Operating System Structures

OS Structures

• Different OS Structures are:
  • Simple
  • Monolithic
  • Layered
  • Microkernel
  • Modular

Simple Structure

• MS-DOS was written to provide the most functionality in the least space
• It is not divided into modules
• MS-DOS has some structure, but interfaces and functionality levels aren't well separated
• Application programs can directly access hardware
• All layers access hardware, so it is not a layered architecture
• It is not well protected, structured, or defined
• Written on Intel 8088, lacking dual mode support

UNIX

• The original UNIX operating system had limited structuring because of hardware functionality
• The UNIX OS has two separable parts:
  • System programs
  • The kernel

The Kernel

• The kernel consists of everything below the system-call interface and above the physical hardware
• It provides the file system, CPU scheduling, memory management, and other OS functions in one level

Traditional UNIX System Structure

• Monolithic structure places many services and functions in one place
• Debugging is difficult
• Scaling and adding new services is difficult as well

Layered Approach

• The OS is divided into layers, each built on top of lower layers
• The bottom layer (layer 0) is the hardware while the highest layer (layer N) is the user interface
• Modularity means layers are selected to use functions/services of only lower-level layers
• Inner layers hide details from outer layers, simplifying design and debugging
• Errors in one layer may not affect others
• Layer definition is complex, making layers hierarchy difficult
• Overhead exists per layer, due to parameter changes per system call

Microkernel System Structure

• The Microkernel (MK) removes non-essential kernel services, implementing them as system applications to make the kernel smaller and more efficient
• Mach was the first example
• The Mac OS X kernel (Darwin) is partly based on Mach
• Communication happens with user modules using message passing

Benefits of using a Microkernel

• Easier to extend
• Easier to port to new architectures
• More reliable (less code runs in kernel mode)
• More secure (less code runs in kernel mode)

Detriments of using a Microkernel

• Performance overhead from User space to Kernel space communication
• Windows NT moved towards partial monolithism

Modules

• Most modern operating systems use loadable kernel modules
• These use an object-oriented approach, where each core component is separate
• Each core component communicates through known interfaces
• Each is loadable as needed within the kernel
• Overall, similar to layers but with more flexibility, such as with Linux and Solaris

Solaris Modular Approach

• Each module can communicate with others directly, making it more flexible than layered approaches

Virtual Machines

• The main idea is to abstract the hardware of a single computer into several execution environments
• A virtual machine carries the layered approach to its logical conclusion
• The virtual machine treats hardware and the OS kernel as though they were all hardware
• A virtual machine provides an interface identical to the underlying bare hardware
• The "host" OS creates the illusion that a process has its own processor and virtual memory
• Each "guest" is provided with a virtual copy of the underlying computer

Virtual Machines History and Benefits

• First appeared commercially in IBM mainframes in 1972
• Provide multiple execution environments (different operating systems) that share the same hardware
• They protect from each other
• Sharing of files can be permitted and controlled
• They commute with each other and other physical systems via networking
• Useful for development and testing
• Consolidation of many low-resource-use systems occurs onto fewer, busier systems
• The "Open Virtual Machine Format" is a standard format of virtual machines which allows a VM to be run within many different virtual machine (host) platforms

Para-virtualization

• Presents a guest system similar, but not identical, to the hardware
• The Guest must be modified to run on paravirtualized hardware
• A guest can be an OS, or Solaris 10 applications running in containers

Virtualization Implementation

• Difficult to implement because an exact duplicate of the underlying machine must be provided
• It typically runs in user mode, creating virtual user mode and kernel mode
• Timing can be an issue, making it slower than a real machine
• Hardware support is needed for better virtualization
• AMD provides "host" and "guest" modes

Operating-System Debugging

• Debugging is finding and fixing errors, or bugs
• OSes generate log files containing error information
• Failure of an application generates a core dump file, capturing the process's memory
• Operating system failure can generate crash dump files containing kernel memory
• Beyond crashes, performance tuning can optimize system performance
• Kernighan’s Law: "Debugging is twice as hard as writing the code in the first place, so if you write the code as cleverly as possible, you are not smart enough to debug it."
• DTrace tool in Solaris, FreeBSD, and Mac OS X allows live instrumentation on production systems

Probes can be used in debugging

• Probes fire when code is executed, capturing state data and sending it to consumers

Operating System Generation

• Operating systems are designed to run on any of a class of machines so the system must be configured or generated for each specific computer site
• A SYSGEN program obtains information about the hardware configuration to generate the OS for a specific machine

Hardware that it gets information about

• CPU speed and type
• Memory available
• Devices interfaced

Booting

• Booting is starting a computer by loading the kernel
• A bootstrap program is code stored in ROM to locate the kernel, loading it into memory, and starting its execution

System Boot

• The Operating system must be made available to hardware so hardware can start it
• a small piece of code, the bootstrap loader, locates the kernel, loads it into memory, and starts it
• In a two-step process, a boot block at a fixed location loads the bootstrap loader
• When power is initialized, execution starts at a fixed memory location
• Firmware contains initial boot code