On modern x86 CPUs, the so called real mode is a backward compatibility operating mode originally derived from which early Intel processor?

Introduction / Context:
x86 processors support several operating modes, including real mode, protected mode, and long mode on newer chips. Real mode is a simple mode used during system startup and is compatible with very early software. This question asks from which original processor this mode is derived.

Given Data / Assumptions:

• Real mode is associated with 16 bit addressing and simple segmentation.
• The Intel 8086 was the original 16 bit x86 processor and did not support protected mode.
• Later processors added protected mode and advanced features but kept real mode for backward compatibility.

Concept / Approach:
The 8086 introduced the basic x86 instruction set and used a 16 bit architecture with segmented addressing. When the 80286 introduced protected mode, Intel retained a mode that behaved like the older 8086 so that existing software would continue to run. This backward compatible mode became known as real mode. On modern x86 chips, the processor still starts in real mode to run firmware and boot loaders before switching to more advanced modes.

Step-by-Step Solution:
Step 1: Recall that the 8086 was the first in the x86 family and used 16 bit registers with segmented memory addressing.Step 2: Remember that it did not have protected mode or hardware memory protection features that came later.Step 3: When later CPUs such as the 80286 and 80386 were introduced, they needed to run old 8086 programs, so they preserved a compatibility mode.Step 4: This compatibility mode, which mimics the 8086 environment, is what is now called real mode.

Verification / Alternative check:
Technical manuals for x86 processors describe real mode as the mode where the CPU behaves like an 8086. Addressing is limited to 20 bit addresses constructed from 16 bit segment and offset values, leading to a one megabyte address space. There is no hardware enforced memory protection or privilege levels. These characteristics match the original 8086 design and confirm that real mode originates from that processor.

Why Other Options Are Wrong:
Option A: The 4004 is a much earlier 4 bit microprocessor that is not part of the x86 family and did not define real mode.Option C: The Pentium Pro is a much later, advanced superscalar processor that built on an already established x86 architecture; real mode predates it by many years.Option D: Real mode is clearly based on earlier architecture and is not a completely new design.

Common Pitfalls:
Some learners assume that real mode is a simplified mode invented later just for booting, rather than recognizing it as the original operating model of the 8086. Another confusion is mixing up the 8086 and 80286, where the 80286 introduced protected mode but still supported real mode for compatibility. Keeping the historical order clear helps when understanding how modern operating systems switch modes during startup.

Final Answer:
The correct answer is The 16 bit Intel 8086 processor, which did not have protected mode.