11 Nov
2000
11 Nov
'00
8:52 p.m.
(On Chuck McManis' design)
Sorry, I didn't express myself clearly. In Chuck's machine the
displayed value comes from a 12 line I/O bus. He could build two
different display devices to attach to the bus: one for everyday use,
with LEDs and bcd-to-7-segment decoders, and one special occasions,
with decoders and drivers and sockets for the nixie tubes that he
borrows from whatever machine they otherwise belong to.
Yes, he could make 4xx an encoded operation (like pdp8 operate
instructions) such that 400 happens to perform a clear. Or he could
make 4xx load the accumulator with 0xx, so that 400 is again a clear;
this may fit the spirit of the design better. In either case he need
not disclose the extra features at first :-)
(On mine)
Yes. But since I'm doing a pure 8-bit machine, including 8-bit memory
address, it's *already* limited significantly. I do currently have one
unassigned opcode, so I could add an I/O instruction if it turns out I
have enough board space. More likely I'll just move on to design #2,
where I can hide complexity in microcode.
Yes, I'm still considering something like that, though I'd probably
keep it simpler, and just leave the PC in the accumulator: PC->AC,
MA->PC. I know this probably seems absurdly stripped down to anyone who
already knows what they're doing, but I don't, so I want to keep the
control very, very basic, to reduce the chance of serious errors.
Could one build
"nixie tube emulators", for instance using seven-segment
LEDs, and then borrow real ones from some other device for special
occasions?
No. Nixie is one of 10 output (highvoltage very low current) and
7Segment is different in every other way. For an era machine 7 segment
using incandescent lamps would be valid! >4 - CLEAR
> Reset the accumulator (and flags).
> Example: 400 - clears the state of the machine.
> Note: This seems a waste of a major opcode.
[....]
You could have a microcoded bit that if set clears the AC. Depends
on the number of bits you have to use. After several
partial designs than included IN and OUT operations, I
decided to punt I/O almost completely. [...]
Limts the machine significantly. I have
provisionally punted the CALL, too, as a separate operation;
[...]
Look at how PDP-5/8 and other did it. [...]
PC-> SUB1, SUB1->PC, PC+1->, fetch PC Suggestion:
Eliminate the ISZ from the PDP8 ops and make IOT and ISZ in to
INPUT(to ac) and OUTPUT(from AC).
I'm fairly sure I'll leave out I/O from this first design, and I'd
prefer to have 4/8 or at worst 6/8 memory operations to keep decoding
simple.
Keep indirect addressing and drop page 0 addressing.
I think you're right. I know my current addressing scheme is too
complicated, and I no longer know why I've been so reluctant to give up
page 0 addressing. Do you have any comment on PC-page (upper bits from
PC, lower bits from instruction) versus PC-relative (PC + lower bits
from instruction, either sign extended or not)?
--
Kevin Schoedel
schoedel(a)kw.igs.net