Why did I lose the mark?

The answer that scoresA logical left shift moves every bit one place to the left, shifts a 0 in at the right-hand end and loses any bit pushed off the left-hand end. If no 1 is lost the value is multiplied by 2. If a 1 is lost from the most significant bit the value is not multiplied by 2: left shifting 1100 0011 (195) by one place gives 1000 0110, which is 134, because the 1 worth 128 has been lost.

The answer that scoresHexadecimal is base 16, so the place values are 16 and 1. 2D -> (2 x 16) + (13 x 1) = 32 + 13 = 45. The same answer can be checked by converting to binary first: 2D is 0010 1101, which is 32 + 8 + 4 + 1 = 45.

The answer that scoresWrite +25 as 0001 1001. Invert every bit to get 1110 0110, then add 1 to get 1110 0111. So -25 is 1110 0111. You can check it: the sign bit is 1, so the value is negative, and inverting 1110 0111 gives 0001 1000, which is 24; adding 1 gives 25, so the value is -25.

The answer that scoresFor an image of 1024 x 768 pixels with a colour depth of 8 bits: 1024 x 768 = 786 432 pixels; 786 432 x 8 = 6 291 456 bits; ÷ 8 = 786 432 bytes; ÷ 1024 = 768 KiB. Use 1024 for every step up between storage units, and give the answer in the unit the question asked for.

The answer that scoresIn packet switching, data is broken down into packets and each packet can take a different route to the destination. A router controls the route that each packet takes, choosing the best route available at the time. The packets may therefore arrive out of order, and once the last packet has arrived the receiving device uses the packet numbers in the headers to reorder them and rebuild the data.

The answer that scoresParallel transmission sends several bits at the same time, one down each wire, so it is faster than serial for the same clock speed, which makes it suitable for short connections. Over a long distance the bits can become skewed, so bits sent together arrive at slightly different times and the data is read incorrectly, and a multi-wire cable suffers more from interference. Serial transmission is used over long distances because there is only one stream of bits, so the data cannot become skewed and only one wire is needed.

The answer that scoresA parity check adds a parity bit to each byte so that the number of 1s is always odd (odd parity) or always even (even parity). The receiver counts the 1s in each byte it receives; if the count does not match the parity being used, an error has occurred and the data is re-sent. The check only works when an odd number of bits have changed, so if two bits in the same byte are corrupted the error is not detected.

The answer that scoresData has to pass through connections and devices that the sender does not control, so it can be intercepted. Encryption converts the data into a form that cannot be understood without the key needed to decrypt it, so even if the data is intercepted it is meaningless to anyone who does not have the key. This is why personal details, passwords and card payments are encrypted before transmission.

The answer that scoresThe program counter holds the address of the next instruction to be fetched, and it is incremented during the fetch stage so that it points to the following instruction. The instruction that has just been fetched is copied from the memory data register into the current instruction register, and the CIR is the register that holds the instruction being decoded and executed.

The answer that scoresRAM is volatile primary storage: it can be read from and written to, it holds the programs and data currently in use, and its contents are lost when the power is turned off. ROM is non-volatile primary storage: it can be read but not changed, and it holds the start-up instructions the computer needs to boot. A computer needs both, because the start-up instructions must always be available and must not be lost, while the programs in use need fast, changeable storage.

The answer that scoresA core is a complete processing unit, so a multi-core processor can process several instructions at the same time. More cores improve performance because more instructions are processed simultaneously, but only if the software is written to make use of more than one core. The performance of a CPU also depends on the clock speed, which is the number of cycles per second, and the size of the cache, which reduces the number of fetches from the slower RAM.

The answer that scoresA MAC address is given to the network interface card at the point of manufacture and cannot be changed. It is 48 bits long, usually written as hexadecimal in six pairs of digits, and it is created from the manufacturer code and the serial code, so it uniquely identifies the hardware. An IP address is different: it is allocated by the network, and it can be static, so it stays the same, or dynamic, so it is allocated each time the device connects.

The answer that scoresSystem software provides the services that the computer requires, and it includes both the operating system and utility software. Application software provides the services that the user requires. A backup program is system software because it maintains the computer, while the word processor that the backup is protecting is application software.

The answer that scoresThe processor finishes the instruction it is executing and saves the contents of its registers and the address of the next instruction. It then looks up the address of the interrupt service routine for that interrupt and jumps to it. When the ISR has finished, the saved registers and the address of the next instruction are restored, and the processor continues from exactly the point at which it was interrupted.

