• Operating system must be made available to hardware so hardware can start it
  * Small piece of code – bootstrap loader, locates the kernel, loads it into memory, and starts it
  * Sometimes two-step process where boot block at fixed location loads bootstrap loader
  * When power initialized on system, execution starts at a fixed memory location
  

• Operating systems are designed to run on any of a class of machines; the system must be configured for each specific computer site.
• SYSGEN program obtains information concerning the specific configuration of the hardware system.
• Booting – starting a computer by loading the kernel.
• Bootstrap program – code stored in ROM that is able to locate the kernel, load it into memory, and start its execution.
  

• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware.
• A virtual machine provides an interface identical to the underlying bare hardware.
• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory.
• The resources of the physical computer are shared to create the virtual machines.
  * CPU scheduling can create the appearance that users have their own processor.
  * Spooling and a file system can provide virtual card readers and virtual line printers.
  * A normal user time-sharing terminal serves as the virtual machine operator’s console.
  

IMPLEMENTAION

• Traditionally written in assembly language, operating systems can now be written in higher-level languages.
• Code written in a high-level language:
  * can be written faster.
  * is more compact.
  * is easier to understand and debug.
• An operating system is far easier to port (move to some other hardware) if it is written in a high-level language.
  
  

BENEFITS

• The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.
• A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.
• The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.
  

EXAMPLES

• Compiled Java programs are platform-neutral bytecodes executed by a Java Virtual Machine (JVM).
• JVM consists of
  - class loader
  - class verifier
  - runtime interpreter
• Just-In-Time (JIT) compilers increase performance

• The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.
• With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers.
  

• System calls provide the interface between a running program and the operating system.
  * Generally available as assembly-language instructions.
  * Languages defined to replace assembly language for systems programming allow system calls to be made directly (e.g., C, C++)
• Three general methods are used to pass parameters between a running program and the operating system.
  * Pass parameters in registers.
  * Store the parameters in a table in memory, and the table address is passed as a parameter in a register.
  * Push (store) the parameters onto the stack by the program, and pop off the stack by operating system.

• Program execution – system capability to load a program into memory and to run it.
  
• I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.
• File-system manipulation – program capability to read, write, create, and delete files.
• Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.
• Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.
• Additional functions exist not for helping the user, but rather for ensuring efficient system operations.
• Resource allocation – allocating resources to multiple users or multiple jobs running at the same time.
• Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics.
• Protection – ensuring that all access to system resources is controlled.

OPERATING SYSTEMS PROCESS MANAGEMENT

• Program execution – system capability to load a program into memory and to run it.
• I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.
• File-system manipulation – program capability to read, write, create, and delete files.
• Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.
• Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.
  

MAIN MEMORY MANAGEMENT

• Memory is a large array of words or bytes, each with its own address. It is a repository of quickly accessible data shared by the CPU and I/O devices.
• Main memory is a volatile storage device. It loses its contents in the case of system failure.
• The operating system is responsible for the following activities in connections with memory management:
  * Keep track of which parts of memory are currently being used and by whom.
  * Decide which processes to load when memory space becomes available.
  * Allocate and deallocate memory space as needed.
  

FILE MANAGEMENT

• A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data.
  
• The operating system is responsible for the following activities in connections with file management:
  * File creation and deletion.
  * Directory creation and deletion.
  * Support of primitives for manipulating files and directories.
  * Mapping files onto secondary storage.
  * File backup on stable (nonvolatile) storage media.

I/O SYSTEM MANAGEMENT

• The I/O system consists of:
  * A buffer-caching system
  * A general device-driver interface
  * Drivers for specific hardware devices

SECONDARY STORAGE MANAGEMENT

• Since main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory.
• Most modern computer systems use disks as the principle on-line storage medium, for both programs and data.
• The operating system is responsible for the following activities in connection with disk management:
  * Free space management
  * Storage allocation
  * Disk scheduling
  

PROTECTION SYSTEM

• Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.
• The protection mechanism must:
  * distinguish between authorized and unauthorized usage.
  * specify the controls to be imposed.
  * provide a means of enforcement.

COMMAND INTERPRETER SYSTEM

• Many commands are given to the operating system by control statements which deal with:
  * process creation and management
  * F/O handling
  * secondary-storage management
  * main-memory management
  * file-system access
  * protection
  * networking
  
  
• The program that reads and interprets control statements is called variously:
  *command-line interpreter
  *shell (in UNIX)
  
• Its function is to get and execute the next command statement.