1. University of the Sulaimani (UOS)
University of the Sulaimani (UOS). Faculty of Science and Sciences Education School of Science Department of Computer
CS11 Digital Logic and Organization
Lecture 6: Memory System in a Computer Rebaz Nawzad

2. Memory System in a Computer
Assumptions:
Graduate level
Operating Systems
Making Choices about operation systems
Why a micro-century? …just about enough time for one concept

3. What is memory?
A term for a device that enables the computer to retain (store) information.

4. What are the types of memory?
Volatile memory (Random Access Memory/RAM; Cache Memory; Virtual Memory)- Contents of memory are erased when power supply is turned off. Also called Temporary storage.
Nonvolatile memory (Read Only Memory/ROM; Flash Memory)- Contents of memory are not erased when power is turned off. Also called Permanent storage.

5. What is Random Access Memory / RAM?
A type of volatile memory that stores information temporarily so that it’s available to the CPU.

6. What are the different types of RAM?
Dynamic RAM- A memory chip that needs to be refreshed periodically or it loses its data
Synchronous DRAM (SDRAM)- Synchronized with the computer’s system clock
Rambus DRAM (RDRAM)- Uses fast bus to send and receive data within one clock cycle. Faster than SDRAM
Double Data Rate SDRAM (DDR SDRAM)- A type of SDRAM that can send and receive data within one clock cycle.

7. What is a memory module?
A narrow printed circuit board that holds memory chips
Three types
Single Inline Memory Module (SIMM)- 30 or 72 pin connectors. Available in 1MB, 4MB, 16MB, and 32MB versions (1980s to the late 1990s.)
Dual Inline Memory Module (DIMM)- 168 or 184 pin connectors. Available in 8MB, 16MB, 32MB, 64MB, 128MB, 256MB, and 512MB versions
Rambus Inline Memory Module (RIMM) (184-pin connector)

8. DIMM
RIMM
NOTEBOOK DIMM
SIMM

9. HOW DOES RAM WORK?
CHIPS CONTAIN MEMORY LOCATIONS CALLED MEMORY ADDRESSES
THE CPU STORES AND RETRIEVES DATA BY GOING TO THE MEMORY ADDRESSES

10. HOW RAM WORKS
MONITOR
WEB
RAM
KEYBOARD
CPU

11. Memory
Memory is a collection of cells, each with a unique physical address
The size of a cell is normally a power of 2, typically a byte today. 11

12. What is cache memory?
Primary cache (Level 1 or L1)- Located within the CPU chip, it is the memory the microprocessor uses to store frequently used instructions and data.
Secondary cache (Level 2 or L2; Backside Cache)- Located near the CPU, it is the memory between the CPU and RAM
Faster than RAM

13. What is virtual memory? FULL Part of the hard disk is reserved as RAM
When RAM modules become full the CPU accesses the hard disk to store and retrieve data
Slower than RAM
FULL

14. Memory Hierarchy
The memory unit is an essential component in any digital computer since it is needed for storing programs and data
Not all accumulated information is needed by the CPU at the same time
Therefore, it is more economical to use low-cost storage devices to serve as a backup for storing the information that is not currently used by CPU

15. Memory Hierarchy
Computer Memory Hierarchy is a pyramid structure that is commonly used to illustrate the significant differences among memory types.
The memory unit that directly communicate with CPU is called the main memory
Devices that provide backup storage are called auxiliary memory
The memory hierarchy system consists of all storage devices employed in a computer system from the slow by high-capacity auxiliary memory to a relatively faster main memory, to an even smaller and faster cache memory

16. MEMORY HIERARCHY
Memory Hierarchy is to obtain the highest possible access speed while
minimizing the total cost of the memory system
Auxiliary memory
Magnetic
tapes
I/O
Main
processor
memory
Magnetic
disks
CPU
Cache
memory
Register
Cache
Main Memory
Magnetic Disk
Magnetic Tape

17. Memory Hierarchy
The main memory occupies a central position by being able to communicate directly with the CPU and with auxiliary memory devices through an I/O processor
A special very-high-speed memory called cache is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate

