In operating systems and programming, which specific error occurs when software attempts to access memory that it is not allowed to use, such as protected or invalid addresses?

Introduction / Context:
Many modern operating systems use memory protection to prevent one process from corrupting the memory space of another or of the kernel. When a program violates these rules, the operating system stops it and reports a specific kind of error. In Unix like systems and in many programming contexts, this error is known by a particular name. This question checks whether you know that name.

Given Data / Assumptions:

• The question mentions software trying to access protected memory.
• Protected memory includes addresses outside the allowed range or regions marked as read only.
• Options include segmentation fault, display time error, I or O error, and runtime error.
• We assume a basic understanding of process memory and error categories.

Concept / Approach:
A segmentation fault, often abbreviated as segfault, occurs when a program attempts to read or write memory outside the segment of memory that it is allowed to access. The term comes from segmented memory architectures but is now used generally for invalid memory access. Display time errors relate to presentation or timing problems and are not standard terms. I or O errors occur when input or output operations fail, such as disk or network failures. Runtime error is a general term that covers any error during execution, not a specific protected memory violation. Therefore, segmentation fault is the correct technical term.

Step-by-Step Solution:
Step 1: Focus on the description of the error: software tries to access protected memory.Step 2: Recall that modern operating systems give each process its own virtual address space and use hardware to enforce access limits.Step 3: Recognise that when a process touches memory outside its permitted region, the hardware raises an exception that the operating system reports as a segmentation fault.Step 4: Compare this with I or O errors, which involve external devices, not memory addresses.Step 5: Note that runtime error is a broad category and does not name the specific protected memory violation. Therefore, segmentation fault is the best match.

Verification / Alternative check:
If you run a C program that dereferences a null pointer or writes beyond the bounds of an array on a Unix like system, the program will often terminate with a message such as Segmentation fault. Debuggers such as gdb also describe these events as segmentation faults. Documentation for signals in Unix lists SIGSEGV as the signal generated on a segmentation violation. These consistent names and behaviours confirm that segmentation fault is the specific term for this error.

Why Other Options Are Wrong:
Display time error is not a standard category of error in operating systems textbooks, and it does not specifically refer to memory access violations. I or O error describes failures during input or output operations, such as disk read errors or network interruptions. Runtime error is a generic term that can include arithmetic errors, logic errors, and memory errors, but it is not specific enough to answer the question. Only segmentation fault directly refers to illegal access of protected memory.

Common Pitfalls:
Some students may choose runtime error because they know that segmentation faults occur at runtime, but they miss the fact that the question is asking for a specific named error type, not a general category. Others may not distinguish between memory errors and I or O errors. To prepare for exams, you should memorise that segmentation fault is the term for protected memory access violations, often linked to bugs in pointer handling or array indexing.

Final Answer:
The correct answer is Segmentation fault, the specific error that occurs when a program attempts to access protected or invalid areas of memory.