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Computer hardware

Different buses and their uses

In computer architecture, a bus is a communication system that transfers data between components inside a computer, or between computers. This expression covers all related hardware components (wire, optical fiber, etc.) and software, including communication protocols.

The internal bus, also known as internal data bus, memory bus, system bus or Front-Side-Bus, connects all the internal components of a computer, such as CPU and memory, to the motherboard. Internal data buses are also referred to as a local bus, because they are intended to connect to local devices. This bus is typically rather quick and is independent of the rest of the computer operations.

The external bus, or expansion bus, is made up of the electronic pathways that connect the different external devices, such as printer etc., to the computer.

Types and uses:

USB

Universal Serial Bus. Designed for: input devices, digital cameras, printers, media players...

Serial ATA

Used by internal storage devices (hard disk). They replace the old ATA connectors.

PCI

Peripheral Component Interconnect, is a local computer bus for attaching hardware devices in a computer. Attached devices can take either the form of an integrated circuit fitted onto the motherboard itself or an expansion card that fits into a slot. Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as USB or serial, TV tuner cards and disk controllers.

PCI Express

Peripheral Component Interconnect Express (also called PCIe), is a high-speed serial computer expansion bus standard designed to replace the older PCI, PCI-X, and AGP bus standards. PCIe has numerous improvements over the older standards, including higher maximum system bus throughput, lower I/O pin count and smaller physical footprint, better performance scaling for bus devices, a more detailed error detection and reporting mechanism, and native hot-plug functionality. More recent revisions of the PCIe standard provide hardware support for I/O virtualization.

Mini PCIe

It is based on PCI Express technoogy. Main point is its small size and its large variety of connectors makes it used for USB2.0 cards, SIM card, Wifi and Bluetooth cards, 3G and GPS cards.

ExpressCard

It is an interface to connect peripheral devices to a computer, usually a laptop computer. ExpressCards can connect a variety of devices to a computer including mobile broadband modems, IEEE 1394 (FireWire) connectors, USB connectors, Ethernet network ports, Serial ATA storage devices, solid-state drives, external enclosures for desktop-size PCI Express graphics cards and other peripheral devices, wireless network interface controllers (NIC), TV tuner cards, Common Access Card (CAC) readers, and sound cards.

What are the differences between hard disk drive (HDD) and solid state drive (SSD)?

The traditional spinning hard drive (HDD) is the basic nonvolatile storage on a computer. Hard drives are essentially metal platters with a magnetic coating which stores the data. A read/write head on an arm accesses the data while the platters are spinning in a hard drive enclosure. An SSD does same jobas an HDD, but instead of a magnetic coating on top of platters, the data is stored on interconnected flash memory chips that retain the data even when there's no power present. HDDs have spinning plates with magnetic coating, while SSDs have no moving parts and instead are using flash memory.

Attribute	SSD (Solid State Drive)	HDD (Hard Disk Drive)
Power Draw / Battery Life	Less power draw, averages 2 – 3 watts, resulting in 30+ minute battery boost	More power draw, averages 6 – 7 watts and therefore uses more battery
Cost	Expensive, roughly $0.10 per gigabyte (based on buying a 1TB drive)	Only around $0.06 per gigabyte, very cheap (buying a 4TB model)
Capacity	Typically not larger than 1TB for notebook size drives; 1TB max for desktops	Typically around 500GB and 2TB maximum for notebook size drives; 6TB max for desktops
Operating System Boot Time	Around 10-13 seconds average bootup time	Around 30-40 seconds average bootup time
Noise	There are no moving parts and as such no sound	Audible clicks and spinning can be heard
Vibration	No vibration as there are no moving parts	The spinning of the platters can sometimes result in vibration
Heat Produced	Lower power draw and no moving parts so little heat is produced	HDD doesn’t produce much heat, but it will have a measurable amount more heat than an SSD due to moving parts and higher power draw
Failure Rate	Mean time between failure rate of 2.0 million hours	Mean time between failure rate of 1.5 million hours
File Copy / Write Speed	Generally above 200 MB/s and up to 550 MB/s for cutting edge drives	The range can be anywhere from 50 – 120MB / s
Encryption	Full Disk Encryption (FDE) Supported on some models	Full Disk Encryption (FDE) Supported on some models
File Opening Speed	Up to 30% faster than HDD	Slower than SSD
Magnetism Affected?	An SSD is safe from any effects of magnetism	Magnets can erase data

What is the purpose of Flash Translation Layer in terms of solid state drives?