18. Memory Hierarchy
CPU logic is usually faster than main memory access time, with the result that processing speed is limited primarily by the speed of main memory
The cache is used for storing segments of programs currently being executed in the CPU and temporary data frequently needed in the present calculations
The typical access time ratio between cache and main memory is about 1to7
Auxiliary memory access time is usually 1000 times that of main memory

20. Memory devices
A memory device is a gadget that helps you record information and recall the information at some later time.
Example:

21. Memory devices (cont.) Requirement of a memory device:
Example:
A memory device must have more than 1 states
(Otherwise, we can't tell the difference)
Memory device in state 0
Memory device in state 1

22. The switch is a memory device
The electrical switch is a memory device:
The electrical switch can be in one of these 2 states:
off (we will call this state 0)         
on (we will call this state 1)

23. Memory cell used by a computer
One switch can be in one of 2 states
A row of n switches:
can be in one of 2n states !

24. Memory cell used by a computer (cont.)
Example: row of 3 switches
A row of 3 switches can be in one of 23 = 8 states.
The 8 possible states are given in the figure above.

25. Representing numbers using a row of switches
We saw how information can be represented by number by using a code (agreement)
Recall: we can use numbers to represent marital status information:
0 = Sunny
1 = Rainy
2 = Snow
3 = cloudy

26. Representing numbers using a row of switches (cont.)
We can represent each number using a different state of the switches.
Example:

27. Representing numbers using a row of switches (cont.)
To complete the knowledge on how information is represented inside the computer, we will now study:
The representation scheme has a chic name:
How to use the different states of the switches to represent different numbers
the binary number system

28. The binary number system
The binary number system uses 2 digits to encode a number:
That means that you can only use the digits 0 and 1 to write a binary number
Example: some binary numbers
0 = represents no value
1 = represents a unit value 1 10 11
1010          
and so on.

29. The binary number system (cont.)
Now you should understand how the different states of these 3 switches represent the numbers 0-7 using the binary number system:

30. A cute binary number joke
Try to understand this joke:
(Read: there are binary 10 (= 2) types of people: those who understand binary (numbers) and those who don't)

31. A cute binary number joke (cont.)
A knock off joke:

32. What does all this have to do with a computer ?
Recall what we have learned about the Computer RAM memory:
The RAM consists of multiple memory cells:
Each memory cell stores a number

33. What does all this have to do with a computer ? (cont.)
The connection between the computer memory and the binary number system is:
The computer system uses the binary number encoding to store the number
Example:

34. What does all this have to do with a computer ? (cont.)
Note: the address is also expressed as a binary number
A computer can have over 4,000,000,000 bytes (4 Gigabytes) of memory.
So we need a 32 bites to express the address

35. Computer memory A computer is an electronic device
Structure of a RAM memory:
The RAM memory used by a computer consists of a large number of electronic switches
The switches are organized in rows
For historical reason, the number of switches in one row is 8

36. Computer memory (cont.)
Details
In order to store text information in a computer, we need to encode:
26 upper case letters ('A', 'B', and so on)
26 lower case letters ('a', 'b', and so on)
10 digits ('0', '1', and so on)
20 or so special characters ('&', '%', '$', and so on)
for a total of about 100 different symbols
The nearest even power 2n that is larger than 100 is:
27 = 128 ≥ 100
For a reason beyond the scope of this course, an 8th switches is added

37. Computer memory (cont.)
This is was a portion of the RAM memory looks like:
What information is stored in the RAM memory depends on:
The type of data (this is the context information)
Example of types: marital status, gender, age, salary, and so on.
This determines the encoding scheme used to interpret the number

38. Computer memory jargon:
bit = (binary digit) a smallest memory device
A bit is in fact a switch that can remember 0 or 1
(The digits 0 and 1 are digits used in the binary number system)
Byte = 8 bits
A byte is in fact one row of the RAM memory
KByte = kilo byte = 1024 (= 210) bytes (approximately 1,000 bytes)
MByte = mega byte = (= 220) bytes (approximately 1,000,000 bytes)
GByte = giga byte = (= 230) bytes (approximately 1,000,000,000 bytes)
TByte = tera byte

