[PLUG] Emulating 1970s computers

[Keith Lofstrom]

> It seems intent on opening a new window.  Oh, well.  Just thought I'd post
> my results.

The GW-BASIC emulator opens a new graphics window for
output.  That is the easiest way to simulate what these
1970s hobby computers actually did.  Pre-IBM-PC hobby 
computers painted video on a screen, not ASCII to a
character buffer or pixels to a frame buffer.

There was no pixel array like a modern graphics card,
and often not even a character array in RAM.  The video
driver system was an electronically baroque step "up"
from a teletype driver.  "Up" in scare quotes.

When you think about it, these ancient machines were NOT
even as powerful as "terminals", text or graphic. 
Memories were absurdly small - 4 kBytes for my PET, and
most of that was devoted to the users's BASIC byte code.

The Commodore Pet had its own monitor, which added physical
hardware expense but also made video generation easier; 
no need to generate standard VHF television signals.
Absurdly baroque and clever electronics worked with
interrupts and code timing to create timing signals for
the deflection coils and the corresponding video
luminance signal.  

The Apple 2 took a different path.  Steve Wozniak used some
screwball circuitry to generate a fake color signal for a
standard NTSC color TV.  Unlike Jack Tramiel's team at
Commodore, Woz did not need to continually readapt his
video design to whatever surplus black and white video
monitors Tramiel pungled up from domestic, foreign, and
Warsaw Pact display makers.

Indeed, the Woz personally designed and prototyped almost
all of the Apple II electronics and software, and got paid
less for doing that (moonlighting from his HP engineer job)
than Steve Jobs paid for a pretty plastic cover.

For both the PET and the Apple, program execution was
interrupted every horizontal scan line interval to take care
of video housekeeping; counting raster lines, for example. 
Programs ran slower, but the microprocessor replaced a lot
of display logic.  

Computers are powerful enough today that a clever ubergeek
could simulate a running Apple 2 or Commodore Pet as logic
signals, with 3D rendered solid models of the keyboard,
video signals, and a case with a simulated television set
and screen image displayed as a graphic image on a Linux
desktop .  Our hypothetical ubergeek can practice on the
Commodore PET, which was simpler, more digital, and had
an easily-rendered bent sheet metal case.

A virtual collection of simulated physical computers from
the 1970s onwards would take up vastly less shelf space
than what remains of my physical collection.

Of course, the simulations should be written in C,  and 
render images to a window on a Gnome or Wayland desktop
screen under Linux, thus making this long discursive
ramble on topic for the PLUG list.   

Keith

-- 
Keith Lofstrom          keithl at keithl.com