Flash drives have limited lifespan due to write cycles (or rather program-erase (PE) cycles) and to exhaust the drive equally, FTL layer is utilised.

Although presenting an array of Logical Block Addresses (LBA) makes sense for HDDs as their sectors can be overwritten, it is not fully suited to the way flash memory works. For this reason, an additional component is required to hide the inner characteristics of NAND flash memory and expose only an array of LBAs to the host. This component is called the Flash Translation Layer (FTL), and resides in the SSD controller. The FTL is critical and has three main purposes: logical block mapping, wear leveling and garbage collection.

The logical block mapping translates logical block addresses (LBAs) from the host space into physical block addresses (PBAs) in the physical NAND-flash memory space. This mapping takes the form of a table, which for any LBA gives the corresponding PBA.

Wear leveling (also written as wear levelling) is a technique[1] for prolonging the service life of some kinds of erasable computer storage media, such as flash memory (used in solid-state drives (SSDs) and USB flash drives).

The garbage collection process in the SSD controller ensures that “stale” pages are erased and restored into a “free” state so that the incoming write commands can be processed. If the data in a page has to be updated, the new version is written to a free page, and the page containing the previous version is marked as stale. When blocks contain stale pages, they need to be erased before they can be written to.

What are difference between volatile/non-volatile, RAM, ROM, EEPROM and where are they used?

Volatile = does not hold data after power off. Non-volatile = holds data even after power off.

Random Access Memory or RAM is a form of data storage that can be accessed randomly at any time, in any order and from any physical location., allowing quick access and manipulation. RAM allows the computer to read data quickly to run applications. It allows reading and writing. It is volatile.

Read-only memory or ROM is also a form of data storage that can not be easily altered or reprogrammed.Stores instuctions that are not nescesary for re-booting up to make the computer operate when it is switched off.They are hardwired. ROM stores the program required to initially boot the computer. It only allows reading. It is non-volatile.

EEPROM (electrically erasable programmable read-only memory) is user-modifiable read-only memory (ROM) that can be erased and reprogrammed. Unlike EPROM chips, EEPROMs do not need to be removed from the computer to be modified. However, an EEPROM chip has to be erased and reprogrammed in its entirety, not selectively. It also has a limited life - that is, the number of times it can be reprogrammed is limited to tens or hundreds of thousands of times. BIOS of PCs are usually written to EEPROM.

What is data retention?

It is how long a device can hold data before it becomes unreadable.

What are difference between asynchronous/synchronous, dynamic/static RAM and where are they used?

The two main forms of modern RAM are static RAM (SRAM) and dynamic RAM (DRAM). In SRAM, a bit of data is stored using the state of a six transistor memory cell. This form of RAM is more expensive to produce, but is generally faster and requires less power than DRAM and, in modern computers, is often used as cache memory for the CPU. DRAM stores a bit of data using a transistor and capacitor pair, which together comprise a DRAM memory cell. The capacitor holds a high or low charge (1 or 0, respectively), and the transistor acts as a switch that lets the control circuitry on the chip read the capacitor's state of charge or change it. As this form of memory is less expensive to produce than static RAM, it is the predominant form of computer memory used in modern computers. Dynamic RAM is used to create larger RAM space system, where Static RAM create speed- sensitive cache. Dynamic RAM consumes less power than Static RAM.

Asynchronous refers to the fact that the memory is not synchronized to the system clock. A memory access is begun, and a certain period of time later the memory value appears on the bus. The signals are not coordinated with the system clock at all, as described in the section discussing memory access. Asynchronous memory works fine in lower-speed memory bus systems but is not nearly as suitable for use in high-speed (>66 MHz) memory systems. A newer type of DRAM, called "synchronous DRAM" or "SDRAM", is synchronized to the system clock; all signals are tied to the clock so timing is much tighter and better controlled. This type of memory is much faster than asynchronous DRAM and can be used to improve the performance of the system. It is more suitable to the higher-speed memory systems of the newest PCs.

