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Magnetic disk:

Description:

Consists of one or more flat, circular platters made of glass, metal or plastic and coated with a magnetic substance. There is an electromagnet that can detect the direction of polarization that had been recorded. Electromagnetic read/write heads are used for this purpose.
There are two types of heads, the removable and fixed head.There are also two types of magnetic disk which are hard disks and floppy disks. Generally both store data by using constant angular velocity, (CAV) format.

Example:

Hard drive.

Advantage:

The mechanical differences between operation of floppy disks and hard disks have important effects on overall capacity, speed, data transfer rate and their reliability.
For hard disks, the disk is allowed to rotate at high speed, thus the tracks can be locate very close together. So the disk can store large amounts of data and retrieves data quickly.For floppy disks is soft and flexible, it is a must to support the disk surface as the data being read and written. The advantage of floppy disks is the flexibility of it, but because of it also that it have much lower capacity that hard disk of the same size.

Optical disk:

Description:

Capable of packing a large amount of data into a small area. Optical storage is intended for archiving and referencing. The data storage is similar to magnetic disk; the data is stored in blocks of the disk. DVD technology quite similar to CD, in term of size and format.But different manufacturing technique in DVD allows the use of both sides of DVD-ROM. So DVD-ROM can store more data than CD-ROM.

Example:

CD-ROM, DVD-ROM,CD-RW.

Advantages:

Large data storage capacity and nonexistent or limited writing ability.

Flash memory:

Description:

Can be erased and reprogrammed. It is used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products.
- stores information in an array of memory cells made from floating-gate transistors
-in single-level cell devices, each cell stores only one bit of bit of information.
- in multi-level cell devices, can store more than one bit.

Example:
Thumb drive.

Advantages:

- no power is needed to maintain the information stored in the chip
- flash memory offers fast read access times
- better kinetic shock resistance than hard disks
- durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water

Disadvantage:
- has a limited life. A USB flash drive can store a limited amount of photographic data in its memory or flashes, and after that, it can lead to data corruption.
- Another pitfall of flash memory is that though it retrieves the data fast, it’s too slow in writing
-Block erasure: as the information is kept in block, it must be erase one block at a time.- Memory wear: has a limited number of erase-write process. Same like has been discussed above, this mean that once it achieve the limit, the drives might be corrupted.

Magneto-optical disk:

Description:

Combine optical and magnetic properties.
Also known as erasable optical disks.store data on sectored tracks using the CAV method.

Example:

Magneto-optical drive.

Advantages:

Share the advantages of optical disk technology: capacity, reasonable cost, and removability.
And also have the read/write capability of magnetic disks.
It may also be stored near magnets without concern that the data may lost.

Disadvantage:

Have much longer seek time and a slower transfer time than magnetic disk.

tutorial 4

UNIX	Aspect of difference	Windows
The virtual memory handler attempts to preserve a pool of free RAM continuously. The precise algorithm varies between variants and versions, but in expansive terms the handler uses three variables to control it's activity; 1) lotsfree: When the free memory pool is above lotsfree the VM handler is idle. If it drops below the handler will begin scanning for unused pages to place on the Free List. 2) Desfree: 'Desirable Free'. Below this point the handler will start work in earnest applying various criteria to reclaim in use pages that (say) have not be accessed for some time. The effort and priority of this work increases as the free memory pool drops. 3) Minfree: Things are getting desperate if the Free memory pool reaches this point. The system may begin thrashing and if the situation is not cleared desperation swapping.	Implementing virtual memory	All the systems processes use virtual memory addresses which are automatically translated to real (RAM) addresses by the hardware. Only core parts of the operating system kernel bypass this address translation and use real memory addresses directly. All processes running under 32 bit Windows gets virtual memory addresses (a Virtual Address Space) going from 0 to 4,294,967,295 (2*32-1 = 4 GB), regardless of how much RAM is actually installed on the computer. Generally, applications use only a small fraction of the memory allocated of private address space. The operating system only assigns RAM page frames to virtual memory pages that are in use. The translation between the 32 bit virtual memory address used by the code running in a process and the 36 bit RAM address is handled routinely and transparently by the computer hardware according to translation tables maintained by the operating system. Any virtual memory page (32 bit address) can be associated with any physical RAM page (36 bit address). When the memory in use by all the presented processes exceeds the amount of RAM available, the operating system will move pages (4 KB pieces) of one or more virtual address spaces to the computer’s hard disk, thus freeing that RAM frame for other purposes. In Windows systems, these “paged out” pages are stored in one or more files called pagefile.sys in the root of a partition
To determine the page size at run time, use the system function sysconf(). Support up to eight different page sizes, from 4kB up to 256MB.	Page sizes	To determine the page size at run time, use the system function GetSystemInfo() from kernel32.dll.
Use signals, such as SIGSEGV, to report these error conditions to programs. It will typically report these conditions to the user with error messages.	Page fault	Windows uses structured exception handling to report page fault-based invalid accesses as access violation exceptions. It would report such problems with less technical error messages.

Resources:
1) http://216.147.18.102/unixfaq/explain_vm.shtml
2) http://en.wikipedia.org/wiki/Page_fault
3) http://www.aumha.org/win5/a/xpvm.php
4) http://support.microsoft.com/kb/555223
5) http://www.aumha.org/win5/a/xpvm.php