AS Level · Topics 3–4
Trace processor operations
Use registers, buses, assembly-style operations, and bit masks.
Lessons distinguish AS topics 1–12 from A Level topics 13–20. Examples include Cambridge pseudocode and Python.
Track data and addresses separately
The program counter identifies the next instruction. During a fetch, the memory address register holds the requested address, the memory data register holds the transferred value, and the current instruction register holds the instruction being decoded. An address is a location, not the value stored there.
Address, data, and control buses carry different information. To explain an instruction, trace which register supplies an address, what memory returns, and how the control unit directs execution. Keep the accumulator's changing value in a trace table.
Use masks to isolate bits
Bitwise AND with a mask keeps only positions where the mask has a 1. OR can set selected bits, while XOR can toggle them. For example, AND with 00001111 extracts the low four bits without affecting their relative positions.
Distinguish a bitwise operation on every bit from a Boolean operation on a whole condition. For a logic circuit, compute intermediate gate outputs before the final output, then test every input combination when constructing a truth table.
Check yourself
What is 10110110 AND 00001111?Show answer
00000110. Only the low four input bits can remain set.
References used to build these lessons
We used these official documents as references when building this course's lessons. They are the sources for the syllabus structure and course requirements.
Cambridge International · 9618 · Exams 2027–2029