Local Attacks

int power(long base, long exponent) {

   int counter;
   int result = 1;
   for (counter = 0; counter < exponent; counter++)
       result *= base;
   return result;

}

/* When processor enters the function body the arguments are already placed in registers r0=5 (base), r1=3 (exponent) */

cmp r1, #0 /* Compare exponent to 0 */ mov r2, #1 /* Place constant 1 in register r2,

                                    this corresponds to result = 1 in C code */

ble .L2 /* Exponent was not less than 0, so no jump to L2 mov r3, #0 /* Place constant 0 in register r3,

                                    this corresponds to variable counter */

add r3, r3, #1 /* Perform r3 = 0 + 1 which results in 1 being stored to r3

                                    this corresponds to first invocation of counter++ in C code */

cmp r3, r1 /* Compare counter (1 in this case) to exponent (3), this will be used by bne instruction below */ mul r2, r0, r2 /* Perform r2 = r0 * r2 which results in 1 * 5 = 5 being placed in r2

                                    this corresponds to first invocation of result *= base in C code */

bne .L3 /* The comparison resulted in counter being not equal to exponent, so we jump back to L3

                                    this corresponds to first invocation of counter < exponent in C code */

add r3, r3, #1 /* Perform r3 = 1 + 1 which results in 2 being stored to r3

                                    this corresponds to second invocation of counter++ in C code */

cmp r3, r1 /* Compare counter (2 in this case) to exponent (3), this will be used by bne instruction below */ mul r2, r0, r2 /* Perform r2 = r0 * r2 which results 5 * 5 = 25 being placed in r2

                                    this corresponds to second invocation of result *= base in C code */

bne .L3 /* The comparison resulted in counter being not equal to exponent, so we jump back to L3

                                    this corresponds to second invocation of counter < exponent in C code */

add r3, r3, #1 /* Perform r3 = 2 + 1 which results in 3 being stored to r3

                                    this corresponds to third invocation of counter++ in C code */

cmp r3, r1 /* Compare counter (3 in this case) to exponent (3), this will be used by bne instruction below */ mul r2, r0, r2 /* Perform r2 = r0 * r2 which results 25 * 5 = 125 being placed in r2

                                    this corresponds to third invocation of result *= base in C code */

bne .L3 /* The comparison resulted in counter being equal to exponent, so we DO NOT jump back to L3 */

mov r0, r2 /* Copy register r2 contents (125) to register r0 */ bx lr /* Jump back to caller */ /* Function returns with 125 placed in r0 this is where caller function should expect the return value */ /* The other registers will still hold whatever values were left there: r1 = 3, r2 = 125, r3 = 3 */  

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.[1] Data thus structured are said to be blocked. The process of putting data into blocks is called blocking, while deblocking is the process of extracting data from blocks. Blocked data is normally stored in a data buffer and read or written a whole block at a time.

 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. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.

 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.[1] Partitioning is typically the first step of preparing a newly manufactured disk, before any files or directories have been created

 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 filesystems 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

In the Linux operating system, JFS is supported with the kernel module (since the kernel version 2.4.18pre9-ac4) and the complementary userspace utilities packaged under the name JFSutils. Most Linux distributions support JFS, unless it is specifically removed due to space restrictions or other concerns.

Hardware

Computer hardware Jargon: CPU, RAM, ROM, HDD, SSD, PCI, PCI Express, USB 2.0, USB 3.0, VGA, HDMI, DVI, LCD, TFT, LED, OLED, AMOLED, CRT, PWM Lecture recording #1 Lecture recording #2 starting 12:30 Lecture slides Random access memory, permanent storage, buses, input devices, display technologies, networking Potential exam questions:  Different buses and their uses Bus is a system which help to transact the date between each component in computer or between computers. It has 2 types of buses in side computer (Asus socket 7) and outside of computers (Pc card or IEEE-448)

 What are the differences between hard disk drive (HDD) and solid state drive (SSD)? [1]

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

A flash translation layer is used to adapt a fully functional file system to the constraints and restrictions imposed by flash memory devices

