Bernd Paysan wrote:
Klaus Fehrle wrote:
Actually, a dual-core CPU is more expensive to fab than two single-core
CPUs. Semiconductor-economies are often counter-intuitive.
Nah, the answer is "it depends". Packaging in a 940 pin ceramics PGA package
with thermal heat spreader is expensive, so if you make a dual-core CPU
with that package, you can be significantly cheaper than two single-core
CPUs.
Silicon itself is rather "cheap" (the raw silicon price for an untested
Athlon64 or Opteron should be somewhere around $20, depending on cache
size). Yield is a question - and a well-kept secret. If you have a yield of
60% for a single-core-CPU (assume the cache has good yield due to
redundancy), the yield of a dual-core is too small, less than 30%. However,
usual production yield should be rather in the >90% range, so a dual-core
wouldn't give you such a bad hit. You'll see that on the next process
transition: my bet is that the dual-core CPUs will show up later.
Testing mostly is a function of pins*time. A dual-core CPU can run the
time-consuming test code just as fast as a single-core CPU (the speed paths
to be evaluated can run in parallel). So the testing cost of a dual-core
CPU is roughly the same as of a single-core CPU.
The net result is that a dual-core Opteron is probably $10 more expensive to
make than a single-core Opteron ($20 at most). Do you expect AMD to sell it
for $10 more? I rather expect three digit figures.
Well, I do follow you to "it depends". The only way to make dual-core
dies manucaturable is a design for a joint production (allowing to make
use of silicon not passing all tests for the target product for lower
grade products - dualcore with lower cachesizes, and singlecore
products).
In terms of yields, look at the wafer processing capacities of Intel (or
AMD, which is easier to calculate) and the volume of CPU they make with
it. A quick back of the envelope calculation easily falsifies the myth
of overall yields >90%. Yield-figures of >90% do exist though, but these
are line yields for layers. Opteron has polysilicon plus nine
interconnect layers. (!) So, this process would hardly be manufacturable
without making a lot of Athlon64 and Sempron products from dies failing
to pass all tests to qualify for Opteron despite redundancy mechanisms
used. (HTT, L2-cache, registers). It is only because a design for
manufacturabilty as described above is utilized.
Now, based on that, the question how much one die qualified to package
an Opteron from really costs needs considerations what it would cost to
fab the byproducts on another process. This cannot be done on the back
of an envelope, but needs modeling. I do not have process data at hand,
but for a careful estimation it is a three-digit figure. Btw pretty much
the same for a mature 130nm process and for a juvenile 90nm precess for
now.
Bottomline again: Semiconductor-economies _are_ often counter-intuitive.
K.