13 Dec
2008
13 Dec
'08
11:05 p.m.
Tony Duell wrote:
Agreed - it's likely to be more of a pain in the backside due to the amount of
wiring, but shouldn't necessarily be any more complex (at least if using SRAM
for memory - I've seen some really convoluted DRAM refresh designs for CPUs of
16 or more data bits)
Way back I had some sort of RS232-capable graphics box called Pluto - I
believe the idea was that you could send it graphics primitive commands via
RS232 and it'd render them on a suitable display (I think it was VGA, but it's
been so long now).
Whilst that's not quite what the OP's after, I think there's some potential
there in that a modern PC with a serial port could be initially and quickly
used to do that task, with the 'real' hardware built later. (I suspect it's
the route I would take, purely as offloading the graphics ability elsewhere
seems to have a certain 'cool factor' about it :-)
Given my
druthers, I'd go with a 32-bit system, but I'm not sure if
that will increase the complexity beyond my abilities. I'm competent
Making a 32 bit system is not inherently any harder than making a 16 bit
one, there are just more data bus lines to wire up, more RAM/EPROM chips,
more bufferes, etc. Avoid those plugblock-type solderless breadboards. You
will have no end
of problems from bad connections, and maybe even stray capacitance.
I've heard of people having no problems with 8 bit CPUs, but I'm sure it does
become an issue at some stage - maybe even for an 8MHz or so 68k.
Solder it up (or wire wrap it) from the start. You can
build it on
stripboard (in fact I would recomend against making a PCB for an
experimental machine, since you want to be able to change things).
One of the things that's kept putting me off doing this myself over the years
is actually *how* to best tackle the design phase. I'm not sure what the best
technology is to use (given the large amount of interconnects), but then I've
never tried wirewrap before.
(if you can get a 68008, are they easy to get now?)
Were they ever? I've got two 68008 ICs (one in the Cumana copro for a BBC
micro, and one in a STE-bus board) and those are the only two I've ever come
across. Compare that to a healthy pile of Z80 and 68000 (and '010 etc.)
processors...
I'm not sure they ever really saw much widespread use (actually, what
commercial machines used them other than the Sinclair QL?)
What does it
take to do video (bitmapped) output, by the way? That
would be especially interesting, in my opinion.
A sup[rising amount of logic. Basically you need an area of memory that
can be accessed both by the CPU and by the video hardware. Typvially RAM
with multiplexers to switch it between the 2 devices.
I think quite a few systems did it on the falling edge of the clock signal,
didn't they? i.e. the memory was only accessed by the CPU on the rising edge,
so (if fast enough) was essentially free on a falling edge. Elegant, but
probably hell to debug and get working as a first project (particular if DRAM
refresh is thrown in there)
cheers
Jules