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

RAM is Random Access Memory. ROM is Read Only Memory. RAM is the memory available for the operating system, programs and processes to use when the computer is running. ROM is the memory that comes with your computer that is pre-written to hold the instructions for booting-up the computer. RAM requires a flow of electricity to retain data (e.g. the computer powered on). ROM will retain data without the flow of electricity (e.g. when computer is powered off). RAM is a type of volatile memory. Data in RAM is not permanently written. When you power off your computer the data stored in RAM is deleted. ROM is a type of non- volatile memory. Data in ROM is permanently written and is not erased when you power off your computer. There are different types of RAM, including DRAM (Dynamic Random Access Memory) andSRAM (Static Random Access Memory). There are different types of ROM, including PROM (programmable read-only memory) that is manufactured as blank memory (e.g. a CD-ROM) and EPROM (erasable programmable read-only memory). There are many differences between RAM and ROM memory but there are also a couple similarities (and these are very easy to remember). Both types of memory used by a computer, and they are both required for your computer to operate properly and efficiently. EEPROM EEPROM , or electrically erasable programmable read only memory, is another step up from EPROM because EEPROM chips do away with some of the drawbacks. For example, EEPROM chips do not need to be removed to be rewritten. Additionally, a portion of the chip can be changed without erasing the entire chip. Furthermore, it does not require special equipment to rewrite the chip.

Volatile memory Non-volatile memory Requires a power source to retain information. Does not require a power source to retain information. When power source is disconnected, information is lost or deleted. When power source is disconnected, information is not deleted. Often used for temporary retention of data, such as with RAM, or for retention of sensitive data. Often used for long-term retention of data, such as files and folders.

 What is data retention? Data retention defines the policies of persistent data and records management for meeting legal and business data archival requirements;

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

Synchronous Circuits: These are the class of sequential circuits which are governed by a global clock signal generated by an oscillator. The state of all elements of a synchronous circuit changes only by an application of a distributed clock signal. So, this makes the state of a synchronous circuit predictable. Also, synchronous clock signals are less susceptible to noise, circuit anomalies and hence safer to design and operate. But they are limited in operation of speed by the propagation delay of the clock signal in reaching all the elements of the clock signal. The time period of a clock signal should be long enough to accommodate longest propagation delay. Practically all the circuits today are synchronous circuits, except the part where speed of the circuit operation is crucial.

Asynchronous Circuits:

Asyncronous circuits change state only through the inputs received by them. So, the operation is quite instantaneous since they dont have to wait for a clock pulse. They are limited by propagation delay of logic gates only. But asynchronous circuits can transition into a wrong state due to incorrect arrival time of 2 inputs. This is called a race condition. Asynchronous circuits are quite difficult to design for a reliable operation. These are used primarily in high speed systems such as Signal Processing hardware.

The basic difference between Static and Dynamic RAM lies mainly in structure and work principal.

•Firstly the main difference in the structure varies due to transistor and capacitor number and setting as just three to four transistors are required for a Dynamic RAM, but six to eight MOS transistors are necessary for a Static RAM.

•Secondly Dynamic RAM memory can be deleted and refreshed while running the program, but in case of Static RAM it is not possible to refresh programs. 

•Data is stored as a charge in a capacitor in Dynamic RAM, where data is stored in flip flop level in Static RAM.

•For refreshing a data another capacitor is required in case of Dynamic capacitor, but no refreshing option is available in Static RAM.
•A Dynamic RAM possesses less space in the chip than a Static RAM.
•Dynamic RAM is used to create larger RAM space system, where Static RAM create speed- sensitive cache.
•Static ram is 4 times more expensive than Dynamic RAM. •Dynamic RAM consumes less power than Static RAM. 

•For accessing a data or information, Static RAM takes less time than Dynamic RAM. •Dynamic RAM has higher storage capacity. In fact it can store 4 times than Static RAM.

 What is cache? What is cache coherence?

Cache is very fast and small memory that is placed in between the CPU and the 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? [2]

Explain how computer mouse works? History of computer mouse.

Ball mouse and optical mouse How does a mouse like this actually work? As you move it across your desk, the ball rolls under its own weight and pushes against two plastic rollers linked to thin wheels (numbered 6 and 7 in the photo). One of the wheels detects movements in an up-and-down direction (like the y-axis on graph/chart paper); the other detects side-to-side movements (like the x-axis on graph paper).

How do the wheels measure your hand movements? As you move the mouse, the ball moves the rollers that turn one or both of the wheels. If you move the mouse straight up, only the y-axis wheel turns; if you move to the right, only the x-axis wheel turns. And if you move the mouse at an angle, the ball turns both wheels at once. Now here's the clever bit. Each wheel is made up of plastic spokes and, as it turns, the spokes repeatedly break a light beam. The more the wheel turns, the more times the beam is broken. So counting the number of times the beam is broken is a way of precisely measuring how far the wheel has turned and how far you've pushed the mouse. The counting and measuring is done by the microchip inside the mouse, which sends details down the cable to your computer. Software in your computer moves the cursor on your screen by a corresponding amount.

