And now that you mention the Enterprise sequel.... it was already projected, effectively with a 6Mhz clock as standard. It had shape and was real, now we know. You can see it on Werner's thread.
Yes, I have read that thread, and I would have loved to have one of those machines!
I would like to create something similar to that today, with SD card instead of the floppy ... but now that I know that I have the bad Nick chip in my Egyptian EP64, I have to wait until I can find an affordable Enterprise (or just motherboard) that has a good Nick chip on it.
Unfortunately Enterprise 911 was not a real sequel, they just inserted a 6MHz z80 CPU, additional RAM, if i remember well it had 320KB, and a 3,5' floppy was built in.
Back in the 1980's, I would have happily seen that as a worthy sequel to the EP128!
As I work with the IS-DOS source code, I have finally remembered that I took a similar path from the old-and-slow M80/L80 to the fast Z80ASM back in 1986.
I have also remembered that I abandoned using the EP128 as my primary development machine sometime back then, precisely because it didn't have enough memory to run a useful sized RAMdisk.
IIRC Gryzor (and maybe Renegade) was actually written on an Amstrad 6128 with both the DK'Tronics 256KB RAM and 256KB Silicon Disc expansion boards. There is an old magazine interview from the time that mentions me using that as my Gryzor development system.
If we check overclocking from Dave's perspective then the best is 6MHz, because by setting bit1 of 0bfh port Dave behaviour will be the same like on a 4MHz machine, but anyway i think 6MHz should be the base of Enterprise instead of 4MHz, and 8MHz is a good choice.
Yes, 6MHz is the obvious choice for an upgrade, precisely for that reason, and it would have been a good Z80 upgrade speed back in the 1980s.
But when thinking of a Z280, which probably wouldn't even have been affordable to put into a home computer at the time, 6MHz seems just a little to slow!
The best thing in Zozo's turbo realisation, that you can switch between the CPU speeds on the fly, and you can turn back to 4MHz when you want.
That is perfect for a Z80 upgrade, but it really wouldn't work out quite as well for a Z180 or Z280 upgrade, because they will run faster than a Z80 at the same clock speed because of the CPU pipelining.
Z180 main advantage, it is still in production (like the normal Z80), easy to get it. Previously I never see any available Z280! (And at this case, the price: 8.99$ + 44.06$ shipping to Hungary, +27% VAT -> 67.37$ total price)
If you want me to get you one or more of those chips, it seems like a small (<0.22KG) package costs me $15.25 to send to Hungary, and tax is 7% here. I don't know if they would charge you VAT at the receiving end if the package is marked as a "gift".
Errrm ... I also still have 2 512KB memory expansion boards left for sale!
I currently don't know, but how about internal I/O registers? There are also problem with Z180, but least can relocated (EXOS 2.4 move these to 40-7Fh, for avoid collosion with EXDOS card).
Yes, I've heard that the internal I/O locations on the Z180 can cause problems.
There is also the unfixable problem that it doesn't (AFAIK) work with the undocumented IXH/IXL/IYH/IYL opcodes that were widely used by Z80 programmers in the 1980s.
The Z280 puts it internal I/O registers into a different 64KB I/O region (I/O addresses are 24-bit), and it does officially support those previously-undocumented IXH/IXL/IYH/IYL instructions.
OTOH, it would need a small PCB designed for it, both because it is a PLCC chip, and also because it needs a TTL 8-bit latch added for the low 8-bits of the address bus. But IMHO that also gives you a good location to add the 20MHz, 16MHz, or 12MHz TTL oscillator that you need to use!