CPU Architecture and the Fetch–Decode–Execute Cycle: Question 7
Syllabus 3.1
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]
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Worked solution
Part (a): The stored-program concept
In a Von Neumann architecture computer, the stored-program concept means that both the program’s instructions and the data those instructions operate on are held together in the same main memory. [1] For the photo frame, this means the slideshow program’s instructions and the actual image data for each photo both live in the same memory, rather than the program being kept somewhere separate from the data it processes. [1]
Part (b): Why only one instruction can be fetched at a time
The CPU communicates with memory using a fixed set of buses: one address bus and one data bus. [1] Because there is only a single address bus, it can carry only one memory address at a time, and because there is only a single data bus, only one instruction (or item of data) can travel back to the CPU at a time. With no second, independent set of buses available, the CPU is limited to fetching one instruction per fetch stage rather than several at once. [1]
Part (c): The consequence of a program counter fault
Normally, the program counter is updated after each fetch so that it holds the address of the next instruction, allowing the program to move forward. [1] If a fault meant the program counter stayed at the same address instead, the CPU would keep fetching, decoding and executing the identical instruction over and over. For the photo frame, this would most likely show up as the slideshow freezing (for example, endlessly repeating a single step) instead of ever reaching the instructions that load and display the next photo. [1]
Final answers
- (a) The stored-program concept means the photo frame’s instructions and image data are both held in the same memory.
- (b) A single address bus and a single data bus can each carry only one address or one instruction at a time, so only one instruction can be fetched per fetch stage.
- (c) The CPU would keep fetching and executing the same instruction repeatedly, most likely freezing the slideshow instead of progressing to the next photo.