13 Jan
2014
13 Jan
'14
8:57 p.m.
On Mon, Jan 13, 2014 at 2:47 PM, emanuel stiebler <emu at e-bbes.com> wrote:
On 2014-01-13 12:25, Ethan Dicks wrote:
It looks like a lot of work, and I understand not want to change it
now that it's done, but given the abundance of full 68000 chips (in
numerous form factors) and the relative scarcity of 68008 chips plus
the bus bandwidth, I wouldn't have designed such an elaborate board
with an 8-bit data bus in the first place. I would have started with
16 bits (not even including the fact that the 68008 has a 1MB address
space, so you would need resort to bank switching or a custom MMU to
wedge all that stuff in there).
The memory space of a medium-sized 68000 design is easy - use A22 and
A23 to quarter the 16MB into 4MB of primary RAM, 4MB of expansion (or
video) RAM, 4MB of I/O, and 4MB of ROM. If you want 12MB of RAM
space, it's only slightly more complicated and might take one more
gate or two (or another term or two on a PAL/CPLD), and 15MB of RAM +
512K I/O and 512K ROM is barely any harder. If you really need more,
consider the 68030. It's an awesome chip, and easier, IMO, to work
with than the 68020 if you need an MMU.
-ethan
I'd seen that design before, but I gotta say
that with all those bells
and whistles, you might as well go full 68000 on it.
I talked to this guy, and he is happy with it as it is. Took him a while to
do it (he did all the schematics/layouts/software himself),
so he is not eager at the moment to change it to the 16bit version.