39. Combining adjacent memory cells
A byte has 8 bits and therefore, it can store:
28 = 256 different patterns
(These 256 patterns are: , , , , )

40. Combining adjacent memory cells (cont.)
Each pattern can are encoded exactly one number:
Therefore, one byte can store one of 256 possible values
(You can store the number 34 into a byte, but you cannot store the number 456, the value is out of range)
= 0
= 1
= 2
= 3
...
= 255

41. Main Memory
Most of the main memory in a general purpose computer is made up of RAM integrated circuits chips, but a portion of the memory may be constructed with ROM chips
RAM– Random Access memory
Integated RAM are available in two possible operating modes, Static and Dynamic
ROM– Read Only memory

42. Main Memory
A RAM chip is better suited for communication with the CPU if it has one or more control inputs that select the chip when needed
The Block diagram of a RAM chip is shown next slide, the capacity of the memory is 128 words of 8 bits (one byte) per word

43. SRAM vs DRAM Summary Tran. Access
per bit time Persist? Sensitive? Cost Applications
SRAM 6 1X Yes No 100x cache memories
DRAM 1 10X No Yes X Main memories,
frame buffers

44. RAM Read/write memory, that initially doesn’t contain any data
The computing system that it is used in usually stores data at various locations to retrieve it latter from these locations
Its data pins are bidirectional (data can flow into or out of the chip via these pins), as opposite to those of ROM that are output only
It loses its data once the power is removed, so it is a volatile memory
It has a directional select signal R/W’; When R/W’=1, the chip outputs data to the rest of the circuit; when R/W’ = 0 it inputs data from the rest of the circuit

45. Random-Access Memory Types Different between SRAM & DRAM
Static RAM (SRAM)
Each cell stores bit with a six-transistor (Diode) circuit.
Retains value indefinitely, as long as it is kept powered.
Relatively insensitive to disturbances such as electrical noise.
Faster and more expensive than DRAM.
Dynamic RAM (DRAM)
Each cell stores bit with a capacitor and transistor.
Value must be refreshed every ms.
Sensitive to disturbances.
Slower and cheaper than SRAM.

46. Random-Access Memory RAM is packaged as a chip.
Key features
RAM is packaged as a chip.
Basic storage unit is a cell (one bit per cell).
Multiple RAM chips form a memory.

47. ROM
ROM is used for storing programs that are PERMENTLY resident in the computer and for tables of constants that do not change in value once the production of the computer is completed
The ROM portion of main memory is needed for storing an initial program called bootstrap loader, witch is to start the computer software operating when power is turned off

48. ROM Data is programmed into the chip using an external ROM programmer
The programmed chip is used as a component into the circuit
The circuit doesn’t change the content of the ROM
Can be used as lookup tables to implement various functions
Used by PCs to store the instructions that form their Basic Input/Output System (BIOS)
When power is removed from a ROM chip, the information is not lost, so it is a nonvolatile type of memory
It has an OE (Output Enable) specific control pin. Both OE and CE must be enabled in order for the ROM to output data; otherwise its data output is tri-stated.

49. ROM Types
Masked ROM – programmed with its data when the chip is fabricated
PROM – programmable ROM, by the user using a standard PROM programmer, by burning some special type of fuses. Once programmed will not be possible to program it again.
EPROM – erasable ROM; the chip can be erased and chip reprogrammed; programming process consists in charging some internal capacitors; the UV light (method of erase) makes those capacitors to leak their charge, thus resetting the chip
EEPROM – Electrically Erasable PROM; it is possible to modify individual locations of the memory, leaving others unchanged; one common use of the EEPROM is in BIOS of personal computers.

50. ROM

51. Nonvolatile Memories Lose information if powered off.
DRAM and SRAM are volatile memories
Lose information if powered off.
Nonvolatile memories retain value even if powered off.
Generic name is read-only memory (ROM).
Misleading because some ROMs can be read and modified.
Types of ROMs
Programmable ROM (PROM)
Eraseable programmable ROM (EPROM)
Electrically eraseable PROM (EEPROM)
Flash memory
Firmware
Program stored in a ROM
Boot time code, BIOS (basic input/ouput system)
graphics cards, disk controllers.

