ELE22MIC Lecture 21

1
ELE22MIC Lecture 21

• 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
• lt1ns, lt1KB, Register
• 2ns, 1MB, Cache
• lt10ns, 64M-1GB, Main memory
• 1s, lt2MB 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 dont 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 gt 85

7
Main Memory

• Main Memory - Currently with PC333 and faster
  memory systems - R/W acces times are lt3ns.
• 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.

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Magnetic Surface Recording
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Mass Storage - Floppy Disks (1)
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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

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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.

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Media Descriptor Byte
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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.

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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.

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Mass Storage - Hard Disks (3)
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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

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File System Layout (1)

• File Systems are stored on disks.
• Most disks are divided up into one or more
  partitions

Partition Tables
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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
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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
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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
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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
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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
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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.

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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 lt 2 GB
• FAT 32 - Used on hard disks gt 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.

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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.

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File Allocation Table

• MSDOS File Allocation Table
• FAT 16 -gt 216 table entries -gt 65536 clusters
• FAT 32 -gt 232 table entries -gt 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.

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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

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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

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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
• Mode0 for system mode
• Mode1 for user mode

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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.

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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.

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Dual-Mode operation (cont) (4)

• Monitor mode (contd)
• 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.

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Dual-Mode operation (cont)
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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.

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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.

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Memory Management Unit
Logical Address Bus
Physical Address Bus
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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.

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Memory Management Unit
Logical Address Bus
Physical Address Bus
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Memory Management Unit
Logical Address
Physical Page NumberWord -gt Physical Address
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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.

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MMU Logical Mem lt Physical (1)

• The simplest form of memory management is when
  the logical space is smaller than the physical
  memory present.

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MMU Logical Mem lt Physical (2)
Physical Page NumberWord Number -gt Physical
Address
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MMU Logical lt 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.

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MMU Logical lt Physical (4)
46
MMU Logical gt 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.

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MMU Logical gt Physical (2)

• F

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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.

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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).

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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

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Protection Hardware (4)

• When executing in monitor mode, the operating
  system has unrestricted access to both monitor
  and users memory
• The load instruction for the base and limit
  registers are privileged instructions

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Performing Privileged I/O

• How does a user program perform I/O, if I/O
  instructions are privileged?
• System calls request OS to do I/Os
• Trap to a specific location using an interrupt
  vector
• Interrupt service routine sets the mode to system
• Then the routine verifies the parameters are
  correct before executing the request
• Set the mode to user before returning control to
  the next instruction following the system call

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Protection Rings on 386
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Operating System Services

• OS services can include
• Program Management
• Load, Execution, Removal
• I/O operations
• File-system manipulation
• Communication
• Error detection handling
• .

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Operating System Services (contd)

• Resource management
• Allocating/De-allocating resources to the
  processes
• Account keeping Recording the usage of
  resources for various purposes (e.g. billing,
  statistics, etc.)
• Protection Ensuring that all access to system
  resources is controlled

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System Calls

• System calls provide the interface between a
  running program and the Operating system
• Three normal methods are used to pass parameters
  between a running program and the Operating
  system
• Through the stack
• Through registers
• Through a table, the table address is in a
  register

