[sdiy] OT: COSMAC Elf 2000

[Phillip Harbison]

Roy J. Tellason wrote:
> But they'll also handle 15V logic levels just fine,  
> which is something that none of the newer series parts
> will do AFAIK.

The 74ACT, 74FCT, and 74HCT series will only handle +5V.
I thought the 74AC, 74FC, and 74HC would handle +15V but
I may be wrong. You definitely give up TTL-compatiblity
with the latter.

> Still a whole different chip though.  :-)

They had more in common than most peecee fans wanted to
admit. Electrically, they're largely similar except for
operating frequencies which were higher for the 8088. I
think both used an active low /RD (read) or /WR (write)
to indicate memory cycle. Not sure about the ready/wait
signal.

> And I don't like 'em much.  Segment registers?  Ugh!

I never cared for the 80x86 family either (preferring the
6809 and 68K); however, if you set those segment registers
to zero, you can ignore them for the most part and still
have the full 64K address space supported by S-100. If
you want to address 1MB, then unfortunately your next
destination is segment register city.

The x386 took a similar strategy to end complaints about
the segment registers. They expanded the displacement to
32-bit (i.e. 4GB segments) so everyone just ignored the
segment registers for the most part. However, you still
paid a penalty for their existence, especially if you
wanted to do memory management (write-protection was a
segment-level rather than page-level feature, go figure).

As a computer architecture purist, it is easy for me to
get disgusted at the amount of bondo and duct tape that
has been grafted onto an already deficient architecture
over the years. I just have to remind myself that the
superscalar x86 processors are a bit like the Russian
dancing bears. The miracle is not how well they dance
but that they dance at all.

> That bus also supported the 68000 (Cromemco) and
> probably a bunch of other chips besides.

For that you needed what was called a "Motel" circuit.
The 68K had RW (read/not-write) and /DS (data strobe)
signals. The equations to generate /RD and /WR were
as simple as:
   /RD = ~ (DS & RW)
   /WR = ~ (DS & ~RW)

So you needed two inverters (for /DS and RW) and two
2-input NAND gates. The Motel circuit for the 6502 and
680x was similar but used CLK instead of /DS.

> Yup.  It's also speed-limited.

Somewhere I have an S-100 bus with active terminators.
I think it was made by Tarbell or Cromemco. This solved
a lot of noise problems and allowed it to run faster.

> If I were building anything for either box I'd put 
> the RAM and CPU on the same board, [...] and let the 
> I/O sit somewhere else if I had to...

Actually, there's very little need for the S-100 bus
or expansion cards unless you want to put synthesizer
circuitry on the cards. You can get just about all
the RAM and I/O you need on one S-100-sized card. I
build my first UNIX computer in 1984 using Multibus
cards (similar to S-100 size) and the entire computer
logic (68K, MMU, 1MB RAM, boot PROM, SCSI, and serial
ports) fit on one card. Initially it only used the
Multibus to get power. I personally don't care much
for bus specs that use edge connectors (preferring
the DIN connectors used in VME and NuBus) but I have
to admit I got a lot of work done with that first
little UNIX box.

> Though my interests aren't necessarily limited to
> synth-type stuff.

Neither are mine. Someday I would like to design and
build a simple computer using nothing but NAND gates
just to demonstrate it can be done. I just doubt that
anyone on synth-diy would care much about it or want
to use it to make music. :-)

> You're working on something that has 2048 processors?

I'm designing a massively-parallel processor based on
the Freescale 8641D (essentially a dual-core G4 with
interesting on-chip peripherals). The building block
is a blade with quad 8641Ds and a switch chip. In the
max configuration, which we call a hyper-cluster, it
will have 2048 processors (4096 G4 cores) which adds
up to a boat load of "blinkenlights".

If you're familiar with Beowulf clusters and similar
architectures, my system is similar but the hardware
is purpose-built. I probably should not say too much
more about it without an NDA in place, and I doubt
that anyone else on synth-diy is interested anyway.

-- 
Phil Harbison