The answer that scoresAn interpreter translates and executes line by line, so it stops at the first error and reports it, which is useful while a program is being developed because the programmer gets feedback immediately. A compiler translates the whole program at once, reports all the errors it finds for the whole program, and produces an executable file that runs fast and can be used on a computer with no development software installed. An interpreter is mostly used when developing a program and a compiler is used to translate the final program.

The answer that scoresAssembly language is a low-level language that uses mnemonics in place of binary opcodes and names in place of memory addresses. Because the processor does not understand mnemonics, an assembler is needed to translate the assembly language program into machine code, replacing each mnemonic with its opcode and each named address with the actual address.

The answer that scoresA compiler translates the whole program at once before executing it, and produces an executable file. If errors are detected it produces an error report for the whole code, so the programmer can fix several errors at once. An interpreter translates and executes the code line by line and stops execution when it finds an error, which is the behaviour described in the wrong answer.

The answer that scoresThe internet is the infrastructure: the global network of interconnected networks, made of cables, routers, switches and servers, over which data travels. The world wide web is the collection of websites and web pages accessed using the internet. The web therefore depends on the internet, but the internet can be used for other services such as email and video calls without any web pages being involved.

The answer that scoresThe browser takes the domain name from the URL and asks the domain name server, which holds a directory matching domain names to IP addresses, and the DNS returns the IP address of the web server. The browser then sends its HTTP or HTTPS request to the web server at that IP address, and the web server returns the HTML for the page, which the browser renders and displays.

The answer that scoresA cookie is a small text file that a website stores on the user's device and that the browser sends back to that site on the next visit. Because HTTP itself keeps no memory of earlier requests, the cookie is how a site recognises a returning user. A session cookie lasts only while the browser is open and keeps one visit coherent, such as holding the contents of a shopping basket; a persistent cookie is stored on the device with an expiry date, so it can remember login details or preferences between visits.

The answer that scoresA virus attaches itself to a file or a program and spreads when that file is opened or that program is run, so it needs a host and usually an action by the user. A worm is a standalone program that copies itself and spreads across a network by itself, without a host file and without the user doing anything, which is why worms can spread very quickly and use up a great deal of bandwidth.

The answer that scoresIn a distributed denial of service attack, a large number of compromised computers are made to send requests to a server at the same time, so that the server is overwhelmed and cannot respond to genuine users. The aim is to make the service unavailable rather than to steal data. Data would be stolen by hacking or by intercepting the data while it is being transmitted.

The answer that scoresThe sensor measures a physical quantity and sends the reading to the microprocessor. The microprocessor compares the reading with the value stored in its program and decides what to do. If something needs to change, it signals an actuator, which carries out the physical action such as switching on a heater or opening a valve. The sensor then measures the change and the loop repeats.

The answer that scoresA robot has a mechanical structure or framework, so it can act physically in the world; it contains electrical components including sensors, microprocessors and actuators, which gather information, decide and move; and it is programmable, so its instructions can be written and changed. A washing machine is an automated machine, but it is not programmable in that sense and its body does not carry out varied physical tasks.

The answer that scoresArtificial intelligence is a branch of computer science dealing with the simulation of intelligent behaviours by computers. Its main characteristics are a collection of data and the rules for using that data, the ability to reason, and the ability to learn and adapt. The behaviour may look intelligent, but the computer is processing data according to rules rather than thinking in the way a person does.

The answer that scoresAn expert system reproduces the decision-making of a human expert using a knowledge base of facts, a rule base of rules written by that expert, an inference engine that applies the rules to the facts, and an interface for the user. It does not learn by itself. Machine learning is when a program has the ability to automatically adapt its own processes and/or data, finding patterns in the data it is given and improving as more data arrives.

The answer that scoresAutomated systems are consistent, work continuously and can operate in conditions that would harm a person, but they are expensive to design and install, they need skilled maintenance, they can reduce the number of workers needed, and they cannot deal with anything outside their programmed rules. A failed sensor may go unnoticed, and if the system stops there may be nobody on site able to take over by hand.

The answer that scoresDecomposition is breaking a problem down into its component parts, such as splitting an online shop into a basket, a payment system and a delivery calculator. Abstraction is removing the detail that is not needed for the problem being solved and keeping only what is, in the same way that a tube map keeps the stations and the lines but throws away the real distances.

The answer that scoresA pre-condition loop (WHILE ... DO ... ENDWHILE) tests its condition before the statements are run, so the statements run only while the condition is true and may never run at all. A post-condition loop (REPEAT ... UNTIL) runs the statements first and tests the condition afterwards, so the statements always run at least once and the loop stops when the condition becomes true.