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Example Gray Code Input
Pseudo-code description Start Loop Using
polling - Get input from port A Mask out all
bits except bit 0 and bit 1 to get Gray code. If
Gray code value has changed from previous Gray
value then process the change in Gray code value,
else loop again. Map Gray code to binary
value Work out difference to previous value 1
or -1. Process the /- 1 difference - add it
to the value to be changed. Save the current
value as the previous value Loop to start
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Gray Code Conversion Software (1)
RAM EQU 2000 org RAM jmp
start 68HC11 Equates REGBAS EQU 1000
Starting address for register block PORTA EQU
00 Port A INPUTPA0..2,
OUTPUTPA3..6, I/O PA7 Buffalo
Equates OUTLHF EQU FFB2 Print left
half OUTRHF EQU FFB5 Print right
half CRLF EQU FFC4 Print CRLF
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Gray Code Conversion Software (2)
PrevGrayCode RMB 1 The Previous Gray
Code CurrBinCode RMB 1 Gray Code
converted to binary PrevBinCode RMB 1
Previous Binary Code COUNTER RMB 1
Number to increment/decrement start LDAA
0 STAA COUNTER LDX
PORTAREGBAS LDAB 0, X ANDB
00000011 Mask all but bottom 2 bits
STAB PrevGrayCode Previous gray value -
initialisation
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Gray Code Conversion Software (3)
Initialisations BSR Gray2Bin
Convert Gray -gt Binary code STAB
CurrBinCode Binary equivalent of gray
value STAB PrevBinCode Previous
Value of binary value LoopStart LDX
PORTAREGBAS LDAB 0, X ANDB
00000011 Mask all but bottom 2 bits
CMPB PrevGrayCode Has the reading
changed? BEQ LoopStart
No, so read again STAB PrevGrayCode
else Gray Code has changed
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Gray Code Conversion Software (4)
Convert Gray code to Binary BSR
Gray2Bin Convert Gray 2 Binary - uses
Acc. B STAB CurrBinCode SUBB
PrevBinCode subtract previous Valid
differences are -1 or 3 down 1, or -3 up 1
CMPB 1 BEQ Up1 CMPB
-1 BEQ Dn1 CMPB -3
BEQ Up1 CMPB 3 BEQ
Dn1 JSR BigStepError If we get
here we have stepped by /-2 BRA
Continue
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Gray Code Conversion Software (5)
Up1 Subroutine Up1 Knob has been turned
clockwise Increment - but limit up to 255 - No
wrapping back to 0 LDAA COUNTER
CMPA FF are we already at the
maximum? BEQ ALREADY_MAX Branch if
already maximum. INCA
else increment STAA COUNTER
and save result ALREADY_MAX BRA
Continue
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Gray Code Conversion Software (6)
Subroutine Dn1 - Knob has been turned
anti-clockwise Dn1 Decrement - limit downto
0 - no wrapping to -1 LDAA COUNTER
CMPA 0 Are we already at
the minimum? BEQ ALREADY_ZERO
Branch if zero DECA
Else Decrement STAA COUNTER and
save result ALREADY_ZERO
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Gray Code Conversion Software (7)
Continue Save new binary value in PrevCode
LDAB CurrBinCode Get new binary code
STAB PrevBinCode make it previous
code also Output the hex value to the
terminal LDAA COUNTER JSR
OutHexByte JSR CRLF
output carriage return - line feed BRA
LoopStart
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Gray Code Conversion Software (8)

Convert Gray code in Accumulator B to
Binary
Gray2Bin CMPB 2 Is Bit 1
set? BHS XorBit1 Branch if 2 or 3
to XorBit1 RTS Do nothing XorBit1
EORB 1 This inverts Bit 0
RTS We return with Acc.B Binary code
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Gray Code Conversion Software (9)
OutHexByte Outputs to console the Hexadecimal
byte passed in Acc A PSHB Save Acc. B
on stack PSHA Save Acc. A twice.
PSHA JSR OUTLHF OUTput Left
HalF - high nybble PULA Recover Acc.A
- byte to display JSR OUTRHF OUTput
Right HalF - low nybble PULA Restore
Acc.A PULB Restore Acc.B
RTS ReTurn from Subroutine
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Acknowledgments

• Next Lecture Swapping Virtual Memory
• Motorola M68HC11 Reference Manual
• The Indispensable PC Hardware Handbook,
  Hans-Peter Messmer, ISBN 0-201-87697-3
• Floppy Disk images from IEEE Electronics
  Engineers Handbook, 4th Edition, Donald
  Christiansen, ISBN 0-07-021862-5.
• Seng Gohs original lecture notes on Memory
  Management