Keyboard: Compress keymaps

[mist64]

Currently, we map the PS/2 scancode to the PETSCII or ISO-8859-15 character.

The PS/2 scancodes are very sparse, so each table is 91 bytes. But there are only 48 non-control keys on ISO keyboards.

By mapping the PS/2 scancode to a value from 0-47 first, and using a 48-entry table, the size of the tables can almost be cut in half.

In addition, the scripts should make sure identical maps (e.g. Ctrl+character on Nordic and US) only end up in the ROM once. So it may even make sense to break the 48 scancodes into "A-Z" (26) and "rest" (22), because A-Z tend to be the same for most keyboards.

[mist64]

WIP https://github.com/commanderx16/x16-rom/tree/compressed_keymaps

[mist64]

I am no longer convinced of this:

• Mapping everything onto 48ish codes might work, but there could be a keyboard layout that I'm not aware of now that doesn't fit into this.
• We have plenty of ROM space now and could use more than one bank for keymaps.
• We will support user-defined keyboard layouts (Support user-defined keyboard layouts #258), so the user can always have the autoboot script load a layout from disk.
• If we reserve ~600 bytes in banked RAM for a loadable keyboard layout anyway, we might as well store compressed layouts in ROM and decompress one into RAM. A quick test got 630 bytes down to 349 bytes for "en_US". This way we can store about 23 keymap pairs (PET & ISO) on 16 KB ROM bank.

[mist64]

#258 just defined a fixed keytab format at a fixed banked RAM location.
The next step would be to store the layouts in ROM compressed using the KERNAL's LZSA support. And then to add more layouts.

LZSA gets the 1302 byte keymaps down to 438-478 bytes (average: 459). Plus 8 bytes index header. That's about 35 keyboard layouts. (The previous calculation assumed we'd compress PET and ISO separately.)

[mist64]

I asked users in the forum
https://www.commanderx16.com/forum/index.php?/topic/2782-keyboard-layout-survey/
and on FB
https://www.facebook.com/groups/CommanderX16/posts/1158745751543184
what keyboard layout they use. I already did that back in 2019, but maybe I can get a better signal now.