My VIC-20 Factory TNT for CoCo?

Insert “my first computer was a Commodore VIC-20” story here.

TL:DNR – I figured out how I might port one of my old VIC-20 games to the CoCo so it actually looks like the VIC-20 game ;-)

The VIC-20 has a 22×23 text screen. The CoCo has a 32×16 text screen. At the time, going from a width of 22 characters to 32 seemed like a huge upgrade. After all, you could scroll up and down to get more lines, but you couldn’t scroll left and right to get more columns.

I knew there was no way I could port any of my old character-based VIC-20 games to the CoCo without altering the screen layouts. Games that were written to use the 22×23 display, such as my Sky-Ape-Er, could not be replicated on the CoCo’s 32×16 text screen with 100% size accuracy.

VIC-20 Sky-Ape Er, screen 1.

Without reprogrammable characters, anything on the CoCo’s text screen would look far less graphical. Each text blog could either be a letter/number/punctuation character, or a 2×2 block of one color plus black.

Even if I did try to use these blocky characters to port any of my VIC-20 games, they would not be accurate since the width and height were different. I would have to redesign them to fit the CoCo screen:

Above, you can see I tried to get the “spirit” of the game on the screen, but there are less steps for each level, and 10 more horizontal spaces on each level, so the gameplay would be quite different. (To get to the top takes 12 jumps on the VIC, but only 8 on the CoCo version.)

With 32×16 there is simply no way I could accurately recreate a 22×23 game.

At least, not in text mode.

I knew the CoCo’s 32 column screen was equivalent to the 256 pixel width hi-res screen, and it would be larger than whatever the VIC’s 22 column screen was. Therefore, I figured the VIC’s 23 row screen must be larger than the CoCo’s 16 row screen.

When I started revisiting my VIC-20 programs, I remember that each screen character was 8×8. That made the 22 columns represent 176 pixels (22*8), and the 23 rows represented 184 (23*8). 176×184 is smaller than the CoCo’s 256×192 screen! I had always thought the CoCo’s text characters were 8×8, but that was just the data used to draw them. There were a few extra lines between each row that I had never considered.

On the CoCo, 32 columns at 8 pixels each is 256 — that is what I would expect. But when it came to rows, if each character were 8 pixels high, that would only be 96 pixels tall (16*8)! I had never bothered to “math” this. The 192 horizontal resolution meant that each of the 16 rows was 12 pixels tall! That means the CoCo text font was really more like 8×12.

This led me to experimenting with the VIC-20 font data. I could display a full VIC-20 display of 176×184 with room to spare on the CoCo’s 256×192 screen. I wrote about this in an earlier article and made this example showing the 22×23 VIC-20 character set drawn on the CoCo screen:

VIC-20 20×32 screen displaying PETSCII on a CoCo

The VIC-20 did not have a graphics mode, and did everything by putting characters on the text screen. You were able to reprogram what those characters looked like. You could do some pretty neat things just by PRINTing or POKEing to the text screen.

This is how I made my Factory TNT game. I drew the screen like this…

Factory TNT for the VIC-20

…then remapped the characters to be my graphics:

VIC-20 Factory TNT.

I wanted to accurately represent that screen on a CoCo, I would have to do it using the CoCo’s hi-res 256×192 graphics screen. I could bring over all the actual character data and recreate it accurately, though the aspect ratio would be off a bit… It might look like this:

CoCo Factory TNT concept

Missing is the “MEN:” and “SCORE:” text labels on the top line. Those font characters would have to be brought over, as well. The VIC would let you remap A, B, C, D, etc. to be different characters, and if you used inverse video (or something like that) while in this “reprogrammed character mode”, they would show up as normal uppercase letters on the screen.

This was just a quick experiment, but perhaps I can also bring over just the VIC-20 font letters needed for “MEN:” and “LIVES:”. Maybe I’d want the whole alphabet so I could add a title screen or instructions. Factory TNT 2024, perhaps?

To be continued… Maybe.

(Okay. Definitely. I have already started working on a multi-part article series about this endeavor.)

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