ELE22MIC Lecture 19 Microprocessor Storage Hierarchy –MAIN MEMORY –MASS STORAGE MEDIA, DISK FILE...

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

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.

Storage HierarchyTypical 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

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

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.

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%

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.

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.

Magnetic Surface Recording

Mass Storage - Floppy Disks (1)


• 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


• 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.

Media Descriptor ByteMedia Descriptor Byte values :

Byte(hex)Medium #sides SPT # tracks Capacity DOS Ver.

fe 8" 1 2 77 77 KB 1.00

fe 5 1/4" 1 8 40 160 KB 1.00

fc 5 1/4" 1 9 40 180 KB 2.00

fe 8" 1 6 77 231 KB 1.00

fe 8" 1 8 77 308 KB 1.00

ff 5 1/4" 2 8 40 320 KB 1.10

fa 3 1/2 " 1 8 80 320 KB 1.00

fa 5 1/4" 1 8 80 320 KB 1.00

fd 5 1/4" 2 9 40 360 KB 2.00

fb 3 1/2 " 2 8 80 640 KB 1.00

fb 5 1/4" 2 8 80 640 KB 1.00

f9 3 1/2 " 2 9 80 720 KB 3.20

f9 5 1/4" 2 15 80 1200 KB 3.00

f0 3 1/2 " 2 18 80 1440 KB 3.20

fd 8" 2 26 77 1960 KB 1.00

f8 hard disk 5 MB+ 2.00

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.


• 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.


• 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

File System Layout (1)

• File Systems are stored on disks.

• Most disks are divided up into one or more partitions

Partition Tables


• 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


• After the Master Boot Record comes the partition tables - information such as Start & Ending Addresses, File System type, etc.

Partition Tables


• 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


Partition Tables

• 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

• 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

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.

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.

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.

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.

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

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

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

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.


• 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.

– 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.


Dual-Mode operation (cont)

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.

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.


LogicalAddressBus

Physical AddressBus


• 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.


LogicalAddressBus

Physical AddressBus

Memory Management UnitLogical Address :

Physical Page Number:Word -> Physical Address

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.

MMU : Logical Mem < Physical (1)

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

Physical Page Number:Word Number -> Physical Address

MMU : Logical Mem < Physical (2)

• 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.

MMU : Logical < Physical (3)

MMU : Logical < Physical (4)

• 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.

MMU : Logical >= Physical (1)

• F

MMU : Logical >= Physical (2)

• 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.

Protection Hardware (1)

• 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).


• 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



• 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

• 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

Performing Privileged I/O

Protection Rings on 386+

Operating System Services

• OS services can include:• Program Management

– Load, Execution, Removal

• I/O operations

• File-system manipulation

• Communication

• Error detection & handling– ….

Operating System Services (cont’d)

• 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

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

Example: Gray Code InputPseudo-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

Gray Code Conversion Software (1)

RAM EQU $2000 org RAM jmp start

; 68HC11 EquatesREGBAS EQU $1000 Starting address for register blockPORTA EQU $00 ; Port A INPUT:PA0..2, OUTPUT:PA3..6, & I/O PA7; Buffalo Equates:OUTLHF EQU $FFB2 ; Print left halfOUTRHF EQU $FFB5 ; Print right halfCRLF EQU $FFC4 ; Print CRLF


PrevGrayCode RMB 1 ; The Previous Gray CodeCurrBinCode RMB 1 ; Gray Code converted to binaryPrevBinCode RMB 1 ; Previous Binary CodeCOUNTER RMB 1 ; Number to increment/decrementstart: LDAA #0 STAA COUNTER

LDX #PORTA+REGBAS LDAB 0, X ANDB #%00000011 ; Mask all but bottom 2 bits STAB PrevGrayCode ; Previous gray value - initialisation


Initialisations: BSR Gray2Bin ; Convert Gray -> Binary code STAB CurrBinCode ; Binary equivalent of gray value STAB PrevBinCode ; Previous Value of binary value

LoopStart: LDX #PORTA+REGBAS 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


; 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


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 resultALREADY_MAX: BRA Continue


; Subroutine Dn1 - Knob has been turned anti-clockwiseDn1: ; 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 resultALREADY_ZERO:


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


************************************************** 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 nothingXorBit1: EORB #1 ; This inverts Bit 0 RTS ; We return with Acc.B = Binary code


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

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 Goh’s original lecture notes on Memory Management

ELE22MIC Lecture 19 Microprocessor Storage Hierarchy –MAIN MEMORY –MASS STORAGE MEDIA, DISK FILE...

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