COSC 243. Memory and Storage Systems. Lecture 10 Memory and Storage Systems. COSC 243 (Computer Architecture)

Transcription

1 COSC 243 1

2 Overview This Lecture Source: Chapters 4, 5, and 6 (10 th edition) Next Lecture Control Unit and Microprogramming 2

3 Electromagnetic Induction Move a magnet through a coil to induce a current Lenz's law An induced current is always in such a direction as to oppose the motion or change causing it 3

4 Magnetic Tape Put a whole load of magnets beside each other in a big long string to make a tape The tape can be read by passing by a coil Make the magnets really small and stick them to the surface of a piece of plastic This is magnetic tape 4

5 Floppy Disks Disks Unwind the tape and join the ends to form a circle Surface passes the head at a tangent to the head If this is the case then if you pull a drive apart then you ll see: The floppy disk head The coils in the head The magnetic surface of the disk 5

6 Floppy Disk Drive Head coil 6

7 Floppy Disk 7

8 Floppy Disks But there are problems The speed that the data passes under the head Is a function of the head s distance from the centre Solutions The amount of media passing under the head is variable Slow the motor as the head moves outwards Multiple Zone Recording (more sectors on outer than inner tracks) Constant linear vs. constant angular velocity Advantages: Disadvantages: 8

9 Multiple Zone Multiple zone recording Also known as zone bit recording (ZBR) or zone- CAV recording (Z-CAV) Compromise between CAV and CLV Disk divided into zones Tracks in different zones have a different number of sectors Number of sectors in a particular zone is constant

10 Floppy Disks If the magnetic field does not fluctuate (a constant signal is on the disk) then no current is induced. So the sequences and cannot be detected Must encode an alternating field but this cannot be done because if you encode an alternating current you cannot encode data! 10

11 FM Encode a timing signal on the disk Frequency Modulation (FM, Single Density) Always encode the clock pulse then encode the bit pulse for 1, no pulse for 0 11

12 MFM Modified Frequency Modulation (double density) Store a 1 as no pulse then pulse Store a 0: If last bit was 0 then Pulse then no pulse Else last bit was 1 so No pulse no pulse FM uses two transitions per bit but MFM uses only one so more data is stored in the same physical space 12

13 Where Are You The next problem is that you don t know how far around the disk you are. Two solutions Hard sectoring (left) Soft sectoring (right) Index Hole Index Hole Sector Hole 13

14 Where Are You Or how far in or out you are (this one s easy) On power-up retract the head to track 0 The head is on a stepper motor Keep track of how far in (or out) of the disk you are Disk head 14

15 Speed Characteristics Seek time Average time for the drive to move the head from one track to another Rotational latency Time it takes for the disk to complete one revolution Access time Worst case: seek time + rotational latency Block transfer time How much data can move from the disk to the computer per second Where is the bottleneck? 15

16 Seek Time: Multi-Platters 16

17 Rotational Latency: Multi-Head Can be addressed with multiple heads per disk Floppy 5.25 Twiggy 17

18 Files The remaining problem is multiple files per disk The universal solution is to divide the disk into tracks and sectors Tracks are concentric rings Also known as cylinders if there are multiple platters Sectors are a wedge of that ring Files are chains of sectors Not necessarily sequential on disk 18

19 Magnetic Disk 19

20 Sectoring The formatting of the disk starts at the index hole and writes bytes to the disk. This includes not just data, but also information at the start of each sector When the disk spins the controller looks for the sector header and then reads-from or writes-to the disk 20

21 Format of a Floppy Disk Track 21

22 Moving Media Characteristics Number of platters Physical disks to write on and read from Single sided or two sided Sides per platter Number of tracks (cylinders) per disk Related to the diameter of a platter and width of a track Number of sectors per track Related to storage density Number of bytes per sector Typically 512 on SATA / ATA / IDE 22

23 CD-ROM Plastic resin disk Covered by lacquered polished aluminium surface Reflective surface has pits. Laser reads presence or absence of reflection Constant linear velocity. Consistent density throughout the surface. A single spiral track Typical spec: Length of spiral: 5.27 km Apparent number of tracks: 20, minutes Data streamed at KB/sec Capacity: MB Encoding: 1 is encoded as a transition between pit and no-pit. 0 is encoded as no transition 23

