How does the CPU differentiate between command and data?

Answer

In general the only difference between commands, or instructions, and data is the context in which each appears. If the CPU is fetching the contents of memory to get the next instruction, then it assumes that the Program Counter register points to commands. If the CPU is executing an instruction that needs to fetch data from memory, the data at the address specified by the instruction are fetched, assuming that the address points to data.

This is what allows a program to be loaded into memory in the first place; the part of the operation system responsible for this operation treats the program as data, loading it into memory as instructed in the file. Then the OS branches to a specified place within that memory and begins fetching instructions there.

This blurring between instructions and data has also been used in the past to allow a program to modify itself as it executes. This is usually considered poor practice; some operating systems, such as HP's OpenVMS, even set up memory page protections to keep this from happening.

First answer by Joe Sewell. Last edit by Joe Sewell. Contributor trust: 940 [recommend contributor]. Question popularity: 1 [recommend question]