The challenge has been set! Earlier
in 2015, the great Allen Huffman started the 4K Programming Challenge.
The aim was to accept submissions for programs written that could run
on the original 1980 4K Color Basic Color Computer. Quite a number of entries were made and one can see the submissions made on his website at: http://subethasoftware.com/4k/ My submission was a recreation of the classic arcade game, Space Invaders. I
have never owned a 4K Color Basic CoCo so it caught my
interest to see how such a computer could be used. Color Basic was a
very simple version of Basic that not only had no support for high res
graphics (4K ram wasn't even enough to open a high res screen!) but it
also lacked the ability to edit a line of Basic code. If you made a
typo in your program, you were forced to retype it from scratch! I
still don't own a 4K Color Basic CoCo so I used the XROAR CoCo
emulator, configured as a 4K Color Basic CoCo as my programming
environment. A
month later, I had a reasonable rendition of Space Invaders using
the blocky 8 color graphics and limited sound effects running in 4K
(actually there was less than 2.5k free RAM after Basic boots). I
wanted to experience Color Basic programming the way it was designed in
1980 so I restricted myself to only Basic... no assembly language code
whatsoever. Below is my program which you can run on any Color Basic CoCo emulator or real Color Basic CoCo. It must be Color Basic (not Extended Color Basic) and if you only have 4K of RAM, do a CLEAR 100 prior to loading the program. 
Space Invaders in 4K Color Basic? Space Invaders on a 4K Color Basic CoCo? Is it possible to replicate the tension and excitement of the arcade original? With
just over 2K of free RAM to start with and a BASIC that doesn't even
allow editing of it's own lines, it seemed doubtful that anything
decent was possible under such constraints. The
challenge was set! An opportunity to relive those early computing days
when BASIC programming was cool and large colored pixels were cutting
edge! I set forth using techniques such as multi-statement lines,
embedded graphics, self modifying BASIC code and a total disregard for
structured programming! This
is my attempt to squeeze a piece of arcade history into a limited
programming environment and show what can be accomplished using
standard 4K Color Basic (no machine language). Let the invasion begin! Unfortunately, the 4K competition never reached it's conclusion so I have made my entry available to others for download. It's in a CASS (cassette) emulator format and runs in the XROAR emulator when it's configure as a Color Basic system. To load it into a real Color Basic System, it will need to be tranferred to tape. On a 4K RAM system, do a CLEAR 100 first then load the program with CLOAD. RUN to start. Use the left and right arrow keys to move your laser base and spacebar to fire. Click on link to download game 
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