Author: ieze_cutie Posted under:
(0) System Boot

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
    -------Firmware used to hold initial boot code
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Author: ieze_cutie Posted under:
(0) System Generation

(computer science) A process that creates a particular and uniquely specified operating system; it combines user-specified options and parameters with manufacturer-supplied general-purpose or nonspecialized program subsections to produce an operating system (or other complex software) of the desired form and capacity. Abbreviated sysgen.
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Author: ieze_cutie Posted under:
(0) Virtual Machine

A virtual machine was originally defined by Popek and Goldberg as "an efficient, isolated duplicate of a real machine". Current use includes virtual machines which have no direct correspondence to any real hardware.[1]Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine. A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine -- it cannot break out of its virtual world.
A virtual machine, simply put, is a virtual computer running on a physical computer. The virtual machine emulates a physical machine in software. This includes not only the processor but the instruction set, the memory bus, any BIOS commands and critical machine hardware such as the system clock and and DMA hardware. Depending upon the machine peripheral devices are generally virtualized including storage devices like floppy drives, hard drives and CD drives. Video, keyboard and mouse support are also common. A virtual machine must look and act just like the real thing so standard software, like operating systems and applications, can run without modification.


  • Implementation


Modes:
>>virtual user mode and virtual monitor mode,
>>Actual user mode and actual monitor mode
Time
>>Whereas the real I/O might have taken 100 milliseconds, the virtual I/O might take less time (because it is spooled) or more time (because it is interpreted.)
>>The CPU is being multi-programmed among many virtual machines, further slowing down the virtual machines in unpredictable ways.

  • benefits

For testers it is important that they test software against the various supported operating systems that an application runs against. The traditional approach is to run multiple physical machines, each with a different operating system. This is bad for several reasons. Space, maintenance, power and feasibility come to mind. Deployment of the software to these various machines can also be an issue. Instead a tester can run multiple virtual machines on one physical machine. Each virtual machine could have a different operating system. The application can be deployed to the virtual machines and tested.

Another advantage of virtual machines is reproducibility. Build and test environments generally need to be well controlled. It would be undo work to have to wipe out a machine and rebuild it after each build or test run. A virtual machine allows the environment to be set up once. The environment is then captured. Any changes made after the capture can then be thrown away after the build or test run. Most emulation software packages offer this in some form or another


Two advantages
>>To provide a robust level of security
>>no direct sharing of resources.
Two solutions
>>To allow system development to be done easily
>>A perfect vehicle for OS research and development.
>>difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.
>>Wine for Linux


  • Examples

A program written in Java receives services from the Java Runtime Environment (JRE) software by issuing commands to, and receiving the expected results from, the Java software. By providing these services to the program, the Java software is acting as a "virtual machine", taking the place of the operating system or hardware for which the program would ordinarily be tailored.

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