What is cache? What is cache coherence?

Cache memory is used to reduce the average memory access times. This is done by storing the data that is frequently accessed in main memory addresses therefore allowing the CPU to access the data faster. This is due to the fact that cache memory can be read a lot faster than main memory.

Cache coherence is the consistency of shared resource data that ends up stored in multiple local caches. When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data, which is particularly the case with CPUs in a multiprocessing system.

What are differences between resistive and capacitive touchscreen?

Resistive touchscreen needs pressure to work by connecting to sheets of plastic and thus conducting electricity. Capacitive touchscreen works by conducting electricity from user's finger.

Resistive touchscreens rely on the pressure of your fingertip—or any other object—to register input. They consist of two flexible layers with an air gap in-between. In order for the touchscreen to register input, you must press on the top layer using a small amount of pressure, in order to depress the top layer enough to make contact with the bottom layer. The touchscreen will then register the precise location of the touch. You can use anything you want on a resistive touchscreen to make the touch interface work; a gloved finger, a wooden rod, a fingernail – anything that creates enough pressure on the point of impact will activate the mechanism and the touch will be registered.

Capacitive touchscreens instead sense conductivity to register input—usually from the skin on your fingertip. Because you don’t need to apply pressure, capacitive touchscreens are more responsive than resistive touchscreens. However, because they work by sensing conductivity, capacitive touchscreens can only be used with objects that have conductive properties, which includes your fingertip (which is most ideal), and special styluses designed with a conductive tip. This is the reason you cannot use a capacitive screen while wearing gloves – the gloves are not conductive, and the touch does not cause any change in the electrostatic field.

In-depth comparison of two

Explain how computer mouse works?

An optical computer mouse uses a light source, typically an LED, and a light detector, such as an array of photodiodes, to detect movement relative to a surface. Modern surface-independent optical mice work by using an optoelectronic sensor (essentially, a tiny low-resolution video camera) to take successive images of the surface on which the mouse operates. The technology underlying the modern optical computer mouse is known as digital image correlation. To understand how optical mice work, imagine two photographs of the same object except slightly offset from each other. Place both photographs on a light table to make them transparent, and slide one across the other until their images line up. The amount that the edges of one photograph overhang the other represents the offset between the images, and in the case of an optical computer mouse the distance it has moved. Optical mice capture one thousand successive images or more per second. Depending on how fast the mouse is moving, each image will be offset from the previous one by a fraction of a pixel or as many as several pixels. Optical mice mathematically process these images using cross correlation to calculate how much each successive image is offset from the previous one. History of computer mouse.

Explain how computer keyboard works?

A keyboard is a lot like a miniature computer. It has its own processor and circuitry that carries information to and from that processor. A large part of this circuitry makes up the key matrix. The key matrix is a grid of circuits underneath the keys. In all keyboards, each circuit is broken at a point below each key. When you press a key, it presses a switch, completing the circuit and allowing a tiny amount of current to flow through. The mechanical action of the switch causes some vibration, called bounce, which the processor filters out. If you press and hold a key, the processor recognizes it as the equivalent of pressing a key repeatedly. When the processor finds a circuit that is closed, it compares the location of that circuit on the key matrix to the character map in its read-only memory (ROM). A character map is basically a comparison chart or lookup table. It tells the processor the position of each key in the matrix and what each keystroke or combination of keystrokes represents. For example, the character map lets the processor know that pressing the a key by itself corresponds to a small letter "a," but the Shift and a keys pressed together correspond to a capital "A."

HowStuffworks article Explain that Stuff article Keyboard History

Explain how cathode ray tube (CRT) based screen technology works and name pros/cons.

[1] The cathode ray tube (CRT) is a vacuum tube containing one or more electron guns, and a phosphorescent screen used to, It has a means to accelerate and deflect the electron beam(s) onto the screen to create the images. The images may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets or others. CRTs have also been used as memory devices, in which case the visible light emitted from the fluorescent material (if any) is not intended to have significant meaning to a visual observer (though the visible pattern on the tube face may cryptically represent the stored data).

