Storage Hierarchy: Memory Types, File Systems, and Demonstration of Gray Code Encoder

1. ELE22MIC Lecture 19 • Microprocessor Storage Hierarchy • MAIN MEMORY • MASS STORAGE • MEDIA, DISK FILE SYSTEMS • Memory Management Unit • Memory Protection and Privilege Levels • 68HC11 Gray Code - Encoder Software Demonstration • Refer: http://thor.ee.latrobe.edu.au/~paulm

2. Mass Storage • It is necessary to be able to store large quantities of information (programs and data) for long periods of time and during periods of no power. • Old forms of storage: • paper tape, punched cards, magnetic tape - small capacity, very slow, hard to use. • Modern forms of storage: • hard disks • floppy disks • optical disks • magnetic tape - huge capacities, very fast, very versatile, easy to use.

3. Storage Hierarchy Typical Memory Heirarchy: Speed, Capacity, Memory type <1ns, <1KB, Register 2ns, 1MB, Cache <10ns, 64M-1GB, Main memory 1s, <2MB Floppy Disk 10ms, 5-300GB, Hard Disk 1s/10min, 650MB-4.7GB CDROM/RW/DVD 100s, 100MB-4TB Tape

4. Storage Hierarchy • Consider the following issues • Speed • Cost • Volatility • Protection Storage systems are organized in hierarchy as follows: Register, Cache, Main memory, Disk, CDRW/Tape

5. Cache • Main Memory Caching • copys data into faster storage systems to improving the system performance. • Modern PC systems currently don’t use memory cache as the overhead is greater than the benefit. • Cache use in hardware - its use ebbs & flows with the technology capacity - it is used when performance improvement can be gained. Used in Intel 386 through Pentium.

6. Cache Memory • Higher Speed than main memory • When CPU accesses memory for code & data that is held in a cache, a cache “hit” can occur and the information is directly accessed from the cache. • If the cache is filled, the MMU (may) determine the least recently used cache memory, and replace it with the newly accessed one. • In most computers the cache hit ratio is > 85%

7. Main Memory • Main Memory - Currently with PC333 and faster memory systems - R/W acces times are <3ns. • Based on capacitive storage elements • When using SDRAM, and later technology, DRAM refresh is managed on the memory controller without system intervention. • SRAM - as used in HCCOM • 85ns cycle time.

8. Secondary Storage- Floppy Disks • A floppy (flexible) disk is a flexible Mylar disk which has been uniformly coated with a ferro-magnetic compound. • Each side is organised in concentric circles called Tracks. • Each track is organised in divided into segments called sectors. • Historically Disks have been available in 8 inch, 5 1/4”, 3 1/2” dimensions.

9. Magnetic Surface Recording

10. Mass Storage - Floppy Disks (1)

11. Mass Storage - Floppy Disks (2) • A 5 1/4” floppy disk can contain: • two sides, • 80 tracks, • 9 sectors per track, • 512 bytes per sector. • The 5 1/4” floppy disk can contain: • 2 x 80 x 9 x 512 bytes = 640k bytes

12. Mass Storage - Floppy Disks (3) • Using similar formatting arrangements the 3 1/2” floppy disk can contain 2MB, but after formatting for IBM format only 1.44 MB. The File Allocation Table occupies this space.

13. Media Descriptor Byte

14. Mass Storage - Hard Disks (1) • Original hard disk interface ST506 was a very popular standard between disk vendors. • Originally developed by Seagate Technology • Data is read & written serially onto the disk surface in a similar manner to floppy disks. • 5mbit/sec data transfer rate • Separate controller & disk.

15. Mass Storage - Hard Disks (2) • In 1983 ESDI was developed. 15mbit/sec. • Higher density - 20 to 50 Sectors per track. • The defect map is stored in the drive. • The number of Cylinders, Heads & Sectors stored in ROM on the hard-disk.

