Tales From The Code Front Stories in words and pictures

And now: something completely different ...

Let's talk keyboards.

Specifically, about keyboards in #Genesis64.
More specifically, about what a headfuck they are.


My early take on a Genesis64 on-screen keyboard

Looking at the #C64 keyboard (with the image above being an exact copy of the layout), you'll notice a slight difference to modern keyboard layouts, like the C= (Commodore) key (although we are blessed with the Windows / Apple key), missing Alt, Tab, Esc ... you name it.


Basic input is easy ...

Getting input working is pretty easy, sort of.

The keyboard driver is a simple affair, capturing the keydown event on the <canvas/> element, everything is fine as long as you stick to the ASCII range (a-z, numbers).

The first tiny obstacle is that you get a Unicode character string back when reading out the KeyboardEvent.key property.
Obstacle, because #Genesis64 uses the same way to render text to screen the #C64 does, by reading out the screen-ram (stored at $0400 by default, and the color-ram from $D800, but let's ignore that).

Even though we can convert the Unicode to ASCII quite easily, getting this to PETSCII is a different matter. While PETSCII codes match with a huge part of the ASCII range, the screen-codes do NOT.

The #C64 uses PETSCII when dealing with strings, but screen-code when storing things in screen-ram. I honestly have no idea why that distinction was made.
Take the letter "A" for example, which resides at $01 as screen-code, but at $41 in PETSCII-verse (see: http://sta.c64.org/cbm64scrtopetext.html for some insights in the screen-code / PETSCII conversion, see also: https://style64.org/petscii/).

So how does it work in #Genesis64? In the laziest and un-heroic way possible, by using Javascript's .indexOf.

public TXTSCREENCODE: string = "@abcdefghijklmnopqrstuvwxyz[£]^_ !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ";

If the KeyboardEvent.key length is 1 I can safely assume that it is a letter or a number and then simply:

let scr: number = this.m_G64Memory.TXTSCREENCODE.indexOf(key);

For everything >= 0, scr is now the screen-code of the pressed letter and the value can be stored directly in screen-ram (at cursor position, of course), should scr be <0 we have to investigate further ...

The fun begins when trying to reach the C= key combinations. While being conveniently shown under the letter on the #C64 keyboard there seems to be no logical connection between the symbol's screen code and the letter. You cannot simply capture the Alt key (as a replacement of the C= key) and add, say 111 to it (PETSCII values, "A" is at 65 and "" is at 176).

Adding to this particular flavor of fun is the fact that there are symbols located on keys that are in a different position on modern keyboards. I made an Excel table to see how to map keys around:

Never one for the easy way, I also use a German keyboard layout with those lovely umlauts and the § sign instead of the pound symbol (£, Shift & "3"). Just from looking at the table I'm getting a headache (as everyone who has looked into the way VICE handles keyboard layouts).

Here are just a few questions I need to find an answer for by myself:

  • Use symbolic or positional layout (ie: "Z"/"Y"), does it make more sense to keep the symbol (C= & key) on that key (hence symbolic C= & "Z" will still result in "", even though the key is in a different place on my keyboard)
  • Keys that have a different shifted representation (Shift & "+" is "*" on my keyboard, but "" on the #C64)
  • Keys not present, like "", "" or "", not to mention Runstop or Restore.
  • "ä", "ö", "ü"?

Right now I'm considering to say "fuck it" and map what can be mapped and resort for everything else to an on-screen keyboard (which is a whole different story).

And on this sober note ...
... have a good one and see you later.