2.STORAGE HIERARCHY

Caching:

->In computer science, a cache (pronounced /kæʃ/) is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch (owing to longer access time) or to compute, compared to the cost of reading the cache. In other words, a cache is a temporary storage area where frequently accessed data can be stored for rapid access. Once the data is stored in the cache, it can be used in the future by accessing the cached copy rather than re-fetching or recomputing the original data.

->Most modern microprocessors contain some form of instruction and data caching where a small but fast bit of memory is used to speed up access to main memory. At its simplest level, a cache can pre-load blocks of memory from main memory so that the processor need not stall when performing a load. This is possible because (a) a processor usually accesses sequential memory locations and (b) loading more than one sequential memory location at the same time is faster than loading each sequential memory location one by one. Hence when the processor accesses an uncached bit of memory, the cache reads a full cache line in the hope it will be used (which it usually is).
Again at its simplest level, a cache must be write-through cache whenever the processor performs a write to main memory. Quite simply, when the write is performed, the relevant cache entry is updated, and a write to main memory is issued. One could say that cache coherency has been maintained ie; the cache accurately reflects the contents of main memory.

Coherency and Consistency:

->(Or "cache consistency") /kash koh-heer'n-see/ The synchronisation of data in multiple caches such that reading a memory location via any cache will return the most recent data written to that location via any (other) cache.Some parallel processors do not cache accesses to shared memory to avoid the issue of cache coherency. If caches are used with shared memory then some system is required to detect when data in one processor's cache should be discarded or replaced because another processor has updated that memory location. Several such schemes have been devised. Coherency defines what value is returned on a read.
->Consistency defines when it is available.

3.HARDWARE PROTECTION

Dual-mode Operation:

Sharing system resources requires operating system to ensurethat an incorrect program cannot cause other programs toexecute incorrectly.
• Provide hardware support to differentiate between at least twomodes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also supervisor mode or system mode) –execution done on behalf of operating system.

Mode bit added to computer hardware to indicate the currentmode: monitor (0) or user (1).

• When an interrupt or fault occurs hardware switches to monitormodeuser monitorinterrupt/faultset user mode.

• Privileged instructions can be issued only in monitor mode.

I/O Protection:

->All I/O instructions are privileged instructions.

->Must ensure that a user program could never gain control ofthe computer in monitor mode (i.e., a user program that, aspart of its execution, stores a new address in the interruptvector).

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 thatdetermine the range of legal addresses a program may access:
–> base register – holds the smallest legal physical memoryaddress.
– >limit register – contains the size of the range.
• Memory outside the defined range is protected.

CPU Protection:

->To prevent a user programs gets stuck in infinite loop and never returning back to the os

1.STORAGE STRUCTURE

Main memory:

Refers to physical memory that is internal to the computer. The word main is used to distinguish it from external mass storage devices such as disk drives. Another term for main memory is RAM.

Magnetic disk:

A memory device, such as a floppy disk or a hard disk, that is covered with a magnetic coating. Digital information is stored on magnetic disks in the form of microscopically small, magnetized needles, each of which encodes a single bit of information by being polarized in one direction (representing 1) or the other (representing 0).

Moving head disk mechanism:

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.

1.Bootstrap Program
In computing, booting is a bootstrapping process that starts operating systems when the user turns on a computer system. Most computer systems can only execute code found in the memory (ROM or RAM); modern operating systems are mostly stored on hard disk drives, LiveCDs and USB flash drive. Just after a computer has been turned on, it doesn't have an operating system in memory. The computer's hardware alone cannot perform complicated actions of the operating system, such as loading a program from disk on its own; so a seemingly irresolvable paradox is created: to load the operating system into memory, one appears to need to have an operating system already installed.

2.Difference of interrupt and trap and thier use.
Trap is actually a software generated interrupt caused either by an error (for example division by zero, invalid memory access etc.), or by an specific request by an operating system service generated by a user program. Trap is sometimes called Exception. The hardware or software can generate these interrupts. When the interrupt or trap occurs, the hardware therefore, transfer control to the operating system which first preserves the current state of the system by saving the current CPU registers contents and program counter's value. after this, the focus shifts to the determination of which type of interrupt has occured. For each type of interrupt, separate segmants of code in the operating system determine what action should be taken and thus the system keeps on functioning by executing coputational instruction, I/O instruction, torage instruction etc.

3. Monitor mode,
or RFMON (Radio Frequency Monitor)mode,allows a computer with a wireless network interface card (NIC) to monitor all traffic received from the wireless network. Unlike promiscuous mode, which is also used for packet sniffing, monitor mode allows packets to be captured without having to associate with an access point or ad-hoc network first. Monitor mode only applies to wireless networks, while promiscuous mode can be used on both wired and wireless networks. Monitor mode is one of the six modes that 802.11 wireless cards can operate in: Master (acting as an access point), Managed (client, also known as station), Ad-hoc, Mesh, Repeater, and Monitor mode.

4.User mode
In User mode, the executing code has no ability to directly access hardware or reference memory. Code running in user mode must delegate to system APIs to access hardware or memory. Due to the protection afforded by this sort of isolation, crashes in user mode are always recoverable. Most of the code running on your computer will execute in user mode.

5.Device Status Table

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

6.Direct memory access
Direct memory access (DMA) is a feature of modern computers and microprocessors that allows certain hardware subsystems within the computer to access system memory for reading and/or writing independently of the central processing unit. Many hardware systems use DMA including disk drive controllers, graphics cards, network cards and sound cards. DMA is also used for intra-chip data transfer in multi-core processors, especially in multiprocessor system-on-chips, where its processing element is equipped with a local memory (often called scratchpad memory) and DMA is used for transferring data between the local memory and the main memory. Computers that have DMA channels can transfer data to and from devices with much less CPU overhead than computers without a DMA channel. Similarly a processing element inside a multi-core processor can transfer data to and from its local memory without occupying its processor time and allowing computation and data transfer concurrency.

7.Difference of RAM and DRAM
Random-access memory (usually known by its acronym, RAM) is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order (i.e., at random). The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.[1] By contrast, storage devices such as tapes, magnetic discs and optical discs rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than data transfer, and the retrieval time varies based on the physical location of the next item. The word RAM is often associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. Many other types of memory are RAM, too, including most types of ROM and flash memory called NOR-Flash.

Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory.

8.Main memory
Refers to physical memory that is internal to the computer. The word main is used to distinguish it from external mass storage devices such as disk drives. Another term for main memory is RAM.

9.Magnetic Disk
Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads. As of 2009, magnetic storage media, primarily hard disks, are widely used to store computer data as well as audio and video signals. In the field of computing, the term magnetic storage is preferred and in the field of audio and video production, the term magnetic recording is more commonly used. The distinction is less technical and more a matter of preference.

10.Storage Hierarchy
The hierarchical arrangement of storage in current computer architectures is called the memory hierarchy. It is designed to take advantage of memory locality in computer programs. Each level of the hierarchy has the properties of higher bandwidth, smaller size, and lower latency than lower levels.Most modern CPUs are so fast that for most program workloads, the locality of reference of memory accesses and the efficiency of the caching and memory transfer between different levels of the hierarchy are the practical limitation on processing speed. As a result, the CPU spends much of its time idling, waiting for memory I/O to complete. This is sometimes called the space cost, as a larger memory object is more likely to overflow a small/fast level and require use of a larger/slower level.