Instructions
- Download disk image, and use as is with modern tools, or image to floppy.
- Click the 80 column keyboard button into position to make it the default.
- Attach two monitors to your C128. Composite on the left, digital RGB on the right.
- Start up (or reset) C128 with disk attached. Boot sector will load and run 80DUAL by default.
- Enjoy screen extended 80 columns!
It works by reinitializing the VDC 8563/8568 registers to display 40 columns offset by 40 columns, and each line thereafter also offset by 40 columns. Since the KERNAL hasn't been told any differently, it will continue to treat the VDC as an 80 column display, but the VDC settings are now to display only the right hand 40 columns. An IRQ routine is installed to copy 5 lines from VDC RAM to VIC-II screen RAM (hardwired to address 1024 currently) and color RAM. The effect is that as text is typed or displayed on the 80-column virtual and physical display, the hidden left-hand side of the screen will also be displayed on the composite monitor.
This is a proof of concept, and has limitations.
- The machine language routines use $1300-$15ff for code and data
- The code uses self-modification techniques so can't be moved to ROM as is
- Character set changes are not handled
- Changing to composite screen is not handled, will continue to be overwritten from VDC RAM
- No graphics support
- Slow, less than 12 frames per second operation
- Not compatible with SNAKE
- Will interfere with programs that write text to VDC screen using own (not ROM) routines. It has a fixed list of ROM addresses to avoid interrupting, and doesn't include RAM.
Why?
- It was a challenge I set myself
- Because it's ridiculous and outrageous, has little practical purpose. I like the bizarre possibilities.
- Related to my rgb64
