CPU Architecture and the Fetch–Decode–Execute Cycle: Computer Science 0478 (Cambridge O Level / IGCSE)

A computer science student is using a CPU simulator that displays the current contents of five named registers while a single machine-code instruction is fetched, decoded and executed. While the control unit is decoding the instruction, one particular register on the screen holds the exact bit pattern of that instruction, unchanged, right up until the next fetch stage begins and a new bit pattern is loaded into it.

Which register is being described?

A vending machine contains a small computer with a Von Neumann architecture. During its stock-check routine, the CPU is about to fetch the next machine-code instruction, which is stored at address 300 in RAM.

(a) Describe, in the correct order, what happens to this address and then to the instruction itself during the fetch stage of the FDE cycle. In your answer, name the specific register or bus involved at each step. [4]

(b) State the name of the CPU component that decodes the fetched instruction during the decode stage, and describe in one sentence what "decoding" an instruction means. [2]

(c) The decoded instruction turns out to be an arithmetic instruction that adds two values together. Identify the CPU component that carries out this addition during the execute stage. [1]

A gaming café is comparing two PCs for customers who edit video. PC Nimbus has a quad-core processor, a clock speed of 3.0 GHz and a 6 MB cache. PC Zephyr has a dual-core processor, a clock speed of 4.2 GHz and a 2 MB cache.

(a) The owner runs a test where each PC exports a video while also scanning for viruses in the background. PC Nimbus finishes both tasks noticeably sooner than PC Zephyr. Explain, in terms of cores, why PC Nimbus can handle these two tasks faster. [2]

(b) When only the video export is run, with no background scanning, PC Zephyr completes the export slightly faster than PC Nimbus, despite having fewer cores. Explain, in terms of clock speed, why this can happen. [2]

(c) Both PCs are then tested exporting a long video built from many small effect files that are reused throughout the project. PC Nimbus completes this export faster than its clock speed alone would suggest. Explain, in terms of cache, why a larger cache can give this advantage. [3]

A rice cooker contains a single microprocessor running one program stored permanently in its memory. A temperature sensor tells the microprocessor when the rice and water inside have reached boiling point, and the microprocessor then automatically switches the cooker from "cook" mode to "keep warm" mode. The owner cannot reprogram this microprocessor to run any other software.

(a) State what is meant by an embedded system. [1]

(b) Explain two reasons, based on the description above, why the rice cooker's microprocessor should be classed as an embedded system rather than as a general purpose computer. [4]

(c) A different kitchen device, a recipe tablet, can run a recipe app, a web browser and a video-calling app, and its owner can install further apps from an app store. State, with a reason, whether this tablet should be classed as an embedded system or a general purpose computer. [2]

An electronics hobbyist buys a single-board computer for a robotics project. The board's spec sheet states: "Single microprocessor, instruction set of 58 machine-code instructions, clock speed 1.4 GHz."

(a) State what is meant by a microprocessor. [1]

(b) State what is meant by the instruction set of a CPU, referring to the spec sheet above. [2]

(c) The hobbyist writes their robot's control program in a high-level language, but the CPU can only directly execute the 58 instructions in its instruction set. Explain why the hobbyist's program must be translated before the CPU can run it. [3]

A CPU is about to fetch an instruction from RAM. The memory address register has already placed the address of that instruction onto one bus. Before any data travels back from RAM, the control unit sends a separate signal to RAM, telling it that the CPU wants to read from that address rather than write to it.

Which bus carries this read signal?

A digital photo frame contains a simple computer with a Von Neumann architecture. Its memory holds both the program that controls the slideshow and the image data for each photo, stored together in the same memory.

(a) State what is meant by the "stored-program concept", referring to how the photo frame's memory is used. [2]

(b) The photo frame's CPU can only fetch one instruction from memory at any single moment. Explain why this is the case, in terms of the buses connecting the CPU to memory. [2]

(c) Explain what would most likely happen to the photo frame's behaviour if a fault caused the program counter to stay at the same address instead of moving to a new address after each instruction is fetched. [2]

A simple electronic scoreboard for a table-tennis match contains a microprocessor. During the match, it repeatedly carries out two operations: adding one point to a player's score, and comparing the two players' current scores to check whether either player has reached 11 points.

(a) Name the CPU component that carries out both of these operations. [1]

(b) State whether each of the following operations is an arithmetic operation or a logical (comparison) operation, and give a brief reason for your answer: (i) adding one point to a player's score [2] (ii) checking whether a player's score is greater than or equal to 11 [2]

(c) State which register normally holds the result immediately after this component completes an operation. [1]

A student runs a single-threaded photo-filter app that cannot split its workload across more than one core. They time how long it takes to apply the same filter on two laptops, both with an identical clock speed of 2.4 GHz:

• Laptop P: octa-core processor (8 cores)
• Laptop Q: dual-core processor (2 cores)

Which statement correctly predicts and explains how long the app takes to run on each laptop?

A manufacturer is designing a digital kitchen scale. It only needs to measure weight, display the reading, and let the user zero the scale. The manufacturer is choosing between two options: building the scale around a small, dedicated embedded microprocessor that runs one fixed program, or building it around a general purpose computer chip capable of running a full operating system and multiple apps.

(a) State and explain two advantages that the dedicated embedded microprocessor option would most likely offer the manufacturer for this kitchen scale, compared with the general purpose computer chip option. [4]

(b) State one advantage that the general purpose computer chip option would offer instead. [1]

(c) Explain one disadvantage, in the context of this simple kitchen scale, of using a general purpose computer chip instead of the dedicated embedded microprocessor. [2]