An optical mouse works in a completely different way. It shines a bright light down onto your desk from an LED (light-emitting diode) mounted on the bottom of the mouse. The light bounces straight back up off the desk into a photocell (photoelectric cell), also mounted under the mouse, a short distance from the LED. The photocell has a lens in front of it that magnifies the reflected light, so the mouse can respond more precisely to your hand movements. As you push the mouse around your desk, the pattern of reflected light changes, and the chip inside the mouse uses this to figure out how you're moving your hand. The mouse was invented by Douglas Engelbart in 1964 and consisted of a wooden shell, circuit board and two metal wheels that came into contact with the surface it was being used on.

 Explain how computer keyboard works? HowStuffworks article Explain that Stuff article Keyboard History http://www.explainthatstuff.com/computerkeyboards.html

Keyboards and typing technology have come a long way over the past couple centuries. The first typing devices were designed and patented in the 1700s while the first manufactured typing devices came about in the 1870s. These machines featured “blind typing” technology, where characters were printed on upside-down pages that remained unseen until completion. Since then, we have seen several updates in design, layout, technology, and function that are more efficient and user-friendly.

 Explain how cathode ray tube (CRT) based screen technology works and name pros/cons. [3]

Sort for cathode-ray tubes, CRT monitors were the only choice consumers had for monitor technology for many years. Cathode ray tube (CRT) technology has been in use for more than 100 years, and is found in most televisions and computer monitors. A CRT works by moving an electron beam back and forth across the back of the screen. Each time the beam makes a pass across the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. By drawing many such lines from the top to the bottom of the screen, it creates an entire screen of images.

Resolution on a CRT is flexible and a newer model will provide you with viewing resolutions of up to 1600 by 1200 and higher, On a CRT the sharpness of the picture can be blemished by soft edges or a flawed focus. A CRT monitor can be viewed from almost any angle Some users of a CRT may notice a bit of an annoying flicker, which is an inherent trait based on a CRTs physical components. Today's graphics cards, however, can provide a high refresh rate signal to the CRT to get rid of this otherwise annoying problem.. Screen (viewable) Size Most people today tend to look at a 17-inch CRT or bigger monitor. When you purchase a 17-inch CRT monitor, you usually get 16.1 inches or a bit more of actual viewing area, depending on the brand and manufacturer of a specific CRT. Physical Size There is no denying that an LCD wins in terms of its physical size and the space it needs. CRT monitors are big, bulky and heavy. They are not a good choice if you're working with limited desk space, or need to move the monitor around (for some odd reason) between computers

 Explain how liquid crystal displays (LCD) work and name pros/cons. [4]

Short for liquid crystal display, LCD technology can be found in digital watches and computer monitors. LCD displays use two sheets of polarizing material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them. Each crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light. Color LCD displays use two basic techniques for producing color: Passive matrix is the less expensive of the two technologies. The other technology, calledthin film transistor (TFT) or active-matrix, produces color images that are as sharp as traditional CRT displays, but the technology is expensive.

resolution

an LCD the resolution is fixed within each monitor (called a native resolution). The resolution on an LCD can be changed, but if you're running it at a resolution other than its native resolution you will notice a drop in performance or quality. Both types of monitors (newer models) provide bright and vibrant color display. However, LCDs cannot display the maximum color range that a CRT can. In terms of image sharpness, when an LCD is running at its native resolution the picture quality is perfectly sharp. On a CRT the sharpness of the picture can be blemished by soft edges or a flawed focus. A CRT monitor can be viewed from almost any angle, but with an LCD this is often a problem. When you use an LCD, your view changes as you move different angles and distances away from the monitor. At some odd angles, you may notice the picture fade, and possibly look as if it will disappear from view.

