Processor Fundamentals and Assembly Language: Question 10
Syllabus 4.1
A student writes four statements about the Von Neumann architecture used in most general-purpose computers, and wants to identify which one is genuinely correct.
Which of these statements correctly describes a feature of the Von Neumann architecture?
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Worked solution
Why A is correct
The defining feature of the Von Neumann architecture is the stored program concept: program instructions and the data they operate on are held together in a single, shared main memory, rather than in separate memories. Because there is only one memory, there is also only one set of address, data and control buses connecting the CPU to it, and both instructions and data are transferred over that same shared set of buses.
Why the other options are wrong
- B (separate memories for instructions and data): this describes the opposite design. The Harvard architecture, which uses two separate memories, each with its own buses, so that instructions and data can be accessed independently. It is not a feature of the Von Neumann architecture.
- C (bidirectional address bus): the address bus is unidirectional, the CPU places an address onto it to specify which memory location to access, and addresses only ever travel from the CPU to memory. It is the data bus that is bidirectional, carrying data or instructions in either direction between the CPU and memory.
- D (simultaneous fetch and data access): because instructions and data share the same memory and the same buses, the CPU cannot fetch an instruction and separately access data at exactly the same moment. Only one transfer can use the shared bus at a time. This limitation is often called the Von Neumann bottleneck, and is the opposite of what option D claims.
Final answer
A. The Von Neumann architecture’s defining feature is that program instructions and data share a single main memory and a single set of buses.