A computer knows whether an address contains an instruction to be executed or data to be used in an instruction primarily based on the context in which the address is accessed and the architecture of the system.
- Program Counter and Instruction Fetching: The CPU has a special register called the program counter (PC) that holds the address of the next instruction to be executed. When the CPU fetches from the address in the PC, it treats the contents at that address as an instruction. The CPU then decodes and executes this instruction. Thus, any address pointed to by the PC is interpreted as containing an instruction
- Unified Memory in Von Neumann Architecture: In most modern computers (Von Neumann architecture), instructions and data share the same memory space. The CPU does not inherently distinguish between instructions and data at the memory level. Instead, it relies on the program counter to know which addresses contain instructions. Everything else can be manipulated as data. This means that an address is interpreted as instruction or data depending on whether the CPU is fetching an instruction (via PC) or accessing data during instruction execution
- Memory Segmentation and OS Role: Operating systems and CPU architectures can mark regions of memory as containing instructions (executable) or data (non-executable). For example, OS memory protection mechanisms can mark code segments as "execute-only" and data segments as "non-executable." This helps prevent the CPU from mistakenly executing data as code and vice versa. If the CPU tries to execute data marked as non-executable, it will cause a fault or crash
- Instruction Decoding: When the CPU fetches an instruction from memory, it decodes the opcode to determine what operation to perform and how many operands to fetch. The operands may be immediate values encoded in the instruction or addresses pointing to data in memory. The CPU uses addressing modes (immediate, direct, indirect, indexed) to locate data operands referenced by the instruction
- Program Loading: The operating system loads a program into a known memory region designated for executable code. The CPU starts execution at the program's entry point, where it expects instructions. The program's logic ensures that instructions and data are stored in their respective places in memory
In summary, the CPU knows an address contains an instruction if that address is currently being fetched as the next instruction via the program counter. Otherwise, if the address is accessed as an operand or data reference during instruction execution, it is treated as data. Memory protection and program structure also help maintain this distinction at runtime. This mechanism relies on:
- The program counter directing instruction fetches
- The CPU decoding fetched bytes as instructions
- The operating system and hardware enforcing memory protections
- The program being loaded with instructions and data in separate or designated regions
Thus, the distinction is not intrinsic to the memory content itself but depends on how the CPU accesses and interprets that memory address at a given time