Refresh Rate

Some users of a CRT may notice a bit of an annoying flicker, which is an inherent trait based on a CRTs physical components. Today's graphics cards, however, can provide a high refresh rate signal to the CRT to get rid of this otherwise annoying problem. LCDs are flicker-free and as such the refresh rate isn't an important issue with LCDs. Dot Pitch Dot pitch refers to the space between the pixels that make up the images on your screen, and is measured in millimeters. The less space between pixels, the better the image quality. On either type of monitor, smaller dot pitch is better and you're going to want to look at something in the 0.26 mm dot pitch or smaller range. Screen (viewable) Size Most people today tend to look at a 17-inch CRT or bigger monitor. When you purchase a 17-inch CRT monitor, you usually get 16.1 inches or a bit more of actual viewing area, depending on the brand and manufacturer of a specific CRT. The difference between the "monitor size" and the "view area" is due to the large bulky frame of a CRT. If you purchase a 17" LCD monitor, you actually get a full 17" viewable area, or very close to a 17".

Physical Size

There is no denying that an LCD wins in terms of its physical size and the space it needs. CRT monitors are big, bulky and heavy. They are not a good choice if you're working with limited desk space, or need to move the monitor around (for some odd reason) between computers. An LCD on the other hand is small, compact and lightweight. LCDs are thin, take up far less space and are easy to move around. An average 17-inch CRT monitor could be upwards of 40 pounds, while a 17&-inch LCD would weigh in at around 15 pounds. Price As an individual one-time purchase an LCD monitor is going to be more expensive. Throughout a lifetime, however, LCDs are cheaper as they are known to have a longer lifespan and also a lower power consumption. The cost of both technologies have come down over the past few years, and LCDs are reaching a point where smaller monitors are within many consumers' price range. You will pay more for a 17" LCD compared to a 17" CRT, but since the CRT's actual viewing size is smaller, it does bring the question of price back into proportion. Today, fewer CRT monitors are manufactured as the price on LCDs lowers and they become mainstream.

 Name screen technologies making use of thin film transistor (TFT) technology? [5]

A thin-film transistor (TFT) is a special kind of field-effect transistor made by depositing thin films of an active semiconductor layer as well as the dielectric layer and metallic contacts over a supporting (but non-conducting) substrate. A common substrate is glass, because the primary application of TFTs is in liquid-crystal displays. This differs from the conventional transistor, where the semiconductor material typically is the substrate, such as a silicon wafer.

 Name uses for light polarization filters? [6] [7] Camera, tv, photography….

	What are the benefits of twisted pair cabling and differential signalling? twisted pair cabling
   Electrical noise going into or coming from the cable can be prevented.[10] 	Cross-talk is minimized differential signalling The technique minimizes electronic crosstalk and electromagnetic interference, both noise emission and noise acceptance, and can achieve a constant or known characteristic impedance, allowing impedance matching techniques important in a high-speed signal transmission line or high qualitybalanced line and balanced circuit audio signal path.

	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.

• Compare FAT32 and NTFS

NTFS

NTFS is the preferred file system for this version of Windows. It has many benefits over the earlier FAT32 file system, including:

The capability to recover from some disk-related errors automatically, which FAT32 cannot.

Improved support for larger hard disks.

Better security because you can use permissions and encryption to restrict access to specific files to approved users.

FAT32

FAT32, and the lesser-used FAT, were used in earlier versions of Windows operating systems, including Windows 95, Windows 98, and Windows Millennium Edition. FAT32 does not have the security that NTFS provides, so if you have a FAT32 partition or volume on your computer, any user who has access to your computer can read any file on it. FAT32 also has size limitations. You cannot create a FAT32 partition greater than 32GB in this version of Windows, and you cannot store a file larger than 4GB on a FAT32 partition.

Bootloaders, kernels

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

BIOS  BIOS (Basic Input/Output System) is read from EEPROM and copied to RAM • Processor starts executing the BIOS code in RAM • BIOS sets up the hardware and probes storage, USB etc for bootable media • BIOS reads master boot record of selected bootable media and boot loader takes over UEFI

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.

 Turn on the Power button.  • CPU pins are reset and registers are set to specific value.  • CPU jump to address of BIOS (0xFFFF0).  • BIOS run POST (Power-On Self Test) and other necessary checks.  • BIOS jumps to MBR(Master Boot Record).  • Primary Bootloader runs from MBR and jumps to Secondary Bootloader.  • Secondary Bootloaders loads Operating System

 Describe the functionality provided by general purpose operating system. See architecture of Windows NT, Android, OS X.

User mode in Windows NT is made of subsystems capable of passing I/O requests to the appropriate kernel mode device drivers by using the I/O manager. The user mode layer of Windows NT is made up of the "Environment subsystems," which run applications written for many different types of operating systems, and the "Integral subsystem," which operates system specific functions on behalf of environment subsystems.

