1) What is the first step in problem-solving? A) Writing code B) Debugging C) Understanding the problem D) Optimizing the solution Answer: C 2) Which of these is not a step in the problem-solving process? A) Algorithm development B) Problem analysis C) Random guessing D) Testing and debugging Answer: C 3) What is an algorithm? A) A high-level programming language B) A step-by-step procedure to solve a problem C) A flowchart D) A data structure Answer: B 4) Which of these is the simplest data structure for representing a sequence of elements? A) Dictionary B) List C) Set D) Tuple Answer: B 5) What does a flowchart represent? A) Errors in a program B) A graphical representation of an algorithm C) The final solution to a problem D) A set of Python modules Answer: B 6) What is pseudocode? A) Code written in Python B) Fake code written for fun C) An informal high-level description of an algorithm D) A tool for testing code Answer: C 7) Which of the following tools is NOT commonly used in pr...
Overview of Mass-Storage Structure
Magnetic Disks
Traditional magnetic disks have the following basic form:
* One or more plates in the form of disks covered with magnetic media. Hard disk plates are made of inflexible metal, while "floppy" disks are made of more flexible plastic.
* Each platter has two working surfaces. Older hard disk drives would sometimes not use the very top or bottom surface of a stack of platters, as these surfaces were more manageable to potential damage.
* Each working surface is splitted into a number of concentric rings called tracks. The collection of all tracks that are the same distance from the edge of the plattes, (i.e. all tracks instantly above one another in the following diagram) is called a cylinder.
* Each track is further splitted into sectors, basically containing 512 bytes of data each, although some modern disks occasionally use larger sector sizes. (Sectors also adds a header and a trailer, adding checksum information among other things. Larger sector
sizes decrease the fraction of the disk consumed by headers and trailers, but increase internal segmentation and the amount of disk that must be marked bad in the case of errors. )
* The data on a hard disk is read by read-write heads. The standard configuration (shown below) uses one head for each surface, each on a separate arm, and managed by a common arm assembly which moves all heads simultaneously from one cylinder to another. (Other arrangements, including independent read-write heads, may speed up disk access, but involve serious technical difficulties.)
* The storage capacity of a traditional disk drive is same to the number of heads (i.e. the number of working surfaces), times the number of tracks for each surface, times the number of sectors for each track, times the number of bytes for each sector. A certain physical block of data is specified by giving the head-sector-cylinder number at which it is located.
* In operation the disk rotates at high speed, such as 7200 rpm (120 rotations per second.) The rate at which data can be transfer from the disk to the computer is composed of several steps:
• The positioning time, a.k.a. the seek time or random access time is the needed time to move the heads from one cylinder to another, and for the heads to settle down after the move. This is typically the slowest step in the process and the main bottleneck to overall shift rates.
• The rotational latency is the total of time needed for the desired sector to
rotate around and come under the read-write head. This can range anywhere from
zero to one full revolution, and on the mean will equal one-half revolution. This is another physical step and is normally the second slowest step behind seek time.
(For a disk rotating at 7200 rpm, the medium rotational latency would be 1/2
rotation / 120 rotations per second, or just over 4 milliseconds, a long time
by computer standards.
• The transfer rate, which is the time needed to move the data electronically from the disk to the computer. (Some authors may also use the term transfer rate to refer to the overall transfer rate, adding seeks time and rotational latency as well as the electronic data transfer rate.)
* Disk heads "fly" over the surface on a very lean cushion of air. If they should unexpectedly contact the disk, then a head crash occurs, which May or may not permanently damage the disk or even destroy it completely. For this reason it is normal to park the disk heads when turn a computer off, which means to move the heads off the disk or to an area of the disk where there is no data kept.
* Floppy disks are normally changeable. Hard drives can also be changeable, and some are even hot-swappable, meaning they can be changed while the computer is running, and a new hard drive inserted in their place.
* Disk drives are joined to the computer via a cable known as the I/O Bus. Some of the common link structures include Enhanced Integrated Drive Electronics, EIDE; Advanced Technology Attachment, ATA; Serial ATA, SATA, Universal Serial Bus, USB; Fiber Channel, FC, and Small Computer Systems Interface, SCSI.
* The host controller is at the computer terminal of the I/O bus, and the disk controller is built into the disk itself. The CPU provides commands to the host controller via I/O ports. Data is carried between the magnetic surface and onboard cache by the disk controller, and then the data is carried from that cache to the host controller and the motherboard
memory at electronic speeds.
Solid-State Disks - New
* As technologies increase and economics change, old technologies are often used in different ways. One example of that in the increasing use of solid state disks, or SSDs.
* SSDs use memory technology as a small fast hard disk. Specific implementations may use either flash memory or DRAM chips protected by a battery to sustain the information through power cycles.
* Because SSDs have no changing parts they are much faster than traditional hard drives, and certain problems such as the scheduling of disk accesses simply do not apply.
* However SSDs also have their weaknesses: They are more expensive than hard drives, normally not as large, and may have shorter life spans.
* SSDs are especially useful as a high-speed cache of hard-disk information that must be retrieved quickly. One example is to store file system meta-data, e.g. directory and anode information that must be accessed quicklyand often. Another variation is a boot disk consists of the OS and some application executables, but no vital user data. SSDs are also used in laptops to make them compact, faster, and lighter.
* Because SSDs are so much faster than traditional hard disks, the throughput of the bus can become a limiting factor, causing some SSDs to be linked directly to the system PCI bus for example.
Magnetic Tapes
* Magnetic tapes were once used for common secondary storage before the days of hard disk drives, but today are used primarily for backups.
* Accessing a particular spot on a magnetic tape can be slow, but once reading or writing commences, access speeds are comparable to disk drives.
* Capacities of tape drives can range from 20 to 200 GB and compression can double that capacity.