Intel’s six-core Xeon processor, known as Westmere, has been seen in a leak of an engineering sample.

Still identified as a mainstream Core i7 chip by software, the Xeon W5590 example runs at a relatively low 2.4GHz clock speed but has 12MB of Level 3 cache shared between all cores and 256KB of Level 2 cache for each core. With Hyperthreading, which runs up to two program threads on a single core, the system shows as many as 12 effective cores.

A second, quad-core Xeon chip from the Bloomfield family, known as the W3580, would drop to 8MB of cache given the reduced number of cores but would increase its clock speed much higher. It’s officially poised to run at 3.33GHz but, through engineering tests, can run up to 4.2GHz on regular fan cooling when overclocked.

Westmere-based Xeons are officially due in early 2010 and are likely to form the basis of the highest-end workstations and servers, including Apple’s Mac Pro or HP’s Z series.