Evolution of Memory Management in MAC OS

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Evolution of Memory Management in MAC OS

• Meenakshi Thuniki

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Memory Management

• Memory management hardware (MMUs, RAM, etc.)
• Operating system memory management (virtual
  memory, protection)
• Application memory management (allocation,
  deallocation, garbage collection)

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Memory Management Hardware

• Memory Management Units
• Random Access Memory

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Operating System Memory Management

• Virtual Memory
• It is an addressing scheme implemented in
  hardware and software that allows non-contiguous
  memory to be addressed as if it is contiguous
• Protection
• Many operating systems support protection of
  memory pages. Individual pages may be protected
  against a combination of read, write or execute
  accesses by a process

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Application Memory Management

• Allocation
• Allocation is the process of assigning resources
• Deallocation
• In manual memory management, to free or
  deallocate an object is to tell the memory
  manager that it is no longer needed
• Garbage Collection
• Garbage collection (GC), also known as automatic
  memory management, is the automatic recycling of
  dynamically allocated memory

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Organization of Memory in Mac OS

• System partition
• System Heap
• System Global Variables
• Application partition
• Application Stack
• Application Heap
• Application Global Variables and A5 world

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Organization of Memory in Mac OS Contd..

• Temporary memory
• Virtual memory
• In system software version 7.0 and later
  Macintosh computers take advantage of the OS
  feature known as virtual memory, by which the
  machine have a logical address space that extends
  beyond the limits of the available physical
  memory
• Addressing modes
• Latest operating system supports 32 bit
  addressing
• Earlier versions supported 24 bit addressing

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LISA OS (Office System)

• Introduced in January 1983
• Motorola 68000 CPU at a 5 MHz clock rate and had
  1 Mibytes RAM
• The Lisa operating system featured cooperative
  (non-preemptive) multitasking and virtual memory
• Virtual memory coupled with a fairly slow disk
  system made the system performance seem sluggish
  at times
• The Lisa supported a sophisticated hardware based
  memory management strategy
• This strategy allowed Lisa programs to believe
  they could access more memory than there really
  was in the Lisa

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Memory Management Unit - Lisa

• 24 bit logical address with 16 Mbytes of logical
  address space and this space is divided into 128
  segments
• Each segment consists of up to 128 Kbytes in
  blocks of 512 bytes
• Upper 7 bits of the 24 bits is segment number and
  the remaining 17 bits are offset within that
  segment
• To access actual locations in the Lisa hardware,
  logical addresses are translated into physical
  addresses by a section of logic on the processor
  board known as the Memory Management Unit (MMU)
• The MMU hardware permits the operating system to
  control the entire relocation process. MMU
  prevents a particular process from accessing
  areas of memory outside of the portion assigned
  to it

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MMU Lisa Contd..

• The Lisas RAM memory occupies 2 Mbytes of
  physical address space. This would imply that
  only 16 segments, each of size 128 Kbytes, could
  be used
• Each segment does not necessarily occupy the full
  128 Kbytes allotted to it in logical address
  space. Each segment can be mapped into as little
  as one 512-byte block and 16 logical segments can
  map into 2-Mbyte physical memory
• Areas larger than 128 Kbytes can be accommodated
  by treating multiple logically contiguous
  segments as one segment
• The translation of a logical address to a
  physical address by the MMU is performed on a
  segment-by-segment basis
• Associated with each segment in the MMU is its
  origin which is the 12-bit block number in the
  physical address space where the corresponding
  segment begins

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MMU Lisa Contd..

• The logical block number from bits 9-16 of the
  logical address is added to the segments origin
  value to produce the physical block number to be
  accessed
• The nine displacement bits from the logical
  address translate directly into the physical
  displacement
• The MMU also performs access checks to verify
  that the requested access is allowed. The MMU
  checks each access to ensure that it does not
  exceed the bounds of the specified segment
• In addition, an attempt to access a segment which
  is not mapped or an attempt to write into a
  read-only segment will generate an access
  violation
• These access violations trap to the operating
  system for handling

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System 6
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MultiFinder

• It provided each application CPU time, and a way
  for windows from different applications to
  co-exist
• When an application was activated, all of its
  windows were brought forward as a single layer
• It also provided a way for applications to
  supply their memory requirements ahead of time,
  so that MultiFinder could allocate a chunk of RAM
  to each according to need

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Connectix

• Connectix Corporation was a software and hardware
  company, noted for having released innovative
  products that were either made obsolete as Apple
  Computer incorporated the ideas into system
  software, or were sold to other companies once
  they become popular
• MODE32 software, which allowed 32-bit addressing
  on "32-bit-dirty" Machintosh systems (later
  bought by Apple and distributed for free, at
  least in part to settle a class-action lawsuit
  brought by customers who demanded to know why
  their 32-bit 68020 microprocessors could not
  access more than 8 megabytes of RAM)

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System 7
First to use MMU. VM features such as protected
address space, memory mapped files, page locking,
shared memory, etc. were not present
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Multitasking

• Multitasking is a method by which multiple tasks,
  also known as processes, share common processing
  resources such as a CPU
• In the case of a computer with a single CPU, only
  one task is said to be running at any point in
  time, meaning that the CPU is actively executing
  instructions for that task
• Multitasking solves the problem by scheduling
  which task may be the one running at any given
  time, and when another waiting task gets a turn

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Multitasking Contd..
Operating systems may adopt one of many different
scheduling strategies, which generally fall into
the following categories

Mac OS used cooperative multitasking to enable
the running of multiple applications
simultaneously.
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Scheduling Categories

• In multiprogramming systems, the running task
  keeps running until it performs an operation that
  requires waiting for an external event (e.g.
  reading from a tape) or until the computer's
  scheduler forcibly swaps the running task out of
  the CPU. Multiprogramming systems are designed to
  maximize CPU usage
• In Cooperative Multitasking / time-sharing
  systems, the running task is required to
  relinquish the CPU, either voluntarily or by an
  external event such as a hardware interrupt. Time
  sharing systems are designed to allow several
  programs to execute apparently simultaneously
• In real-time systems, some waiting tasks are
  guaranteed to be given the CPU when an external
  event occurs. Real time systems are designed to
  control mechanical devices such as industrial
  robots, which require timely processing

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Finder and MultiFinder

• The Finder is the default application program
  used on the MAC OS and MAC OSX operating systems
  that is responsible for the overall
  user-management of files, disks, network volumes
  and the launching of other applications
• As such the Finder acts like the shell on other
  operating systems, but using a graphical user
  interface
• The Finder is the first program a user interacts
  with after booting a Mac (and potentially logging
  in), and as such it is responsible for the
  general look and feel of the machine
• Finder on System 1.0 4.1 could only run one
  application at a time, through special
  application shells such as Switcher
• Early versions of the Finder would shut down
  whenever another program was launched, due to the
  single-tasking nature of the original Mac OS

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Finder and MultiFinder Contd..

• System 5 added MultiFinder, an add-on replacement
  for the Finder which could run several programs
  at once. Time was given to the background
  applications only when the foreground (or
  "running") applications gave it up (cooperative
  multitasking)
• System 5 and 6 came with option of selecting
  Finder or MultiFinder. If MultiFinder were
  selected, the Finder and its functions continued
  to run when an application was launched. The
  MultiFinder environment allowed users to see past
  the windows of running applications to view
  Finder icons such as the Trash, or the windows of
  other applications running in the background
• This was at the expense of more RAM, so it was
  not always possible to multitask if one of the
  applications required too large a portion of the
  available RAM
• In System 7.0, the MultiFinder replaced the
  original Finder, and Apple renamed it to Finder

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MAC OS 9

• MAC OS 9 and earlier allocated static blocks of
  memory to applications whether or not
  applications needed it
• Example if you are working on a 100 MB poster
  in Photoshop, the application still stakes claim
  of available RAM
• This increases the use of virtual memory, which
  in turn slows memory performance
• Prior to MAC OS X used Time Slice architecture,
  where every application is allotted time to the
  processor

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Time Slice Architecture

• Every application is allotted time to the
  processor. Applications in this environment are
  required to cooperatively share processor time
• Furthermore, the memory is unprotected. That is,
  an application could write into critical memory
  space that may be in use by the operating system
  kernel
• The result is the infamous bomb alert on the
  desktop with the button highlighted for restart
• Mac OS X brings to the table a preemptive
  multitasking system
• This means the kernel schedules time to each
  (process) application and governs how much time
  it is allotted.
• The kernel will also assign address space to the
  application
• The application will not be allowed to write
  outside of this without the kernel taking action.
  This protected memory environment will change
  everything for Mac users

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MAC OS X

• Unix style memory management and pre-emptive
  multitasking
• This new memory management system allowed more
  programs to run at once and virtually eliminated
  the possibility of one program crashing another
• Virtual memory has been incorporated in the
  underpinnings of the OS, and the OS dynamically
  allocates RAM to applications as they need it

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References

• http//en.wikipedia.org/wiki/MultiFinder
• http//en.wikipedia.org/wiki/Lisa_OS
• http//en.wikipedia.org/wiki/System_6
• http//en.wikipedia.org/wiki/System_7_(Macintosh)
• http//developer.apple.com/documentation/
• http//developer.apple.com/documentation/mac/pdf/M
  emory/pdf.html
• http//www.memorymanagement.org/glossary/m.htmlme
  mory.management
• http//developer.apple.com/technotes/tn/tn1176.htm
  l
• http//members.safe-t.net/jwalker/programming/lisa
  Legacy/
• http//kb.iu.edu/data/agww.html
• http//patimg1.uspto.gov
• http//ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp
  arnumber1457132isnumber31335