[vcf-midatlantic] Original Macintosh Architecture Questions

David Riley fraveydank at gmail.com
Fri Jun 25 02:31:18 UTC 2021

On Jun 24, 2021, at 4:05 PM, John Heritage <john.heritage at gmail.com> wrote:
> Thanks David!

Absolutely! I love the hardware design of the original Mac; it's amazingly minimal given the constraints of technology at the time, with some really clever use of PALs to shrink logic down; the only really custom chip in there is the IWM, which was necessary because pretty much no one else used Apple's GCR format, though the SWIM controller had an even neater logic implementation that could also handle MFM using the same decoder components, which is why it was PC compatible (same reason USB floppy drives can read HD Mac disks and 720K PC disks but not 800K Mac disks; Mac HD disks were MFM).

> That diagram (Apple Mac Logic) explains perfectly why the 1990 Mac Classic and the Atari ST were often 20-25% faster than the original Mac despite all being 8 MHz 68000s.
> The Original Mac gave 4 cycles of memory for CPU followed by 4 cycles of memory for everything else.

Yeah, and that's what the 68000 datasheet would lead you to believe you want for the bus, but it's not always the best. It's such a strange bus, and it shows you how the "MHz myth" started early; the 68000 had high clock speeds, but it didn't do much per clock (though having a true 16-bit ALU helped).

> The ST gave 2 cycles of memory for CPU followed by 2 cycles of memory for everything else (glue/shifter/mmu).
> The upgraded Macs basically gave 6 cycles of memory for the CPU then 2 cycles for everything else by doubling memory speed.
> The 68000 would stall for some (many?) instructions with the 4/4 cycle but is able to more or less run at full speed on the 6/2 (Mac) or 2/2 (ST) cycling.

Yup.  Aside from the Portable, most of the 68000 Macs weren't really all that different architecturally; the Plus added a SCSI controller and the Classic got some better timings, and the SE had a PDS slot, but other than that and some extra memory space, they're all almost the same machine (though the ROM contents make a difference).

> Last question on the Mac Audio --
> Is it fair to say that if you wanted audio on the Mac, you basically wanted to digitally sample a sound/take a sound sample and then have the CPU shape it so it would output correctly?  (i.e. costing some CPU cycles)     

You could say that, but it would probably be more accurate to say that when the Mac came around, not many other machines had real sampled audio out, so the sound quality was pretty good for the time already.  But it wasn't perfect, as related in this excellent story from Andy Hertzfeld's recollections of the development of the original Mac: https://www.folklore.org/StoryView.py?project=Macintosh&story=Boot_Beep.txt

In any case, any application with high quality audio for the time (here I'm thinking something like Dark Castle, which actually has really great sound effects) probably either just lived with the iffy quality or pre-equalized the audio, but given that Dark Castle sounds pretty similar on a machine with a real sound chip like something from the II series, I doubt it makes that much of a difference. Keep in mind that a PWM through a properly tuned RC filter isn't going to sound a whole lot different from an actual DAC (again, delta-sigma DACs work basically on this principle, just a whole lot faster).

That site, by the way, is replete with stories you won't find anywhere else (aside from Andy's book, which is the contents of the site plus a few more stories and lots of pictures).  I really loved reading through it.

- Dave

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