The CRT uses an evacuated glass envelope which is large, deep (i.e. long from front screen face to rear end), fairly heavy, and relatively fragile. As a matter of safety, the face is typically made of thick lead glass so as to be highly shatter-resistant and to block most X-ray emissions, particularly if the CRT is used in a consumer product.

Explain how liquid crystal displays (LCD) work and name pros/cons.

[2]

Name screen technologies making use of thin film transistor (TFT) technology?

[3]

Name uses for light polarization filters?

A polarizer or polariser is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization, polarized light. Polarizers are used in many optical techniques and instruments, and polarizing filters find applications in photography, liquid crystal display technology and 3D watching. Polarizers can also be made for other types of electromagnetic waves besides light, such as radio waves, microwaves, and X-rays.

Polarizer photo Polarizer 3D

What are the benefits of twisted pair cabling and differential signalling?

Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk between neighboring pairs.

Differential signaling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires (typically twisted together) or traces on a circuit board. The receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground. Since the receiving circuit only detects the difference between the wires, the technique resists electromagnetic noise compared to one conductor with an un-paired reference (ground). The technique works for both analog signaling and digital signaling.

Active matrix vs passive matrix in display technology

Active-matrix display : An active-matrix display, also known as a TFT (thin-film transistor) display, uses a separate transistor to apply charges to each liquid crystal cell and thus displays high-quality color that is viewable from all angles.

Passive-matrix display : A passive-matrix display uses fewer transistors, requires less power, and is less expensive than an active-matrix display. The color on a passive-matrix display often is not as bright as an active-matrix display. Users view images on a passive-matrix display best when working directly in front of it.

Storage abstractions

What is a block device?

In computing (specifically data transmission and data storage), a block, sometimes called a physical record, is a sequence of bytes or bits, usually containing some whole number of records, having a maximum length, a block size.Data thus structured are said to be blocked. The process of putting data into blocks is called blocking. Most file systems are based on a block device, which is a level of abstraction for the hardware responsible for storing and retrieving specified blocks of data

What is logical block addressing and what are the benefits compared to older cylinder-head-sector addressing method in terms of harddisks?

Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disk drives. LBA is a particularly simple linear addressing scheme; blocks are located by an integer index, with the first block being LBA 0, the second LBA 1, and so on.

Cylinder-head-sector, also known as CHS, is an early method for giving addresses to each physical block of data on a hard disk drive. CHS addressing is the process of identifying individual sectors on a disk by their position in a track, where the track is determined by the head and cylinder numbers.

LBA allows a maximum addressing capacity of 2TB although CHS allows 8GB

What is a disk partition?

Disk partitioning is the creation of one or more regions on a hard disk or other secondary storage, so that an operating system can manage information in each region separately. Partitioning is typically the first step of preparing a newly manufactured disk, before any files or directories have been created. The disk stores the information about the partitions' locations and sizes in an area known as the partition table that the operating system reads before any other part of the disk. Each partition then appears in the operating system as a distinct "logical" disk that uses part of the actual disk.

What is a file system?

In computing, a file system (or filesystem) is used to control how data is stored and retrieved. Without a file system, information placed in a storage area would be one large body of data with no way to tell where one piece of information stops and the next begins. By separating the data into individual pieces, and giving each piece a name, the information is easily separated and identified. Taking its name from the way paper-based information systems are named, each group of data is called a "file". The structure and logic rules used to manage the groups of information and their names is called a "file system".

What is journaling in terms of filesystems and what are the benefits? Name some journaled file systems in use nowadays.

A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the intentions of such changes in a data structure known as a "journal", which is usually a circular log. In the event of a system crash or power failure, such file systems can be brought back online quicker with lower likelihood of becoming corrupted.

Depending on the actual implementation, a journaling file system may only keep track of stored metadata, resulting in improved performance at the expense of increased possibility for data corruption. Alternatively, a journaling file system may track both stored data and related metadata, while some implementations allow selectable behavior in this regard

Bootloaders, kernels

What is the role of BIOS/UEFI in x86-based machines?

