If you want to wrangle 64 Bit data you need to have acording operations - Wasn't it your intention to create a 64 Bit uP ? So, without a 64 Bit ALU and instructions to handle 64 Bit this Z80 will still be an 16 Bit uP just with an 64 Bit data bus. And for the Speed thing: it would be way better if the data bus is extended to 128 Bit, to reduce Memory access. With a 64 Bit bus in a 32 Bit CPU the theoretical bus load is just half. Or simpler: the CPU can be twice as fast with the same memory. Just tell me why ? Because it has been the tame with old 8 Bit uPs ? Come on thats just the stupid 'thats the way it always has been' idea. Address bus shuld be a) as wide as needed and b) if possible not exeeding the Size of the basic data unit. For a) just tell me where you need more than 4 Gig, or better where you need _way_ more than 4 Gig, because just 4 or 10 times this address space is easyer done with paging methods (wait for b)). And for b) you need special logic to perform address calculation, you can't hold one address in one regular register, you need more silicon to implement the adressing logic, adress calculation and (hidden) adressing register - you also need an additional ALU. If you don't need it don't spent the silicon. Be carefull, now you're entering historical teretorry. MS-DOs was designed to use the full 1 Meg address space of the 8086/88. And could could well boot with something like 900 and more K user mem available. The 'Barrier' (640 or what ever) was given to DOS by the BIOS of the respective machine. And the PC-Bios had just to obey the CGA memory. You are just talking to one of them - and FYI we continue to use systems with two digit years and they are Y2K ready :) Again, please think a bit - the x86 segmentation is a real great atempt to give - give more than 64 K Mem to a 16 Bit uP - allow an easy relocatable memory sceme - give a reliable memory base to a multi user/multi tasking OS I never had a problem with that - in fact I was realy happy about it (its just a kind of 16:16 base/offset addressing - remember /390 machines still have only one adressing base+offset - a proof for my belive, that base/offset is the only adressing mode needet anywhere). All the ranting about x86 are senseless and I often found (after taking out all the me-to-dont-like-it people) that these are the same guys who cant go well with 9 Bit machines or can't work with 7 Bit lines. Just remember b) you NEED more effort, especialy things that don't fit into your standard modell. Just think, why was the Z80 so much better than the 8080 ? The 8080 implemented 16 Bit calculations, as needed for adress manipulation only as a kind of 'add-on', while the Z80 was more like a 16 Bit CPU. And here is also your additional effort visible - to give the flexibility to handle a address you need to implement 128 Bit things and operation - or you end up with a methods like the 64180 MMU or the x86 segmentation Or worse things like the page registers on the 65816 (I love the CPU, but I still think they should have used a x86 like sceme). And if your thinking about MMUs to expand a 64 Bit adress to 128, you just cut the adressrage down to 64 Bit, since external address enhancements are no design detail of a CPU. Not taken - I still hope you will see it :) Please ? An Alpha board with CPU is only a bit more expensive than a PC board with an equal Pentium. At least thats the street price over here in Munich - we have some PC part shops that are also selling Alpha boards and CPUs right aut of the blister. FPGA/GAL designs (and languages) some of them I think are already below the 3ns range. AFAIK you could aquire the Z80 design already as a FPGA description - add whatever you need and dump it into the FPGA. With 3nS you are able to get a 33 MHz Z80 right out of the box. Just don't forgett, a ready to use Z180 is 25% faster :) If you still want do try, you should realy dig into FPGA stuff (thanks Allison for the idea) here you get a nice sandbox to try your designs. And most of the evaluation kits are like for free. One more thing, when you want to design a 64 Bit CPU you will get something like 200 pins, so just forgett about any kind of DIL. Faster 180s and of course the 380s. But Zilog has to build things that will sell, so they are more into integrating special I/O devices than more CPU power. Gruss H. -- Ich denke, also bin ich, also gut HRK