16. Mass Storage - Hard Disks (3)

17. Mass Storage - Hard Disks (4) • Circa 1985 IDE was developed. • Transfer rate of 16M byte/sec. • Integrated hard-disk controller. • Enhanced IDE (EIDE) transfer rates: • UDMA - 33 MB/s; ATA 66, - 66MB/s • ATA 100 - 100MB/s • SCSI-Ultra320 SCSI 320MB/s, 2-4ms seek time • Serial ATA - SATA 1.5Gbit/s, 250GB capacity

18. File System Layout (1) • File Systems are stored on disks. • Most disks are divided up into one or more partitions Partition Tables

19. File System Layout (2) • Sector 0 of the disk is called the Master Boot Record (MBR) and is used to boot the computer (or run a boot loader to select the desired Operating System. eg: LILO, partition commander, etc). Partition Tables

20. File System Layout (3) • After the Master Boot Record comes the partition tables - information such as Start & Ending Addresses, File System type, etc. Partition Tables

21. File System Layout (4) • Each Partition contains a root directory, user files & directories - structure is dependent of file-system type. This example here shows a unix style file system. Partition Tables

22. File System Layout (5) • BOOT SECTOR - Occupies first sector of floppy disk/partition. Contains: jump to start of boot loader code, OEM name, bytes per sector, sectors per allocation unit (Cluster), number of FATs, number of root directory entries, number of logical sectors, medium descriptor byte, sectors per FAT, sectors per track, number of heads, number of hidden sectors, program to boot the Operating system Partition Tables

23. File System Layout (6) • MSDOS FAT - File Allocation Table: • The root directory points to the starting cluster of each file. • The entry in the FAT corresponding to that cluster points to the next cluster used for that file. • The entry -1 is used to signify end of file cluster chain. • The location of boot files IO.SYS, MSDOS.SYS & COMMAND.COM is fixed Partition Tables

24. Format (1) • Disks can have a logical structure imposed upon them called a format. • The format or file-system type is identified by numeric values in the partition table. • The format tells the operating system what to expect where • Where to find the root directory • Where is each file extent can be found • How to locate the operating system at boot time.

25. Format (2) • There is a huge list of different format types including: • FAT 12 - still used on IBM format floppy disks • FAT 16 - Used on hard disks < 2 GB • FAT 32 - Used on hard disks > 2GB • ISO9660 - Used on CDROMs (MS Jolliet) • NTFS - Newer NTFS allows for encrypted data. • Ext3fs - Used on newer unixes (allows journalling) • Minix, QNX, SCO,….etc • See fdisk under Linux for more info.

26. Root Directory • The root directory contains a list or table describing file attributes such as: names, sizes, dates & times, and where each file commences on the physical disk. • Where each file extent is found is specified using a File Allocation Table (in MSDOS) or an I-Node (under most Unixes). • Warning various fdisk programs number partitions differently. Even the one OS may use different labelling conventions. Be exceptionally cautious deleting partitions.

27. File Allocation Table • MSDOS File Allocation Table • FAT 16 -> 2^16 table entries -> 65536 clusters • FAT 32 -> 2^32 table entries -> 4177918 clusters • A cluster is a group of sectors allocated by one FAT entry - determined at format time - vary from 1 sector (512 bytes) to 128 sectors (64k) per cluster • Each FAT entry forms a link in a linked list pointing to the next cluster entry. A terminal value indicates end of cluster chain. • The Directory Entry includes a pointer to the first cluster and the file size.

28. Files • Each file is an abstraction • It is a block of bytes managed by the operating system. • Identified by complete path-name or file-name • Path refers to [device:] [directory/folder..] filename . ext • Operating systems services such as open, close, read, write and seek hide the underlying mechanisms and formats - Sectors, Tracks, controllers, CRCs / checksums, speed of rotation, head position, etc

29. Hardware Protection To ensure proper system operation protection is required for any shared resource. • Dual-Mode Privileged operation • I/O protection • Memory protection • CPU protection

30. Dual-Mode Privileged operation (1) • Ensure that by sharing system resources an incorrect program can not cause other programs to execute incorrectly • Provide hardware support to differentiate at least two modes of operation • A Mode bit is used in the hardware to indicate the current mode • Mode=0 for system mode • Mode=1 for user mode

31. Dual-Mode operation (cont) (2) • In User Mode: Direct execution of privileged operations are prevented. • Instead privileged operations are done on behalf of a user. • In user mode, privileged instructions such as I/O, set clock, etc. are not allowed. • A user calls the operating system through a call-gate to perform any privileged operation. The call gate prevents direct damage to operating system structures/stack.