BIOS (Basic Input/Output System) performs during boot up process of a computer and prepares it for the OS and its' programs.

UEFI (Unified Extensible Firmware Interface) is a replacement for BIOS. It offers several advantages over previous firmware interface, like:

• Ability to boot from large disks (over 2 TB) with a GUID Partition Table (GPT)
• CPU-independent architecture
• CPU-independent drivers
• Flexible pre-OS environment, including network capability
• Modular design

Explain step by step how operating system is booted up, see slides for flowchart.

1. The first thing a computer has to do when it is turned on is start up a special program called an operating system. That process is called booting. The instructions for booting are built into BIOS (Basic input/output system).

2. The BIOS will tell your computer to look into the boot disk (usually the lowest-numbered hard disk) for a boot loader and the boot loader will be pulled into memory and will be started.

3. The boot loader's job is to start the real operating system and it does it by looking for a kernel, loading it into memory and starting it.

5. Once the kernel starts, it has to look around to find the rest of the hardware and get ready to run programs ( It does this by poking not at ordinary memory locations but rather at I/O ports — special bus addresses that are likely to have device controller cards listening at them for commands). The kernel doesn't poke at random; it has a lot of built-in knowledge about what it's likely to find where, and how controllers will respond if they're present. This process is called autoprobing.

6. After the kernel is up and running, it's the end of first stage. After the first stage, the kernel hands control to a special program called "init" which spawns housekeeping processes. The init's process's first task is to look that your hard disks are okay

7. Init's next step is to start several daemons. A daemon is a program like a print spooler, a mail listener or a WWW server that lurks in the background, waiting for things to do. These special programs often have to coordinate several requests that could conflict. They are daemons because it's often easier to write one program that runs constantly and knows about all requests than it would be to try to make sure that a flock of copies (each processing one request and all running at the same time) don't step on each other.

8. The next step is to prepare for users. Init starts a copy of a program called getty to watch your screen and keyboard (and maybe more copies to watch dial-in serial ports).

9. The next step is to start up various daemons that support networking and other services. The most important of these is your X server. X is a daemon that manages your display, keyboard, and mouse. Its main job is to produce the color pixel graphics you normally see on your screen.

10. When the X server comes up, during the last part of your machine's boot process, it effectively takes over the hardware from whatever virtual console was previously in control. That's when you'll see a graphical login screen, produced for you by a program called a display manager.

BIOS

• Basic input and output system
• Located at memory location 0xFFFF0
• Boot firmware designed to be run at startup
• POST- power on self test
• Run-time service

 initial configuration 
 selects which device to boot from

• Alternatively, Extensible Firmware Interface (EFI)

Describe the functionality provided by general purpose operating system.

See architecture of Windows NT, Android, OS X.

What are the main differences between real mode and protected mode of x86-based processor?

To preserve backward compatibility, all the further processors have had to include a Real mode. In the real mode, the memory addressability is limited to 1MB and it cannot take advantage of additional hardware support for virtual memory etc because interrupts are disabled.

With the advent of the 286 processor, a new mode was included - that is the protected mode. The name "protected" actually derives from the fact that processes run in different address spaces and that they are essentially protected from one another. If a process tries to access another's address space, it traps to the MMU hardware. In this mode, the full hardware support is enabled, and the processor can address the entire 32 bit address space.

Programming languages

What are the major steps of compilation?

1. Lexer The first phase of scanner works as a text scanner. This phase scans the source code as a stream of characters and converts it into meaningful lexemes. Lexical analyser represents these lexemes in the form of tokens as: <token-name, attribute-value>

2. Parser The next phase is called the syntax analysis or parsing. It takes the token produced by lexical analysis as input and generates a parse tree (or syntax tree). In this phase, token arrangements are checked against the source code grammar, i.e. the parser checks if the expression made by the tokens is syntactically correct.

3. Type checker Semantic analysis checks whether the parse tree constructed follows the rules of language. For example, assignment of values is between compatible data types, and adding string to an integer. Also, the semantic analyser keeps track of identifiers, their types and expressions; whether identifiers are declared before use or not etc. The semantic analyser produces an annotated syntax tree as an output and an intermediate code is produced.

