In PC power supply design, what voltage rail is primarily used by Pentium-class processors for core operation?

Introduction / Context:
Early x86 systems such as the 286/386/486 used 5V logic, but as transistor geometries shrank and power density became a concern, later CPUs required lower voltages. Pentium systems marked a transition to a new standard supply rail for CPU cores.

Given Data / Assumptions:

• System in question: Pentium-class PCs of the 1990s.
• We want the voltage rail directly powering the CPU core.

Concept / Approach:

The Intel Pentium processors and later x86 CPUs required 3.3V logic levels rather than 5V, to reduce power consumption and heat. Other rails like +5V and +12V continued to power peripherals (disks, fans, legacy cards), but the processor core shifted to +3.3V.

Step-by-Step Solution:

Verification / Alternative check:

Technical datasheets show Pentium 75–200 MHz CPUs required 3.3V ± 0.3V. Later Pentium MMX introduced split voltages, but standard Pentiums still used 3.3V.

Why Other Options Are Wrong:

• +12V: used by motors, fans, and drives, not CPU cores.
• +5V: legacy CPUs pre-Pentium, not Pentium itself.
• +8V: no such standard rail in PC ATX/AT supplies.
• None: incorrect because +3.3V is correct.

Common Pitfalls:

• Assuming Pentium still used 5V like 486s; this caused compatibility issues in some upgrades.

Final Answer:

+3.3 volts.