The answer that scoresWrite one row for every pass through the loop and fill in every variable after it has changed. If four values are input, the table needs four rows, each showing the values of the total and the count at that point, followed by a final row showing what the algorithm outputs.

The answer that scoresValidation is an automatic check that the data is sensible and allowed, such as a range check that an exam mark lies between 0 and 100, or a presence check that a surname field is not empty. Verification is a check that the data has been entered accurately, using a visual check or a double entry check.

The answer that scoresSet the maximum to the first value in the list, Maximum ← Score[1], and then compare the remaining values with it from the second element onwards. That way the maximum always starts as a value that really is in the list, and the algorithm works whatever the numbers are.

The answer that scoresThe correct statement is Score ← 10, which stores the value 10 in the variable Score. The assignment operator ← replaces whatever was in the variable, so Score ← Score + 1 increases Score by one by working out the right-hand side first and then storing the result.

The answer that scoresDeclare the variable as REAL, because an average can contain a fractional part: DECLARE AverageMark : REAL. Use INTEGER only for values that are whole numbers by their nature, such as the number of pupils in a class or a count of correct answers.

The answer that scoresInitialise before the loop and update inside it: Total ← 0 comes before the FOR statement, and Total ← Total + Number sits inside the loop. The same rule applies to a counting variable, which must be set to 0 before the loop and increased by 1 inside it.

The answer that scoresUse the function inside an expression, for example OUTPUT Average(Total, Count) or AverageScore ← Average(Total, Count). Use CALL only for a procedure, such as CALL DisplayHeading, which returns no value.

The answer that scoresChoose one form and complete it. A pre-condition loop is WHILE Count < 10 DO ... ENDWHILE, which may run zero times. A post-condition loop is REPEAT ... UNTIL Count = 10, which always runs at least once and stops when the condition becomes true.

The answer that scoresProductCode is the primary key, because every product has its own code and no two records share one. ProductName cannot be the key: a shop may sell two different products that share a name, so the field would not identify one record uniquely. A primary key must also have a value in every record and must never change, which a product code satisfies.

The answer that scoresPrice must be a real, because it has a fractional part: 24.50, 18.00 and 39.99 are all prices. Integer is right for a whole-number field such as QuantityInStock. The same test explains why a telephone number is text/alphanumeric rather than integer - it is never added up, and an integer would drop a leading zero.

The answer that scoresValidation checks that data is sensible before it is stored: a range check rejects a quantity of 700, a length check rejects a product code of three characters and a presence check rejects an empty name field. It cannot make the data correct, because a price of 24.50 typed as 24.05 passes every check and is still the wrong price.

The answer that scoresORDER BY Price DESCENDING puts the records in order of price with the largest price first, so the most expensive product is at the top of the output. ORDER BY Price ASCENDING would give the cheapest first. The WHERE clause, not the ORDER BY, is what decides which records appear at all.

The answer that scoresSELECT SUM(Price) FROM Stock adds up the price field and gives the total. SELECT COUNT(Price) FROM Stock counts how many records were selected and gives a number of records. Both return a single number rather than a list, which is why they are easy to confuse.

The answer that scoresA NAND gate outputs 0 only when both inputs are 1, and 1 in the other three rows of its truth table, which makes it the exact opposite of AND. The circle on the output of the symbol is the NOT that reverses AND's output. For inputs 0 and 1, an AND gate gives 0 and a NAND gate gives 1.

The answer that scoresAn XOR gate outputs 1 when the two inputs are different, so it gives 1 for inputs 0 and 1 and for inputs 1 and 0, and 0 when both inputs are 1 or both are 0. An OR gate, by contrast, outputs 1 when both inputs are 1. The extra curved line across the inputs of the XOR symbol is what marks this difference.

The answer that scoresA truth table needs one row for every possible combination of the inputs: 2 to the power 3 = 8 rows for three inputs, and 2 to the power 2 = 4 rows for two inputs. Write them in binary counting order, 000, 001, 010, 011, 100, 101, 110, 111, so that every combination appears exactly once.

The answer that scoresCircuits must be drawn for the statement given, without simplification. If the statement says the output is 1 when A and B are both on, or when A and C are both on, the circuit is two AND gates feeding an OR gate. Simplifying that to A AND (B OR C) would give the same output but is not the circuit the statement describes.

The answer that scoresBracket the output of every gate so the order is unambiguous. If the circuit has A and B into an AND gate and the output of that gate into an OR gate with C, the expression is Q = (A AND B) OR C. If the circuit has B and C into an OR gate and the output of that gate into an AND gate with A, it is Q = A AND (B OR C).