4. Optimizer The next phase does code optimization of the intermediate code. Optimization can be assumed as something that removes unnecessary code lines, and arranges the sequence of statements in order to speed up the program execution without wasting resources (CPU, memory).

5. Code generator In this phase, the code generator takes the optimized representation of the intermediate code and maps it to the target machine language. The code generator translates the intermediate code into a sequence of (generally) re-locatable machine code. Sequence of instructions of machine code performs the task as the intermediate code would do.

Data encoding

What is bit? Nibble? Byte? Word?

Bit is a basic unit of information that can hold either True or False value (1 or 0).

Nibble is half of an octet.

Byte is a unit of eight bits. Comes from the number of bits used to encode a single character of text in a computer

Word is a length of bits the processor-architecture can process in bits (8-bit, 32-bit etc)

Write 9375 in binary, hexadecimal?

Binary or base two counting system starts from right with 0 and continues left with each step being to the power of 2.

13^2	12^2	11^2	10^2	9^2	8^2	7^2	6^2	5^2	4^2	3^2	2^2	1^2	0^1
8192	4096	2048	1024	512	256	128	64	32	16	8	4	2	1
1	0	0	1	0	0	1	0	0	1	1	1	1	1

To find out the decimal number (9375) in binary, look if it contains the number equal or smaller. 8192 is smaller than 9375, mark down 1. 9375-8192=1183 contains 1024, mark 1, etc. Number 9375 base ten is 0x10010010011111 in binary, where the '0b' represents base two. From binary to decimal conversion works the other way around. If perplexed, see the explanatory video on Khan academy.

Hexadecimal or base 16 system goes from 0 until 9, then starts with A (10 base 10) until F (15 base 10).

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F

Conversion from base two to hex is similar to decimal to base two conversion. First, we figure out the multiples of 16:

16^0	16^1	16^2	16^3	16^4
1	16	256	4096	65 536
F	9	4	2	<-

9375 has two multiples of 4096 (9375 - 2*4096 = 9375 - 8192 = 1183), 1183 has 4 multiples of 256 (1183 - 4*256 = 159), 159 has 159 - 9*16 = 15 (15 in decimal is F in hex) and we come to 249F. As with base 2, you can convert this way from hex to decimal. If still don't get it, watch a video on Khan academy.

Write 0xDEADBEEF in decimal?

Following the table above we calculate, 13x16^8 + 14x16^7 + 10x16^6 + 13x16^5 + 11x16^4 + 14x16^3 + 14x16^2 + 15x16^1 = A BIG NUMBER!

How are single precision and double precision floating point numbers stored in binary according to IEEE754 standard?

Floating-point multiplication

What is bitrate?

Bitrate is the number of bits that are conveyed or processed per unit of time.

The bit rate is quantified using the bits per second unit (symbol: "bit/s"), often in conjunction with an SI prefix such as "kilo" (1 kbit/s = 1000 bit/s), "mega" (1 Mbit/s = 1000 kbit/s), "giga" (1 Gbit/s = 1000 Mbit/s) or "tera" (1 Tbit/s = 1000 Gbit/s).[2] The non-standard abbreviation "bps" is often used to replace the standard symbol "bit/s", so that, for example, "1 Mbps" is used to mean one million bits per second.

One byte per second (1 B/s) corresponds to 8 bit/s.

Microcontrollers

What distinguishes microcontroller from microprocessor?

http://www.zseries.in/embedded%20lab/8051%20microcontroller/difference%20between%20microprocessor%20and%20microcontroller.php#.VpPl2fl97IU

Hardware description languages

What are the uses for hardware description languages?

In electronics, a hardware description language (HDL) is a specialized computer language used to program the structure, design and operation of electronic circuits, and most commonly, digital logic circuits.

A hardware description language enables a precise, formal description of an electronic circuit that allows for the automated analysis, simulation, and simulated testing of an electronic circuit. It also allows for the compilation of an HDL program into a lower level specification of physical electronic components, such as the set of masks used to create an integrated circuit.