52. Auxiliary Memory
The main memory construction is costly. Therefore, it has to be limited in size. The main memory is used to store only those instructions and data which are to be used immediately. However, a computer has to store a large amount of information. The bulk of information is stored in the auxiliary memory. This is also called backing storageor secondary storage. They include hard disk, floppy disks, CD-ROM, USB flash drives, etc.
When the electricity supply to the computer is off, all data stored in the primary storage is destroyed. On the other hand, this is not true for secondary storage. The data stored in secondary storage can be stored for the desired time.

53. SOME
T I P S
FOR
AUXILIARY MEMORY

54. Disk Geometry Disks consist of platters, each with two surfaces.
Each surface consists of concentric rings called tracks.
Each track consists of sectors separated by gaps.
tracks
surface
track k
gaps
spindle
sectors

55. Disk Geometry (Muliple-Platter View)
Aligned tracks form a cylinder.
cylinder k
surface 0
surface 1
surface 2
surface 3
surface 4
surface 5
spindle
platter 0
platter 1
platter 2

56. Computing Disk Capacity
Capacity = (# bytes/sector) x (avg. # sectors/track) x
(# tracks/surface) x (# surfaces/platter) x
(# platters/disk)
Example:
512 bytes/sector
300 sectors/track (on average)
20,000 tracks/surface
2 surfaces/platter
5 platters/disk
Capacity = 512 x 300 x x 2 x 5
= 30,720,000,000
= GB

57. Disk Operation (Single-Platter View)
The disk surface
spins at a fixed
rotational rate
By moving radially, the arm can position the read/write head over any track.
The read/write head
is attached to the end
of the arm and flies over
the disk surface on
a thin cushion of air.
spindle
spindle
spindle
spindle
spindle

58. Disk Operation (Multi-Platter View)
read/write heads
move in unison
from cylinder to cylinder
arm
spindle

59. Disk Access Time
Average time to access some target sector approximated by :
Taccess = Tavg seek + Tavg rotation + Tavg transfer
Seek time (Tavg seek)
Time to position heads over cylinder containing target sector.
Typical Tavg seek = 9 ms
Rotational latency (Tavg rotation)
Time waiting for first bit of target sector to pass under r/w head.
Tavg rotation = 1/2 x 1/RPMs x 60 sec/1 min
Transfer time (Tavg transfer)
Time to read the bits in the target sector.
Tavg transfer = 1/RPM x 1/(avg # sectors/track) x 60 secs/1 min.

60. Disk Access Time Example
Given:
Rotational rate = 7,200 RPM
Average seek time = 9 ms.
Avg # sectors/track = 400.
Derived:
Tavg rotation = 1/2 x (60 secs/7200 RPM) x 1000 ms/sec = 4 ms.
Tavg transfer = 60/7200 RPM x 1/400 secs/track x 1000 ms/sec = 0.02 ms
Taccess = 9 ms + 4 ms ms
Important points:
Access time dominated by seek time and rotational latency.
First bit in a sector is the most expensive, the rest are free.
SRAM access time is about 4 ns/doubleword, DRAM about 60 ns
Disk is about 40,000 times slower than SRAM,
2,500 times slower then DRAM.

61. Cache memory When CPU needs to access memory, the cache is examined
If the word is found in the cache, it is read from the fast memory
If the word addressed by the CPU is not found in the cache, the main memory is accessed to read the word

62. Cache memory
The performance of cache memory is frequently measured in terms of a quantity called hit ratio
When the CPU refers to memory and finds the word in cache, it is said to produce a hit
Otherwise, it is a miss
Hit ratio = hit / (hit+miss)

63. Cache memory
The basic characteristic of cache memory is its fast access time,
Therefore, very little or no time must be wasted when searching the words in the cache
The transformation of data from main memory to cache memory is referred to as a mapping process, there are three types of mapping:
Associative mapping
Direct mapping
Set-associative mapping

64. Cache memory
To help understand the mapping procedure, we have the following example:

65. THE END
Any Questions?