This writing is inspired by somethingI just saw in a YouTube video by 8-Bit Show and Tell:
In this video, Robin shows off an Easter egg found in the CoCo 2’s BASIC (the same one also found in the CoCo 1 since it was the same BASIC). He was wondering if there was any way to change the screen color (and I remember seeing someone ask this in the CoCo Facebook group but did not realize it was the Show and Tell guy). Australian programmer Nick Marantes gives him a “POKE 359,0:SCREEN 0,1” command to change the screen color… SCREEN can change the screen color? Surely I must have known this back then, but I had completely forgotten.
Thus, this article.
Nuclear Green CoCo Screen
The “nuclear green” screen of the Radio Shack TRS-80 Color Computer is certainly iconic. Black text on a green 32 column screen.
The Motorola MC6487 VDG chip contained a number of graphics modes, but only one text mode: 32×16 characters. The semigraphics modes supported up to 8 colors on the screen at the time, but text mode was limited to two colors: black on green. Well, except it was really a dark green on a light green. Here’s a zoomed in look, showing what color the text is compared to the black border:
I guess I sorta remembered this, because there was a way to invert the video to get light text on a dark background, and that background clearly wasn’t black:
I remember reading an article (probably in Rainbow Magazine) that showed how you could remove the VDG chip, pull up a pin, then plug it back in to the socket and get inverted video. I did that on my CoCo 1 since I thought that looked alot better (but not as good as if it was white on black like an Apple II or TRS-80 Model III).
The Xroar emulator has a menu option to toggle this inverted video.
But there was also a weird “pink” mode that I saw used in some early CoCo games. I recall reading a POKE or something that let you toggle it. You can also do that with the SCREEN command in Extended Color BASIC.
From the manual (page 96):
SCREEN type, color set displays the current graphics or text screen
type is 0 (text screen) or 1 (graphics screen)
color set is 0 or 1
Note: If type or color set is any positive number greater than 1, your computer uses 1.
Although not explained in the manual, you could select two color sets for the text mode. 0 was the normal dark green on nuclear green. 1 is the weird pink mode. But, when BASIC returns to the OK prompt, it resets the mode to 0, so you can’t use this to type things in (without a special POKE to bypass this reset). But, you can enter it in a program and then loop using a GOTO:
As soon as you hit BREAK and return to BASIC, the color resets to the green mode. The inverted mode looks like:
I don’t recall ever using this mode, though I do recall at least trying it out with the POKE.
I do not know how to invert the video in software, but I expect there is a POKE to do that.
Anyway, there you go… alternate color sets for CoCo BASIC that you can access from BASIC using the SCREEN command.