Steve Ciarcia described "A Penny Pinching Address State Analyzer" in BYTE magazine, February 1978 - just connect the high and low bytes of the address bus to one pair of DACs, optionally the databus to another, and feed to the X, Y and (optionally) Z inputs of your 'scope:


The illustration shows the response to a carriage return in the Basic interpreter of a SCELBI 8H, but clearly the idea applies to any 8-bit micro.

(via this discussion.)

Back in the days of my one-megahertz KIM-1 I ran much the same experiment, except the code I liked to watch was the FIG Forth compiler as it loaded screens of source code (a process which could last tens of seconds). The visual effect was... dazzling? enigmatic? blindingly rapid? thought-provoking? mesmerizing? All of these things!! Someday I need to make a video.

The setup was very simple, using just a couple of weighted-resistor 8-bit DACs and my trusty Tek 465 analog 'scope running in X-Y mode.

CPU address lines A7-A0 caused horizontal deflection of the dot. For example, during the (numerous!) accesses to zero-page, the dot would illuminate various points along the very bottom edge of the screen. Stack accesses (ie; page one) illuminated points along the line just above the bottom edge. And so on, throughout the entire 64K address space. Accesses in page $FF would place the dot somewhere along the very top edge of the screen.

You'd have thought the 'scope was inhabited by a bunch of manic fireflies! The speed was utterly beyond perception -- 1,000,000 points plotted per second -- but there was very clearly a method to the madness. Naturally the most-frequently-accessed areas seemed to be most brightly illuminated. These included the top of the Forth data and return stacks (whose locations tended to vibrate) and HERE -- the top of the Forth dictionary, which could be seen to slowly climb as compilation proceeded.

The location of FIG's NEXT code sequence, low in page two, was static, of course, but IIRC it was the brightest area of all. NEST UNNEST BRANCH and a dozen or so others would also have been on the list of favorites. As a backdrop, the rest of the screen was mostly dark except for some seemingly-random speckles.

I encourage anyone with access to an analog 'scope to try this. You don't need to run Forth, of course. And I suppose a digital scope would work, but I doubt it could equal the vibrancy of actual CRT phosphor and the accompanying illusion of z-axis modulation.

cheers,
Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

I'd watch that video! One thing about the scope idea, it struck me, is that you (might) get to see the movement from one address to another - you're not just getting spots, but lines. So subroutines will have lines going to them, and routines which touch areas in zero page a lot would be tied to those areas with lines.

In principle, an emulator could offer this graphical entertainment too. But let's make it look like an analogue scope! Perhaps a little like http://www.masswerk.at/spacewar/

Cheers
Ed

Yes, that's exactly right -- the dot left a faint trace as it transited from one location to another. So, in addition to what I've already described -- bright horizonal segments a few (or dozens) of bytes wide -- there was also a ghostly web of diagonal lines interconnecting the hots spots.

Diagonal lines were not part of the original intent, and in that sense were an imperfection. They could've been eliminated if CPU Phase 1 were connected to the scope Z axis so as to dim the dot while everything -- the CPU address lines, the DAC and the scope deflection circuits -- had a chance to settle. But it turns out concealing the diagonal lines would've been a loss.

In the Hacker News discussion someone mentioned using an ordinary AM radio to listen to a computer via the RF noise it generates. I never tried that trick, but I can well believe that the human ear would discern patterns of non-trivial complexity. That's what our brains and senses are evolved to do! Thus it's no surprise that "mis-applying" our wetware to watch or listen to computer code results in an experience which plainly reveals order and is meaningful, even though its exact specifics can't be perceived.

Thanks for bringing this OT topic to our attention, Ed!

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Cheers Jeff. I've certainly done the AM radio thing, but not to the extent of playing music. (http://www.cbttape.org/funny/S36040.mp3, via http://amfone.net/Amforum/index.php?topic=13610.0;wap2)

I gotta try to "one-up" you, Ed. The DEC PDP-8 here is a much more capable instrument than the IBM, although the skill of the programmer could definitely be an important factor.

Mike B.

Brilliant! Those PDPs were (are) very nice machines. (My own experiment would have been with my UK101 - you need something unshielded...)

Just found a short video of starting up an LGP-30 - this was the drum-based machine from 1958 which features in The Story of Mel - and it has a beautiful front panel which includes a scope - showing three waveforms which change as you type and as the machine computes: