Unix Architecture

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Unix Architecture

• Typical computer system consists of
• Hardware
• Operating system
• Applications and utilities
• Typical OS includes a user interface or command
  interpreter
• Unix is somewhat unique in that the command
  interpreter is not integrated with the OS but is
  a separate program

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Unix Architecture
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Levels of Unix Software

• Unix uses a layer approach of defining software
• Layered approach is basis for Unix security
• At the lowest level, the level closest to the
  hardware, are the basic hardware interface
  modules
• Process scheduling
• Memory management
• Interrupt handling
• Low level device control
• Disk system management and data buffering
• Process synchronization and inter-process
  communication (IPC)

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Unix Functional Layer Model
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Kernel Services Layer

• The next level provides all the kernel services
• Mapping between user requests and device driver
  actions
• The user system call is converted to a kernel
  service call that actually performs the requested
  service
• Process creation and termination
• I/O services
• File/disk access services
• Terminal handling services

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System Call Interface Layer

• The system call interface layer converts a
  process running in user mode to a protected
  kernel mode process
• This allows the program to invoke protected
  kernel routines to perform system functions

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User Process Layer

• The uppermost layer consists of user processes
  running
• Shells
• Unix commands
• Utility programs
• User application programs
• User processes
• Protected from from other users
• Have no access to Unix kernel routines except
  through the system call interface
• Cannot directly access kernel memory space

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Kernel vs User Space

• In addition to the software levels, Unix also
  features two "rings of protection" from
  inadvertent programming errors or malicious
  attacks that might damage other user's processes
  or the kernel's processes and data structures
• The inner protected ring is known as kernel space
• The outer ring is called user space

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User Space

• User space is the area in memory where user
  processes are run
• This consists of memory starting above the kernel
  and includes the rest of available memory
• This memory is protected the system prevents one
  user from interfering with another
• Only kernel process can access a user process
• A process operating in this memory region is said
  to be operating in user mode

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Kernel Space

• Kernel space is the region of memory where all
  kernel services are provided via kernel processes
• Any process executing in kernel space is said to
  be executing in kernel mode
• Kernel space is a privileged area the user has
  access to it only through the system call
  interface
• A user does not have direct access to either all
  machine instructions or devices
• A kernel process does have direct access to both

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• Also, a kernel process can modify the memory map,
  an operation frequently required to perform
  process scheduling
• A user process becomes a kernel process when it
  executes a system call and starts executing
  kernel code

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Data Flow Between Kernel and User Space

• Since users and the kernel do not share memory
  address space, the mechanism for moving data
  between them is important
• When a system call is executed, the arguments to
  the call and the corresponding kernel routine
  identification are passed from user space to
  kernel space
• Kernel routine ID is usually passed either via a
  hardware machine register or via the stack
• System call arguments are passed in the u area
  (user area) of the calling process

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• u area of a process (user structure and kernel
  stack) contains information about the process the
  kernel needs while the process is running
• Open files, root, current directory, arguments
  to current system call, and process text, stack,
  and data segment sizes
• A pointer to the process table entry containing
  information for scheduling such as priority
• User file descriptor table and information about
  open files
• Kernel stack for the process (empty when
  executing in user mode)
• Remember, user cannot access kernel space but
  kernel space can access user space

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Unix Command Interface and Commands

• Unix command interface is a separate program
  referred to as the shell
• The shell provides the user interface between the
  user and the kernel
• Unix commands and utilities, like the shell, are
  also separate programs
• They are part of a Unix distribution, and are
  considered part of Unix but not part of the
  kernel

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Unix System Services

• Unix provides a number of system services such as
  systems administration, system reconfiguration,
  and file system maintenance
• Other services include
• Customization of system parameters
• Rebuilding of kernel with user drivers
• Creation and definition of user accounts
• Set up and maintenance of access control
  parameters for files and peripherals
• Most services are built into the kernel as system
  calls

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Unix System Calls

• System calls provide a programming interface that
  allows user programs to access kernel functions
• There are a large number of system calls that
  perform functions to manage system resources such
  as memory, disk storage, and peripherals
• System calls are defined in a run-time library
  that provides a mapping of the system call
  interface to the kernel routine(s) that perform
  the system functions
• The shell also provides functionality equivalent
  to many of the system calls

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System Usage Accounting Services

• Unix provides some general accounting services
• As each process terminates, an accounting record
  is made available describing the resources used
  by the process
• Name of the process
• Amount of user and system CPU time used
• Elapsed (wall clock) time
• Average amount of memory used
• Number of disk I/O operations done
• UID and GID of the process
• Terminal from which the process was started

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My Best Unix System Programming Book
W. Richard Stevens, 65.95