The first spawn of the 6x86MX line - the PR166, PR200 and PR233 - run Windows applications at speeds analogous to a PII running at 166MHz, 200MHz and 233MHz respectively, even though the actual clock frequencies are 66MHz, 166MHz and 187.5MHz respectively, according to Cyrix. PR stands for 'performance rating' and is based on tests conducted using Ziff-Davis's Winstone benchmark suite which simulates a series of common Windows applications.
Cyrix expects to deliver a PR-266 version of the chip in the fourth quarter of 1997 when IBM moves its fabrication to a 0.25-micron process; Cyrix has no fabrication facility of its own. A PR-300, probably based on a 250MHz clock frequency, will be available in the first quarter of 1998.
However, Richard Baker, regional marketing manager for PC products at AMD, which uses 'true' megahertz ratings, said his company will produce a 266MHz K6 in July or August and a 300MHz K6 in the fourth quarter of the year, putting it ahead of the curve. AMD is considering using a 100MHz system bus for the 300MHz part, while Cyrix is almost certain to stay with a 75MHz bus for the 6x86MX PR-300.