The Great Symbian

• Privileged instructions can be issued only in kernel mode.

• Must ensure that a user program could never gain control of the computer in kernel mode. Otherwise, undesirable actions can be done e.g. a user program that, as part of its execution, stores a new address in the interrupt vector.

<--MEMORY PROTECTION-->

• Must provide memory protection at least for the interrupt vectorand the interrupt service routines.

• In order to have memory protection, add two registers that determine the range of legal addresses a program may access:

– Base Register – holds the smallest legal physical memory address.

– Limit Register – contains the size of the range

• Memory outside the defined range is protected.

<--CPU PROTECTION-->

• Timer

– interrupts computer after specified period to ensure operating system maintains control.

– Timer is decremented every clock tick.

– When timer reaches the value 0, an interrupt occurs.

• Timer commonly used to implement time sharing.

• Time also used to compute the current time.

• Load-timer is a privileged instruction.

• Storage systems organized in hierarchy.
– Speed
– Cost
– Size
– Volatility



<--STORAGE HEIRARCHY(MEMORY HEIRARCHY)-->

Click here to see full image size

• Caching – copying information into faster storage system; main memory can be viewed as a cache for secondary storage.

–improve performance where a large access-time or transfer-rate disparity exists between two components) Memory caching: add cache (faster and smaller memory) between CPU and main memory

--> When need some data, check if it’s in cache

-->If yes, use the data from cache

--> If not, use data from main memory and put a copy in cache

– Disk caching: main memory can be viewed as a cache for disks

• Magnetic Tapes

Magnetic tape is a medium for magnetic recording generally consisting of a thin magnetizable coating on a long and narrow strip of plastic. Nearly all recording tape is of this type, whether used for recording audio or video or for computer data storage. It was originally developed in Germany, based on the concept of magnetic wire recording. Devices that record and playback audio and video using magnetic tape are generally called tape recorders and video tape recorders respectively. A device that stores computer data on magnetic tape can be called a tape drive, a tape unit, or a streamer.
Magnetic tape revolutionized the broadcast and recording industries. In an age when all radio (and later television) was live, it allowed programming to be prerecorded. In a time when gramophone records were recorded in one take, it allowed recordings to be created in multiple stages and easily mixed and edited with a minimal loss in quality between generations. It is also one of the key enabling technologies in the development of modern computers. Magnetic tape allowed massive amounts of data to be stored in computers for long periods of time and rapidly accessed when needed.
Today, many other technologies exist that can perform the functions of magnetic tape. In many cases these technologies are replacing tape. Despite this, innovation in the technology continues and tape is still widely used.

Device-status table contains entry for each I/O deviceindicating its type, address, and state.

Click here to see full size of image

• Interrupts and Traps. A great deal of the kernel consists of code that is invoked as the result of a interrupt or a trap.
  
• While the words "interrupt" and "trap" are often used interchangeably in the context of operating systems, there is a distinct difference between the two.
  
• An interrupt is a CPU event that is triggered by some external device.
  
• A trap is a CPU event that is triggered by a program. Traps are sometimes called software interrupts. They can be deliberately triggered by a special instruction, or they may be triggered by an illegal instruction or an attempt to access a restricted resource.
  When an interrupt is triggered by an external device the hardware will save the the status of the currently executing process, switch to kernel mode, and enter a routine in the kernel.
  
• This routine is a first level interrupt handler. It can either service the interrupt itself or wake up a process that has been waiting for the interrupt to occur.
  When the handler finishes it usually causes the CPU to resume the processes that was interrupted. However, the operating system may schedule another process instead.
  When an executing process requests a service from the kernel using a trap the process status information saved, the CPU is placed in kernel mode, and control passes to code in the kernel.
  
• This kernel code is called the system service dispatcher. It examines parameters set before the trap was triggered, often information in specific CPU registers, to determine what action is required. Control then passes to the code that performs the desired action.
  When the service is finished, control is returned to either the process that triggered the trap or some other process.
  
• Traps can also be triggered by a fault. In this case the usual action is to terminate the offending process. It is possible on some systems for applications to register handlers that will be evoked when certain conditions occur -- such as a division by zero.