 What are the main differences between real mode and protected mode of x86-based processor?  If your computer is in real mode, software communicates directly with the computer's ports and devices. For example, when you print a document, the software sends the data stream directly to the port that holds the printer. However, this paradigm doesn't work in a multitasking OS. Imagine what would happen if multiple programs sent data streams to the ports simultaneously. Ports are dumb, and they have no ability to filter or arrange data streams to match the sending programs.  If your computer is in protected mode, the system's ports and devices are protected from the applications that use them. The software thinks it's sending data to a port, but it's a virtual port. The OS is grabbing the data stream and managing it, to ensure that all applications have equal access and to ensure that data from each application is appropriately preserved.

 What happens during context switch?

 In a switch, the state of the first process (assuming that the first process is the process in execution and is to be switched) must be saved somehow, so that, when the scheduler gets back to the execution of the first process, it can restore this state and continue.  The state of the process includes all the registers that the process may be using, especially the program counter, plus any other operating system specific data that may be necessary. This data is usually stored in a data structure called a process control block (PCB), or switchframe.  In order to switch processes, the PCB for the first process must be created and saved. The PCBs are sometimes stored upon a per-process stack in kernel memory (as opposed to the user-mode call stack), or there may be some specific operating system defined data structure for this information.  Since the operating system has effectively suspended the execution of the first process, it can now load the PCB and context of the second process. In doing so, the program counter from the PCB is loaded, and thus execution can continue in the new process. New processes are chosen from a queue or queues (often referred as ready queue). Process and thread priority can influence which process continues execution, with processes of the highest priority checked first for ready threads to execute.

 What is the purpose of paged virtual memory?

 In computing, virtual memory is a memory management technique that is implemented using both hardware and software. It maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory. Main storageas seen by a process or task appears as a contiguous address space or collection of contiguous segments. The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit or MMU, automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities to provide a virtual address space that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer.  The primary benefits of virtual memory include freeing applications from having to manage a shared memory space, increased security due to memory isolation, and being able to conceptually use more memory than might be physically available, using the technique of paging.

Programming languages

 What are the major steps of compilation?

1, Lexical analysis (scanning): the source text is broken into tokens. Syntactic analysis (parsing): tokens are combined to form syntactic structures, typically represented by a parse tree.

2, The parser may be replaced by a syntax-directed editor, which directly generates a parse tree as a product of editing. Semantic analysis: intermediate code is generated for each syntactic structure.

3, Type checking is performed in this phase. Complicated features such as generic declarations and operator overloading (as in Ada and C++) are also processed. Machine-independent optimization: intermediate code is optimized to improve efficiency.

4,Code generation: intermediate code is translated to relocatable object code for the target machine.

5,Machine-dependent optimization: the machine code is optimized.

 What are the differences between interpreted, JIT-compilation and traditional compiling?]]

Traditional Compiled languages  are written in a code that can be executed directly on a computer’s processor. This is because a compiler has translated the code into the computer’s “native” language up front, well before the program is even run. This process can take many passes before it is optimized as machine code, but the output is always code that’s ready to be executed—and that executes efficiently, as a result. Some compiled languages include: • C • C++ • C#

INTERPRETED LANGUAGES • An interpreted language is any programming language that isn’t already in “machine code” prior to runtime. Unlike compiled languages, an interpreted language’s translation doesn’t happen beforehand. Translation occurs at the same time as the program is being executed. Some interpreted languages include: • Java • JavaScript • PHP • Perl • Python • Ruby

Just in Time” (JIT) Compilers • JIT compilers are next-generation compilers, but they don’t just run code—they improve it over time. • Java has a JIT compiler as part of the Java Virtual Machine (JVM); C# has one within the .NET framework; and Android has a JIT in its Dalvik Virtual Machine (DVM)

 What is control flow? Loops? Conditional statements?

control flow (or alternatively, flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an imperative programming language from a declarative programming language.

Loop is a sequence of instruction s that is continually repeated until a certain condition is reached. Typically, a certain process is done, such as getting an item of data and changing it, and then some condition is checked such as whether a counter has reached a prescribed number.

conditional statements, conditional expressions and conditional constructs are features of a programming language, which perform different computations or actions depending on whether a programmer-specified boolean conditionevaluates to true or false. Apart from the case of branch predication, this is always achieved by selectively altering the control flow based on some condition.