (Originally)
6 levels in common use
Not a hierarchy
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

62. RAID 0 No redundancy Data striped across all disks
Round Robin striping
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

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

64. RAID 2 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
Lots of redundancy
Expensive
Not used

65. RAID 3
Similar to RAID 2
Only one redundant disk, no matter how large the array
Simple parity bit for each set of corresponding bits
Data on failed drive can be reconstructed from surviving data and parity info
Very high transfer rates

66. RAID 4 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

67. RAID 5 Like RAID 4 Parity striped across all disks
Round robin allocation for parity stripe
Avoids RAID 4 bottleneck at parity disk
Commonly used in network servers
N.B. DOES NOT MEAN 5 DISKS!!!!!

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

69. RAID 0, 1, 2

70. RAID 3 & 4

71. RAID 5 & 6

72. Data Mapping For RAID 0

73. 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

74. CD Operation

75. CD-ROM Drive Speeds Audio is single speed
Constant linier velocity
1.2 ms-1
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

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

77. Random Access on CD-ROM
Difficult
Move head to rough position
Set correct speed
Read address
Adjust to required location
(Yawn!)

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

79. Other Optical Storage CD-Recordable (CD-R) CD-RW WORM Now affordable
Compatible with CD-ROM drives
CD-RW
Erasable
Getting cheaper
Mostly CD-ROM drive compatible
Phase change
Material has two different reflectivities in different phase states

80. DVD - what’s in a name? Digital Video Disk Digital Versatile Disk
Used to indicate a player for movies
Only plays video disks
Digital Versatile Disk
Used to indicate a computer drive
Will read computer disks and play video disks
Dogs Veritable Dinner
Officially - nothing!!!

81. DVD - technology Multi-layer Very high capacity (4.7G per layer)
Full length movie on single disk
Using MPEG compression
Finally standardized (honest!)
Movies carry regional coding
Players only play correct region films
Can be “fixed”

82. DVD – Writable Loads of trouble with standards
First generation DVD drives may not read first generation DVD-W disks
First generation DVD drives may not read CD-RW disks
Wait for it to settle down before buying!

83. CD and DVD

84. High Definition Optical Disks
Designed for high definition videos
Much higher capacity than DVD
Shorter wavelength laser
Blue-violet range
Smaller pits
HD-DVD
15GB single side single layer
Blue-ray
Data layer closer to laser
Tighter focus, less distortion, smaller pits
25GB on single layer
Available read only (BD-ROM), Recordable once (BR-R) and re-recordable (BR-RE)

85. Optical Memory Characteristics

86. Magnetic Tape Serial access Slow Very cheap Backup and archive
Linear Tape-Open (LTO) Tape Drives
Developed late 1990s
Open source alternative to proprietary tape systems

87. Linear Tape-Open (LTO) Tape Drives
Release date
2000
2003
2005
2007
TBA
Compressed capacity
200 GB
400 GB
800 GB
1600 GB
3.2 TB
6.4 TB
Compressed transfer rate (MB/s) 40 80
160
240
360
540
Linear density (bits/mm)
4880
7398
9638
13300
Tape tracks
384
512
704
896
Tape length
609 m
680 m
820 m
Tape width (cm)
1.27
Write elements 8 16

88. Let's go through processing unit