CPU clocking — A 486DX2 CPU runs internally at 50 MHz. What is the external bus (front-side bus) speed?

Introduction / Context:
Many x86 processors use multipliers to derive an internal core frequency from a lower external bus clock. The 486DX2 family doubled the bus clock to achieve higher internal speeds while retaining compatible motherboard bus timings.

Given Data / Assumptions:

• Processor is an Intel 486DX2 running at 50 MHz internally.
• “DX2” denotes 2× internal clock multiplier.
• We want the external (FSB) frequency.

Concept / Approach:

For a DX2, internal core frequency = 2 * external bus frequency. Therefore, knowing the internal clock allows us to compute the FSB by dividing by 2.

Step-by-Step Solution:

Verification / Alternative check:

Historic system specs list common 486DX2 models like 486DX2-50 (25 MHz bus, 50 MHz core) and 486DX2-66 (33 MHz bus, 66 MHz core).

Why Other Options Are Wrong:

10 MHz is not a standard 486 bus for a DX2-50; 50 MHz is the core, not the bus; 100 MHz would imply 4×, which is a later architecture; “None” is wrong because 25 MHz is exact.

Common Pitfalls:

Confusing core clock with bus clock; mixing DX2 vs DX4 multipliers; overlooking that memory and I/O timings correlate with the bus clock, not the multiplied core frequency.

Final Answer:

25 MHz.