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| is a program (software system) all time present on the machine from power-on until off |
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| major goals of an operating system ? |
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| better use of hardware and good user environment |
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| build the memory hierarchy pyramid |
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reg
cache
M.M.
elec. disk
mag. disk
opt disk
tape
higher you go on the pyramid, the more speed you have, also the cost per bit increases
capacity increases as you descend the pyramid |
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| high probability that you will visit the same region in memory |
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| Consider the following cache for a main memory system: Cache access time is 25ns, penalty time is 300ns, and Hit ratio of 75% What is the average access time ? |
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| DMA, tell me about it daniel son. |
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| is a hardware mechanism that allows I/O to/from memory transfers without frequent involvement of the CPU |
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| is when a sequence of jobs is run without any user interaction. Only 1 program runs at a time; means the operating system handles the programs sequentially. |
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| def of "multi-programming" ? |
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| is a processing technique that loads many jobs in the main memory that will run concurrently |
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| is a processing technique that allows many users to get the CPU alternatively |
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| Interrupt transfers control to the interrupt service routine generally, through the interrupt vector |
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| contains addresses of all the interrupt service routines |
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| s a software generated interrupt caused by either an error or a user request for an operating system service |
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| is when a user program calls the operating system to run a privileged instruction |
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| a method to distinguish between the execution of operating system code and user defined code. There are two modes, kernel mode (mode bit = 0), to run operating system code; and user mode (mode bit = 1), to run user applications. Kernel mode is when the operating system is running, and user mode is when a user program is running. A system call is when a user program calls the operating system to perform privileged instructions on its behalf. |
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| key points about Memory Protection |
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Definition
Must provide memory protection at least for the interrupt vector and the interrupt service routines
In order to have memory protection, at a minimum add 2 registers that determine the range of legal addresses a program may access. 1) base register holds the smallest legal physical memory address 2) limit register contains the size of the memory range
Memory outside the defined range is protected
When executing in kernel mode, the operating system has unrestricted access to both kernel and user’s memory; user mode has restricted access as shown below
The load instructions for the base and limit registers are privileged instructions |
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| memory management unit is a hardware device that performs the run-time mapping from virtual (or logical) to physical addresses |
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| all I/O instructions are privileged instructions (the device controller communicates with the device driver in the operating system to perform these instructions – see 4. I/O structure, low speed devices |
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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
timer is also used to compute the current time
“load timer” is a privileged instruction |
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