32. Dual-Mode operation (cont) (3) • When an interrupt or error occurs, HW switches to monitor mode • Privileged instructions can be issued only in monitor mode • Monitor mode (Supervisor/system/ privileged mode): Hardware only allows privileged instructions (i.e. machine instructions which can cause harm) to be executed in monitor mode such instructions can be requested and are provided by system calls of the OS.

33. Dual-Mode operation (cont) (4) • Monitor mode (cont’d) • That is, a user may make a system call to the service provided by the OS. In doing this, a user is required to place the necessary parameters for each call in well defined registers (locations). • The execution of such specialized function (system) calls within a users program causes the users program to execute a special trap instruction, thereby switching from user mode to monitor mode and transfers control to the OS.

34. Dual-Mode operation (cont)

35. I/O Protection • All I/O instructions are privileged instructions • Must ensure that a user program could never gain control of the computer in monitor (supervisor) mode.

36. Memory Management Unit • The MMU hardware acts basically a sophisticated look-up table, configurable by the processor. • The MMU provides the translation from logical to physical addresses. • A word is defined as the basic addressable unit of a processor. • In a 16 bit processor, a word is 16 bits, in a 32 bit processor a word is 32 bits.

37. Memory Management Unit Logical Address Bus Physical Address Bus

38. Memory Management Unit • In systems using memory management, words of memory are grouped together to form pages. • An address can be considered to consist of a page number and a word number within that page. • The MMU translates the page number to a new page number, but leaves the word number unmodified.

39. Memory Management Unit Logical Address Bus Physical Address Bus

40. Memory Management Unit Logical Address : Physical Page Number:Word -> Physical Address

41. Memory Protection • Some registers within the MMU are used to distinguish the address of one program from another, e.g. • Base register – holds the smallest legal physical memory address • Limit register – contains the size of the range available to the program • Memory outside the defined range is protected - Accessing memory outside the selected range causes a GP Fault.

42. MMU : Logical Mem < Physical (1) • The simplest form of memory management is when the logical space is smaller than the physical memory present.

43. MMU : Logical Mem < Physical (2) Physical Page Number:Word Number -> Physical Address

44. MMU : Logical < Physical (3) • In this instance, the processor's logical address becomes the word number within a page and the MMU supplies the page number. The processor selects which page in physical memory its logical address corresponds to. • The physical page number and the word number together form the physical address for memory.

45. MMU : Logical < Physical (4)

46. MMU : Logical >= Physical (1) • For processor's with address spaces equal to or larger than the physical space, the memory management scheme becomes: • The MMU contains a translation table. • The translation table is accessible and configurable by the processor.

47. MMU : Logical >= Physical (2) • F

48. Protection Hardware (1) • Some processors, like the 8086 and 68020, have a special interface for a MMU (68851 in the case of the '020). • Other processors, like the Intel 80386, Motorola 68030 and 68040, have MMUs built-in. • If the processor was not designed to use an MMU, it will have no special support.

49. Protection Hardware (2) • The MMU must therefore be treated as a peripheral (I/O) device by the processor. • Thus the MMU must appear in the processor's address space. • The MMU must appear in the logical space of the processor and not the physical space of the system, otherwise the MMU may be 'lost’ (mapped out of addressable memory).

50. Protection Hardware (3) • However, since the MMU is in the logical space, it is no longer protected from tampering or corruption by a crashing program. • To solve this, some processors have two (or more) states of operation. • Supervisor mode • AKA Kernel / system / monitor mode • User mode