[mschaef]

Truly from another era.

If you're not familiar, basementcat is right... DTACK grounded refers to the DaTa ACKnowledgment pin on a Motorola 68000. It's the signal that (when grounded) lets the CPU know that data it has requested from memory is ready to be read off the data bus. Systems with slow memory need to be careful that they ground the pin only when the memory has responded.

However, if your memory system can outrun the CPU, it was possible to just ground the pin and assume that the memory always responded in time to satisfy the CPU's read requests. The centerpiece of "DTACK Grounded" was a set of Motorola 68000 CPU boards that (initially) did just that. The memory parts they used were expensive for the time and small, but they were fast, allowed DTACK to be grounded, and allowed the overall design of these CPU boards to be very simplistic and inexpensive. For a while, these boards were most likely the most accessible path to a 16/32-bit microprocessor like the 68000.

What was also interesting was the way that these boards were used. They were sold as attached processors for Commodore PET's and Apple ][ machines. The software would then patch the internal 8-bit BASIC implementation to delegate math operations to the attached processor. Believe it or not, the speed improvement offered by the 68000 was significant enough to offset all of the other complexity around this implementation choice. The net was an accelerated and mostly compatible BASIC.

Later in the newsletter, the author talks about pairing an Intel 8087 with a 68000 to get better floating point. (The 8087 was a remarkable chip for the time.) The 8086 that was needed to run the 8087 is referred to as a 'clock generator'. I guess the net architecture here was to be a 6502 Host CPU, connected to a 68000 attached processor using an 8086 and attached 8087 to accelerate floating point.

Meanwhile, PC clones had sockets for 8087 chips, Apple was releasing relatively inexpensive 68000 hardware, and the 80386 was well on the way. The writing was on the wall for the DTACK grounded approach to accelerating 8-bit microcomputers, but it must have been interesting while it lasted.

[cmrdporcupine]

Yeah the era of "memory can outrun the CPU" was brief and glorious. The approach 80s microcomputers used for graphics required it -- multiplexing the video between the VDP (C64 VIC-II, Atari ST Shifter, etc.) and the CPU on odd bus cycles. Nice and fun.

By the end of the decade the CPU was running 2-3x the speed of the fastest RAM.

Now things are soooo complicated.

Not sure about this alternate reality where Apple's 68000 machines were cheap :-) (I say this as an Atari ST owner).

68000 has kind of aged well despite not being made anymore -- is perhaps now the only "retro" architecture which can be targeted by a full modern compiler. You can compile Rust, C++20, whatever, and have it run on a machine from 1981. That's kinda cool.

[fredoralive]

Well, compared to the first wave of 68000 machines, which were generally high end workstations from the likes of Sun and Apollo, a $2500 Macintosh is cheap. Apples belief in this whole “profit margin” thing did mean it couldn’t compete on price with the Amiga and ST though…

[cmrdporcupine]

I mostly jest. In the late early 90s the prices of 68k Macs actually dropped into the very affordable range. The II series were great machines, priced well, stable, etc. The shift to PowerPC ruined the classic Mac, IMO.

In that era I had a 486/50 running (early) Linux and my mother had a Mac LC II. I actually really enjoyed using that machine.

[markus_zhang]

Just curious why do you think the shift to PPC ruined the classic Mac? I never owned a Mac before but I did buy an iBook G4 because I somehow got fascinated by the PPC machines.

[cmrdporcupine]

The PPC architecture is fine enough. The problem was their "operating system" was written as a 68k OS with no memory protection and a weird memory model generally, and for almost a decade they ran with 68k emulation in order to make it all work.

And it crashed constantly. Very unreliable machines.

They did crash here and there in the 68k days, but overall they worked pretty good. Albeit cooperative multitasking, etc.

But in the mid-90s, with System 7.6, it was like walking through landmines. e.g. I helped admin an office with a bunch of them and you couldn't run Netscape and FileMaker at the same time because they just wrote all over each other's memory and puked.

System 8 and 9 improved things markedly but the reputation was still there.

Meanwhile they had these grandiose OS rewrite projects that all failed until they ended up buying NeXT... and then spent 5 years turning NeXTstep into OS X.

In retrospect Apple could have skipped the whole PPC era and done much better for themselves by just switching to x86 (and then ARM as they've done now) after a brief foray through ColdFire.

Or just jumped straight to ARM instead -- they were an ARM pioneer with the Newton! -- rather than betting the farm on the IBM/Motorola PowerPC alliance, which ultimately ended falling badly with power hungry chips that couldn't keep up with x86.

[markus_zhang]

Thanks for sharing. I never used one before so don't know how good/bad it was. My iBook runs OS X so it is pretty good.

It's a bit embarrassing as the 68k emulation was part of the reason that I got fascinated. But I just want to learn binary translation, not really use them, anyway.

I think Apple in the early 90s threw things on the wall and hope something stuck. Bad for consumers, nightmare for admins but good for engineers who managed to make the throw.

[krallja]

https://www.rocelec.com/part/01t4w00000PPjBiAAL-MC68020EH16E... - the '020 is still "Rochester Active"!

[mschaef]

> Yeah the era of "memory can outrun the CPU" was brief and glorious.

I don't think I fully recognized at first what was happening when wait states, page mode DRAM, and caches started appearing in mainstream computers. :-)

> Not sure about this alternate reality where Apple's 68000 machines were cheap :-) (I say this as an Atari ST owner).

Yeah... I should have cast a broader net. The Atari ST machines were much better deals IIRC. In any event, DTACK grounded PoV was that the 68000 was targeted at minicomputer scale machines, so anything that fit on a desk at all was arguably going to be inexpensive. (Years later, I did embedded work on 68K class machines intended to run in low power environments. They had to be "intrinsically safe" in potentially flammable industrial control environments. That architecture had a long path from 'minicomputer class' to where either wound up.)

The other thread this reminds me of is a bit later, Definicon was selling boards like the DSI-780. These were PC AT boards with an onboard 68020/68881 and local memory. Computationally intensive jobs could be offloaded to that board, which was supposedly like a VAX-11/780 on your desk. In some ways, it served a similar role to the DTACK attached processors, but at a slightly later point in time.

Like the DTACK grounded products, the window of time in which these products had value was oh so short, relatively speaking.