5. Problems with Von Neumann (2)

The illustration below shows the Von Neumann or stored program architecture.

Problem 2

Both data and programs share the same memory space.

This is an issue because it is quite easy for a poorly written or faulty piece of code to write data into an area holding other instructions, so trashing that program.

The operating system, which handles memory management, needs to make sure that all the services currently running on the CPU have their own private memory areas. But the management is not perfect and so programs do crash sometimes.

Problem 3

Another issue is that the rate at which data needs to be fetched and the rate at which instructions need to be fetched are often very different. And yet they share the same bottle-necked data bus.

Good cache design helps, but the difference between data and instruction rates makes it more difficult to optimise the flow.