Zero Electronics "Zero 18" 40/80 column card for VIC 20

Assumingly same as Kalawsky 40/80 column card.


Power-up/reset keys:

	RETURN		Use normal display

	Right shift	80 column mode

	(0)		25 rows
	 1		24 rows
	 2		23 rows
	 3		21 rows
	 4		19 rows
	 5		17 rows
	 6		16 rows
	 7		15 rows
	 8		14 rows


SYS commands:

SYS 42997 ($a7f5)	Set screen geometry. "POKE 211 ($d3),height : POKE 243 ($f3),width" first.
			Height value is from 0 to 7 and matches the corresponding power-up key.
			The screen won't be centered horizontally.

SYS 43000 ($a7f8)	Add screen memory to BASIC free memory, gaining 512 bytes.

SYS 43003 ($a7fb)	Lock topmost screen row. Up to 7 (40 columns) or 4 (80 column) rows can be locked
			by repeating the SYS command.


Memory addresses:

MC6845 address register is at 38912 ($9800).
MC6845 data register is at 38913 ($9801).

Screen memory is located at 43008 ($a800). Writing to memory is done normally, but reading is done by
first reading dummy value from the correct address, then reading screen contents from 38944 ($9820).


Hardware:

The board has 2716 EPROM for code, 2732 EPROM for fonts, 2016 RAM for screen memory and MC6845 CRT controller to
generate video addresses and sync signals. In addition to those there are several TTL logic chips and some passive
components.