A-Level Computer Science: Systems, Data and Networks

Half of A-Level Computer Science is about the machine rather than the algorithm, and most of that half is worked rather than recalled. So these ten questions are calculations and scenarios: how many cycles a four-stage pipeline needs for four instructions, what minus five looks like in eight-bit two's complement, what a stored mantissa and exponent actually come to in denary, which normal form a table with a partial dependency is in, and what a given SQL join with a WHERE clause returns.

The wrong options are the specific slips those calculations invite. Sixteen cycles is what you get if you forget the pipeline overlaps. The inverted bits without the added one is a two's complement answer that stops one step early. In the floating-point question, dividing instead of multiplying gives 0.09375, reading the exponent as a plain multiplier instead of a power of two gives 4.5, and the explanation names each one rather than simply restating the right answer.

The rest covers the Von Neumann bottleneck and what actually causes it, which layer of the TCP/IP stack routes packets, which key encrypts a message when you want only the recipient to read it — the commonest error there is reaching for the sender's private key, which is what signs rather than what conceals — what a data controller is responsible for, and what a page table does.

Notation is board-neutral throughout: plain pseudocode and standard SQL, with every table, query and bit pattern written out inside the question. Boards set their own reference languages, and none of them is assumed here.

Nothing is taken from any exam board specification, past paper, mark scheme or textbook.

Zestly is an independent study tool. It is not affiliated with any exam board and it is not an exam centre.

• Work out how many cycles a pipelined processor needs for a given number of instructions
• Convert a negative denary value to two's complement, and read a stored mantissa and exponent back as a denary value
• Identify which normal form a described table satisfies, and say which dependency breaks the next one
• Say what a given SQL join with a WHERE clause returns
• Place a named networking job at the right layer of the TCP/IP stack
• Choose the correct key for confidentiality in an asymmetric exchange, and say what the other keys are for
• State what a data controller is responsible for, and what a page table does

A 12-bit floating-point format stores an 8-bit mantissa in two's complement, with the binary point immediately after its sign bit, followed by a 4-bit exponent in two's complement. A stored number has mantissa 01100000 and exponent 0011. What denary value does it represent? — one of ten worked questions written for this set.