A hardware description language looks much like a programming language such as C; it is a textual description consisting of expressions, statements and control structures. One important difference between most programming languages and HDLs is that HDLs explicitly include the notion of time.

What is latch?

A latch is an example of a bistable multivibrator, that is, a device with exactly two stable states. These states are high-output and low-output. A latch has a feedback path, so information can be retained by the device. Therefore latches can be memory devices, and can store one bit of data for as long as the device is powered. As the name suggests, latches are used to "latch onto" information and hold in place. Latches are very similar to flip-flops, but are not synchronous devices, and do not operate on clock edges as flip-flops do.

What is flip-flop?

A flip-flop is a device very like a latch in that it is a bistable multivibrator, having two states and a feedback path that allows it to store a bit of information. The difference between a latch and a flip-flop is that a latch is asynchronous, and the outputs can change as soon as the inputs do (or at least after a small propagation delay). A flip-flop, on the other hand, is edge-triggered and only changes state when a control signal goes from high to low or low to high. This distinction is relatively recent and is not formal, with many authorities still referring to flip-flops as latches and vice versa, but it is a helpful distinction to make for the sake of clarity.

There are several different types of flip-flop each with its own uses and peculiarities. The four main types of flip-flop are : SR, JK, D, and T.

What is mux (multiplexer)?

A multiplexer (or mux) is a device that selects one of several analog or digital input signals and forwards the selected input into a single line.[1] A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output.[2] Multiplexers are mainly used to increase the amount of data that can be sent over the network within a certain amount of time and bandwidth.[1] A multiplexer is also called a data selector.

An electronic multiplexer makes it possible for several signals to share one device or resource, for example one A/D converter or one communication line, instead of having one device per input signal.

What is register? Register file?

Registers are a special, high-speed storage area within the CPU. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register. A register may hold a computer instruction , a storage address, or any kind of data (such as a bit sequence or individual characters). A register must be large enough to hold an instruction - for example, in a 32-bit instruction computer, a register must be 32 bits in length. In some computer designs, there are smaller registers - for example, half-registers - for shorter instructions. Depending on the processor design and language rules, registers may be numbered or have arbitrary names.

A register file is an array of processor registers in a central processing unit (CPU). Modern integrated circuit-based register files are usually implemented by way of fast static RAMs with multiple ports. Such RAMs are distinguished by having dedicated read and write ports, whereas ordinary multiported SRAMs will usually read and write through the same ports.

The instruction set architecture of a CPU will almost always define a set of registers which are used to stage data between memory and the functional units on the chip. In simpler CPUs, these architectural registers correspond one-for-one to the entries in a physical register file within the CPU. More complicated CPUs use register renaming, so that the mapping of which physical entry stores a particular architectural register changes dynamically during execution. The register file is part of the architecture and visible to the programmer, as opposed to the concept of transparent caches.

What is ALU?

An arithmetic logic unit (ALU) is a digital electronic circuit that performs arithmetic and bitwise logical operations on integer binary numbers. This is in contrast to a floating-point unit (FPU), which operates on floating point numbers. An ALU is a fundamental building block of many types of computing circuits, including the central processing unit (CPU) of computers, FPUs, and graphics processing units (GPUs). A single CPU, FPU or GPU may contain multiple ALUs.

The inputs to an ALU are the data to be operated on, called operands, and a code indicating the operation to be performed; the ALU's output is the result of the performed operation. In many designs, the ALU also exchanges additional information with a status register, which relates to the result of the current or previous operations.

What is floating-point unit?

A floating-point unit (FPU) is a part of a computer system specially designed to carry out operations on floating point numbers. Typical operations are addition, subtraction, multiplication, division, square root, and bitshifting. Some systems (particularly older, microcode-based architectures) can also perform various transcendental functions such as exponential or trigonometric calculations, though in most modern processors these are done with software library routines.

In general purpose computer architectures, one or more FPUs may be integrated with the central processing unit; however many embedded processors do not have hardware support for floating-point operations.

What is a cache?

Clearing Computer Cache. The cache (pronounced "cash") is a space in your computer's hard drive and in RAM memory where your browser saves copies of previously visited Web pages. Your browser uses the cache like a short-term memory

What is a bus?

