CPU clocking — A 486DX2 CPU runs internally at 50 MHz. What is the external bus (front-side bus) speed?
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A10 MHz
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B25 MHz
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C50 MHz
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D100 MHz
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ENone of the above
Answer
Correct Answer: 25 MHz
Explanation
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:
Given: f_internal = 50 MHz.DX2 multiplier: 2×.Compute bus: f_bus = f_internal / 2 = 25 MHz.Hence, the external logic/bus runs at 25 MHz.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.