[TUHS] Off-topic: any logic designers here?

[TUHS] Off-topic: any logic designers here?

[Warren Toomey]

Hi all, sorry for this off-topic posting.

I just came across this series of videos where a guy built a CPU out
of 7400 series chips on a breadboard: https://eater.net/8bit/

Anyway, I thought it would be fun to do something similar but with
a bigger address space and as few chips as I could get away with.

I've got a design that works in Logisim, but I've never actually
built anything before with real chips. So, if there's someone on the
list who could quickly look at what I've done and point out problems
or gotchas (or indeed, what ROM/RAM chips to use), that would be great!

The design so far: https://github.com/DoctorWkt/eeprom_cpu

Thanks in advance, Warren

[TUHS] Off-topic: any logic designers here?

[Peter Jeremy]

On 2017-Aug-08 12:52:36 +1000, Warren Toomey <wkt at tuhs.org> wrote:
>Hi all, sorry for this off-topic posting.

Well, just promise to port Unix to it and it won't be off-topic.

>Anyway, I thought it would be fun to do something similar but with
>a bigger address space and as few chips as I could get away with.

Well, using a FPGA or microcontroller will reduce the chip count further.

You suggest that you won't use a 74LS181 because they are no longer made.
Whilst that's true, they are still readily available as NOS.  Overall, I
would suggest that using a 74LS181 (or 4) is a cleaner solution than
(ab)using a couple of EPROMs.  (The other option would be a PAL or GAL but
I suspect finding and programming them would be more difficult).

-- 
Peter Jeremy
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[TUHS] Off-topic: any logic designers here?

[jason-tuhs]

> Hi all, sorry for this off-topic posting.
>
> I just came across this series of videos where a guy built a CPU out of 
> 7400 series chips on a breadboard: https://eater.net/8bit/
>
> Anyway, I thought it would be fun to do something similar but with a 
> bigger address space and as few chips as I could get away with.

Doubly off-topic, so doubly sorry...

But for anyone who wants to go the other way, there's this:

http://www.megaprocessor.com/


  -Jason

[TUHS] Off-topic: any logic designers here?

[Arthur Krewat]

I've used static RAM in the past for projects and it's worked out quite 
well. No refresh cycles, easy to deal with.

As an example (not saying you need 1MB of ram, or that this would fit 
your project...) 
http://www.mouser.com/ds/2/100/CY62167E_16_Mbit_1_%20M_%2016_%202%20M_8_Static_RAM-319235.pdf

And only $13.20 each.

side note: A friend of mine and I built a 68000-based computer using 
128Kbit static RAM chips back in the late 80's. I wrote the 68K 
assembler for it in C, and eventually wanted to boot-strap UNIX to it, 
but never got that far. Back then, static RAM was expensive - these were 
"free" ;)

On 8/7/2017 10:52 PM, Warren Toomey wrote:
> Hi all, sorry for this off-topic posting.
>
> I just came across this series of videos where a guy built a CPU out
> of 7400 series chips on a breadboard: https://eater.net/8bit/
>
> Anyway, I thought it would be fun to do something similar but with
> a bigger address space and as few chips as I could get away with.
>
> I've got a design that works in Logisim, but I've never actually
> built anything before with real chips. So, if there's someone on the
> list who could quickly look at what I've done and point out problems
> or gotchas (or indeed, what ROM/RAM chips to use), that would be great!
>
> The design so far: https://github.com/DoctorWkt/eeprom_cpu
>
> Thanks in advance, Warren
>

[TUHS] Off-topic: any logic designers here?

[Clem Cole]

i agree with Peter, WRT the '181 you can certainly find them in small
qualities in the after market, I may have a handful of them myself.   One
thing you are missing is a small barrel shifter, which the '181 does not
support, but for a 8 bit shift it should not be terrible.   Vaxen and such
built their independently, but it is worth while IMO, as it allows in a
single clock tick to perform pretty substantial arithmetic, although
because it's a sq law the cost in die area is huge. If you look at the 68K
for instance, the big area in the middle everyone sees in the pictures is
the shifter (same for the Alpha chips).

Clem

On Thu, Aug 10, 2017 at 4:16 AM, Peter Jeremy <peter at rulingia.com> wrote:

> On 2017-Aug-08 12:52:36 +1000, Warren Toomey <wkt at tuhs.org> wrote:
> >Hi all, sorry for this off-topic posting.
>
> Well, just promise to port Unix to it and it won't be off-topic.
>
> >Anyway, I thought it would be fun to do something similar but with
> >a bigger address space and as few chips as I could get away with.
>
> Well, using a FPGA or microcontroller will reduce the chip count further.
>
> You suggest that you won't use a 74LS181 because they are no longer made.
> Whilst that's true, they are still readily available as NOS.  Overall, I
> would suggest that using a 74LS181 (or 4) is a cleaner solution than
> (ab)using a couple of EPROMs.  (The other option would be a PAL or GAL but
> I suspect finding and programming them would be more difficult).
>
> --
> Peter Jeremy
>
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[TUHS] Off-topic: any logic designers here?

[Warren Toomey]

On Tue, Aug 08, 2017 at 12:52:36PM +1000, Warren Toomey wrote:
>Anyway, I thought it would be fun to do something similar but with
>a bigger address space and as few chips as I could get away with.

All, many thanks for the feedback. I've taken it all on-board and
hopefully it will help when I get to building the thing :)
Cheers, Warren