24 CD-ROM 24

25 The Memory Hierarchy Tradeoffs Shorter access time Greater cost per bit Greater capacity Smaller cost per bit Longer access time Going down the hierarchy diagram Decreasing cost per bit Increasing capacity Increasing access time Decreasing frequency of access by CPU Registers (B) Cache (MB) Main Memory (GB) Magnetic Disk (TB) Optical Disk (TB) Tape (TB) Memory Hierarchy 25

26 Properties of Main Memory Random access (RAM) Same access time to access any location Synonymous with read-write memory Cycle time Minimum time required between successive reads or writes Access time Delay between the start and finish of a memory operation Volatility Usually measured in nanoseconds (10-9 ) or picoseconds (10-12 ) Semiconductor RAM memories lose data on power off Price Static RAM (SRAM) or Dynamic RAM (DRAM)? 26

27 Static RAM Uses latches to store bits i.e flip-flops Very fast Used within the CPU for cache and registers More expensive than DRAM Lecture 7 - Memory and Storage Systems 27

28 Dynamic Ram Value stored as a charge on a capacitor No charge = 0 Charge = 1 A MOSFET connects each capacitor to a grid of wires for addressing Capacitors are leaky Need to be periodically refreshed A read operation drains the capacitor DRAM is smaller and cheaper than SRAM But slower 28

29 A Typical Dynamic RAM 29

30 Other Types of Memory ROM (Read Only Memory) Masked PROM (Programmable ROM) Fuses that can be blown once EPROM (Erasable PROM) UV light whole-chip erasable EEPROM (Electrically Erasable PROM) Electrically byte erasable Flash Block erasable 30

31 Locality of Reference Programs tend to access code and data that is closeby in terms of memory addresses In other words only a small part of the program and data is being accessed in a short time interval The small part changes as the program executes Memory is slow compared to instruction times Memory becomes more expensive as it gets faster Compromise Include a small amount of very high speed (expensive) memory to temporarily hold a portion of the memory being accessed 31

32 Look Through Cache If the addressed location is in cache, it is used. Otherwise a block of memory is transferred from memory to cache and CPU Stale Data (the cache has the wrong value) Data stored in cache RAM and address in tag RAM Cache RAM CPU Cache Controller Main Memory Tag RAM 32

33 Look Aside Cache Main memory and cache see the memory operation Cache terminates it early if it has the result Stale data (the cache has the wrong value) CPU Cache RAM Cache Controller Main Memory Tag RAM 33

34 Fully-Associative Cache Main memory is divided into lines of (often 64) bytes Each can be stored anywhere in the cache RAM Tag RAM stores which memory lines are where Line m Line 2 Line 1 Line 0 Main Memory Line m Line 2 Line 1 Line 0 Cache RAM 34

35 Direct Map Cache Main memory is divided into pages (of cache RAM size) Each page is divided into lines All Line-0 are cached in the same cache RAM etc Tag RAM stores which memory pages are where Line m Line 2 Line 1 Line 0 Main Memory Line m Line 2 Line 1 Line 0 Line m Line 2 Line 1 Line 0 Cache RAM 35

36 Set Associative Cache Direct map into a set then associate within that set Tag RAM stores what is where Line m Line 2 Line 1 Line m Line m Line 2 Line 2 Line 1 Line 1 Line 0 Line 0 Line m Line 2 Line 1 Line 0 Line m Line 2 Line 1 Line 0 Line 0 Line are Fully-Associative Pages are Direct Mapped 36

37 Cache Write Policy Write-Back (Write-Behind) CPU writes go to cache Cache then writes to main memory Dirty Data in cache Only done when the system bus is free Complex and expensive Write-Through CPU writes go to main memory and cache 37

38 Cache Eviction Policy LRU least recently used FIFO first in first out LFU least frequently used 38

39 Cache Memory Performance is measured by hit ratio Typical hit ratios are How do you compute the performance of such a machine? 39

40 The Memory Hierarchy Registers Fast, SRAM, internal to CPU Cache SRAM, internal or external to CPU Main Memory Slower, DRAM, cheap, external to CPU Magnetic Disk Very slow, semi-permanent storage, very cheap Optical Disk Movable, slow, permanent, nearly free Tape Ancient history now? Registers (B) Cache (MB) Main Memory (GB) Magnetic Disk (TB) Optical Disk (TB) Tape (TB) Memory Hierarchy 40