If memory serves, athlon captured 50% of the market for PC's used as servers.
My memory is different. Athlon never made any significant gains in servers, even though AMD did work on a dual-processor Athlon platform.
Opteron, on the other hand, made significant gains in the server market. AMD was at 25% server market share at one point, which was very good for them. And for quad-processor servers, AMD was doing even better, though that segment of the server market is very low volume. Maybe that was the 50% you were thinking about.
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