Internal and External Memory

1. Er. Nawaraj Bhandari
Topic 3
Internal and External
Memory
Computer
Architecture

2. Computer Memory System
 Computer memory is any physical device capable of storing
information temporarily or permanently.
 For example, Random Access Memory (RAM), is a volatile memory that
stores information on an integrated circuit used by the operating
system, software, and hardware.
 The complex subject of computer memory is made more manageable
if we classify

3. Computer Memory System
 Key Characteristics of Computer Memory Systems

4. Computer Memory System
 A memory is just like a human brain. It is used to store data and
instructions.
 The memory is divided into large number of small parts called cells.
 Each location or cell has a unique address which varies from zero to
memory size minus one. For example if computer has 64k words, then
this memory unit has 64 * 1024=65536 memory locations.
 The address of these locations varies from 0 to 65535.

5. Computer Memory System
Generally memory are categorized in 4 different types.
 Semiconductor Main Memory
 Cache Memory
 Magnetic disk
 RAID
 Optical Memory

6. Semiconductor Memory Types
Memory Type 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)
Electrically
Erasable PROM
(EPROM)
Read-mostly memory
UV light, chip-level
Electrically Erasable
PROM (EEPROM)
Electrically, byte-level
Flash memory Electrically, block-level

7. Semiconductor Memory
 RAM
 Random access Memory
 Read/Write
 Volatile
 Temporary storage
DRAM(Dynamic RAM)
SRAM(Static RAM)

8. RAM

9. Read Only Memory (ROM)
 Permanent storage
 Nonvolatile
 Microprogramming (study later)
 Library subroutines
 Systems programs (BIOS)
 Function tables

10. Types of ROM
 Written during manufacture
 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

11. Cache Memory
 Cache memory is used in order to achieve higher performance of CPU by
allowing the CPU to access data at faster speed.
 It is placed closest to the processor in the computer assembly.
 It is way too costly.
 It is also a type of memory but keeping in mind the cost factor it cannot be used
as a primary memory.
What is Cache Memory?

12. Cache Memory

13. Cache write policies
When we write ,should we write to cache or memory?
 Write through cache – write to both cache and main memory. Cache and
memory are always consistent.
 Write back cache – write only to cache and set a “dirty bit”. When the block
gets replaced from the cache ,write it out to memory

14. Levels of Cache
Fastest Fast
Less Fast
Slow

15. Cache Organization
Address
Address
buffer
Control Control
Data
Data
buffer
System bus

16. Mapping Techniques
 Direct mapping
 Associative mapping
 Set associative mapping

17. Direct mapping
 Simplest technique
 In this , each block of main memory is mapped into only one
possible cache line .
i = j modulo m
where , i= cache memory
j= main memory
m=no. of lines in the cache

18. ASSOCIATIVE MAPPING
It overcomes the disadvantage of direct mapping.
It permits each main memory block to be loaded
into any line of the cache .

20. SET ASSOCIATIVE MAPPING
 The relationship which is followed here is
m= v*k
i= j modulo v
Where ,
i= cache set no.
j= main memory
m= no. of lines in the cache
v= no. of set
k= no. of lines in each set
This is called k-way set associative mapping .

22. Replacement algorithms
 Optimizing instructions.
 To manage cache information on computer.
 In direct mapping.
Each block only maps to one cache block .
 Associative and set associative mapping.

23.  Least recently used(LRU)
 First in first out(FIFO)
 Least frequently used(LFU)
 Random
Replacement algorithms

24. Random
 Randomly selects a block.
 Discards it to make space.
 Does not keep track of access history.
 This eliminates the overhead cost of tracking page references.

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

26. 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

27. 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

28. 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

29. Disk Data Layout

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

31. Winchester Hard Disk (1)
 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

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

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

34. Timing of Disk I/O Transfer

35. RAID
 Redundant Array of Independent Disks
 Redundant Array of Inexpensive Disks
 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

36. RAID 0
 This configuration has striping but no redundancy of data.
 It offers the best performance but no
fault-tolerance
 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

37. RAID 1
 Also known as disk mirroring,
 this configuration consists of at least two drives that
duplicate the storage of data.
 There is no striping.
 Read performance is improved since either
disk can be read at the same time.
 Write performance is the same as for single disk storage.
 Recovery is simple
 Swap faulty disk & re-mirror
 No down time
 Expensive

38. 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

39. RAID 3
 Similar to RAID 2
 This technique uses striping and dedicates one drive to storing parity
information.
 Since an I/O operation addresses all drives at the same time,
 RAID 3 cannot overlap I/O.
 For this reason, RAID 3 is best for single-user
systems with long record applications.
 Very high transfer rates

40. RAID 4
 This level uses large stripes,
which means you can read records
from any single drive.
 This allows you to use overlapped I/O for
read operations.
 Since all write operations have to update
the parity drive, no I/O overlapping is possible.
RAID 4 offers no advantage over RAID 5.

41. RAID 5
 The parity information is striped across each drive, allowing
the array to function even if one
drive were to failParity striped
across all disks
 Commonly used in network servers
 RAID 5 requires at least three disks,
but it is often recommended to use
least five disks for performance
reasons

42. RAID 6
 Two parity calculations
 The use of additional parity allows the array to
continue to function even if two disks fail
simultaneously.
 However, this extra protection comes at a cost.
RAID 6 arrays have a higher cost per gigabyte (GB)
and often have slower write performance
than RAID 5 arrays.

43. RAID 0, 1, 2

44. RAID 3 & 4

45. RAID 5 & 6

46. Optical storage
• Optical storage is any storage method in which data is written and
read with a laser for archival or backup purposes.
• Typically, data is written to optical media, such as CDs and DVDs
• Optical media is more durable than tape and less vulnerable to
environmental conditions
• On the other hand, it tends to be slower than typical hard drive
speeds, and to offer lower storage capacities.
• A number of new optical formats, such as Blu-ray and UDO (ultra
density optical), use a blue laser to dramatically increase capacities.

47. Magnetic Disk Layout(Hard Disk)

48. Magnetic Disk Layout(Hard Disk)

49. Magnetic Disk Layout(Hard Disk)

50. Exam Questions
Q: Assume you have a USB memory stick attached to your
computer. After working with the files on the USB memory, a so
called friend simply removes the USB memory from the
computer. Now, how does your mood depend on the used
update policy (write through or write back)?
A: If my USB is a write-back, data is only updated in the cache
and maybe not in the memory. Hence, my mood is towards
anger.
B. If my USB is a write-through, data is always updated. Hence,
my mood is not changed if a so called friend removes the USB.

51. Exam Questions
Q.Main types of technology utilized in computer memory system
Ans: Primary and Seconday Memory System

53. ANY QUESTIONS?