In computer architecture, a bus (related to the Latin "omnibus", meaning "for all") is a communication system that transfers data between components inside a computer, or between computers.

Publishing work

What are the differences between copyright, trademark, trade secret?

“Intellectual property is something that is created by the mind.” Typically, we think of ideas as being created by the mind – but intellectual property does not protect bare ideas: rather, it is the expression or symbolic power/recognizability of the ideas that are protected. Thus, it is the design of the rocket that is patented, not the idea of a rocket. It is the painting of the lake that is copyrighted, not the idea of a lake. And it is the consumer recognizable logo that is trademarked, not the idea of a logo. Intellectual property protects how we express and identify ideas in concrete ways – not the idea itself.

In particular:

Patents: protect functional expressions of an idea – not the idea itself. A machines, method/process, manufacture, compositions of matter, and improvements of any of these items can be patented. Thus, I can patent a design for the nozzle on a rocket, or the method of making the rocket, or the method of making the rocket fuel, or the metal in which the rocket fuel is stored, or a new way of transporting the rocket fuel to the rocket. But I cannot patent the broad “idea” of a rocket.

Copyrights: protect the specific creative expression of an idea through any medium of artistic/creative expression – i.e., paintings, photographs, sculpture, writings, software, etc. A copyright protects your painting of a haystack, but it would not prohibit another painter from expressing their artistry or viewpoint by also painting a haystack. Likewise, while Ian Fleming was able to receive a copyright on his particular expression of the idea of a secret agent (i.e., a debonair English secret agent), he could not prevent Rich Wilkes from receiving a copyright on his expression of the idea of a secret agent (i.e., a tattooed bald extreme athlete turned reluctant secret agent).

Trademarks: protect any symbol that indicates the source or origin of the goods or services to which it is affixed. While a trademark can be extremely valuable to its owner, the ultimate purpose of a trademark is to protect consumers – that is, the function of a trademark is to inform the consumer where the goods or services originate. The consumer, knowing the origin of the goods, can make purchasing decisions based on prior knowledge, reputation or marketing.

Trade secret: is a formula, practice, process, design, instrument, pattern, commercial method, or compilation of information which is not generally known or reasonably ascertainable by others, and by which a business can obtain an economic advantage over competitors or customers.[

While each category is distinct, a product (or components/aspects of a product) may fall into one or more of the categories. For example, software can be protected by both patents and copyrights. The copyright would protect the artistic expression of the idea – i.e., the code itself – while the patent would protect the functional expression of the idea – e.g., using a single click to purchase a book online. Likewise, it is likely that the software company will use a trademark to indicate who made the software.

An additional example would be a logo for a company. The logo may serve as a trademark indicating that all products affixed with the logo are from the same source. The creative and artistic aspects of the logo may also be protected by a copyright.

Algorithms and data structures

What is time complexity of algorithm?

In computer science, the time complexity of an algorithm quantifies the amount of time taken by an algorithm to run as a function of the length of the string representing the input. Time complexity is commonly estimated by counting the number of elementary operations performed by the algorithm, where an elementary operation takes a fixed amount of time to perform.

What is space complexity of algorithm?

Space complexity is a measure of the amount of working storage an algorithm needs. That means how much memory, in the worst case, is needed at any point in the algorithm. It represents the total amount of memory space that a "normal" physical computer would need to solve a given computational problem with a given algorithm.

What's a good algorithm?

It executes as fast as possible. It takes as less space as possible. It is adaptable to computers. It is simple. It is elegant (well written).

History

What is Moore's law? What is Rock's law?

Moore's law is the observation that the number of transistors in a dense integrated circuit doubles approximately every two years. The observation is named after Gordon E. Moore, the co-founder of Intel and Fairchild Semiconductor, whose 1965 paper described a doubling every year in the number of components per integrated circuit, and projected this rate of growth would continue for at least another decade. In 1975, looking forward to the next decade, he revised the forecast to doubling every two years. Rock's law or Moore's second law, named for Arthur Rock or Gordon Moore, says that the cost of a semiconductor chip fabrication plant doubles every four years. As of 2015, the price had already reached about 14 billion US dollars.