|I think your very close here. It is said that the 9800 Mobility even uses the same ASIC as the X800 420 Core. In fact, Dave Bauman over at Guru3D looked at the chip and said it was identical to the full fledged R420 with the exception that is had four pipelines disabled by control points on the package. Read more here in this thread:
Hi, Dave Baumann from Beyond3D
Just saw this in my referrals so I thought I'd drop in a bit of info.
I've actually just completed testing a Mobility Radeon 9800 (the review should be up in a few days) and during my testing I noted that the Vertex performance of M18 was inline with having 6 vertex units, as opposed to 4 that ATI had initially told me - this is further evidence that M18 is using the R420 core.
Note that M18 has 8 pipelines enabled (2 quads), however R420 actually supports 16 pipelines (4 quads), so there are actually 8 pipelines that are disabled on M18, not just 4. However, these things will be qualified to run at the temps produced by 8 pipes, if you try and enable another 4 or even 8 then effectively you'd be up to doubling the quantity of heat outputted by the graphics core and the cooling solution may not cope (leaving you with a melted notebook!). Flashing it with a desktop BIOS would also alter the fan spin speeds and probably loose the power management functionality.
Its is not a given that the pipelines will even work, since the R420 die is very large it is prone to defects - as the pixel pipelines are the largest part of a die this is were a defect is most likely to occur and so some chips are sold with the quad that the defective part is in disabled, whilst the others will work as normal.
I've actually just got off the phone with ATI's Jon Carvill (Mobile PR) where we've been discussing a few things about M28 / M28 Pro. Although Jon wasn't sure, going by the driver revision of the M28 that I've tested (the review should turn up a little after the M18 review) I think M28 will actually be based on R480 rather than R423 as R480 has improved power and thermal controls.