I don?t think there was a physical constraint or a necessity for AMD to add an extra pin for AM2 functionality over the current s939. As about ~20% of the pins in the current s939 K8?s are redundant (they were there to allow for future expansion, DDR2? Well not anymore because of the introduction of s940 AM2). .
AM2 as far as I am aware is supposed to support a vast array of future expansion options. I cant see that extra pin counting for much, except for forced incompatibility with current s939 motherboards.
Dainas, I totally disagree with your statements. s939 has had a superb run, looking at a compatibility stand point, and this is all thanks to AMD?s intuitive decision to replace the northbridge chipset with an integrated on die variant. Intel have to change chipsets a lot which means new motherboard purchases just because the operating characteristics of certain processors change. Where as AMD have all of that incorporated on die, which means changes to the chip stay on die and do not effect Motherboard/Chipset support. AMD did not change the s939 operating frequency of the HTT, and also stuck with ?DDR? memory controllers, which resulted in pretty much motherboard support / compatibility across the whole s939 range (including the dual core migration). This means the only variable is the BIOS, and as most motherboard manufactures released compatible BIOS?s, this means that you can just about use most s939 motherboard?s with any s939 chip.
Some people don?t understand that the chipsets for example NF4 used on today?s s939 motherboards are literally just hubs, which allow all the motherboards I/O devices to communicate down one HTT tunnel. Which then allows for a more efficient motherboard design, and future designs for that matter, as PCB layers, tracers and electrical noise is reduced.