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The March 1988 issue of RUN contains the following tip from a reader on page 20:

Attention: C-128/1902A Owners!

If you use a Commodore 1902A video monitor with your C-128, here's a way to get a clearer and cleaner screen image by increasing the resolution of 80-Column mode:

POKE 54784,9:POKE54785,232

Since I'm a confirmed RUN Script 128 user, I found the default resolution a little hard on my eyes after an hour or so. So now I include the above Pokes within the boot programs of all my 80-column programs.

—John Ryan, Biloxi, MS

I tried typing these commands in the 80-column mode of a C128 emulated with VICE, but the only thing that seems to happen is that the text area shifts a bit. What do these POKEs do on a real Commodore 128 and Commodore 1902A monitor? Do they really "increase the resolution" of the screen, making it "clearer and cleaner", and if so, how exactly does this work and why didn't Commodore configure the computer this way by default?

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  • 1
    It adds an additional blank scan line between each row of text. If a monitor has "issues" with vertically adjacent pixels blurring together, this would improve things.
    – Brian H
    Jan 21, 2021 at 15:13

1 Answer 1

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10

In the Commodore 128, the Video Display Controller (VDC) was used for the 80-column mode. Two variants of the VDC were used, the 8563 and the 8568.

Looking at the POKEs, the address 54784 ($D600) is the index register for the VDC, while 54785 ($D601) is the data register.

So those two POKEs write the value 232 ($E8) to register 9 of the VDC.

Register 9 is the "Character Total Vertical" register, so that changes the total height of the character.

Therefore "increasing the resolution of 80-Column mode" is probably a clumsy way of saying "characters are now taller", probably with an additional blank line of pixels between two lines of characters (I don't have a real C128, so I cannot try this out). If the 1902A monitor by default had enough overscan to still completely display all characters, then that would make the overall picture bigger, and therefore probably easier to read.

It probably would not have worked on all monitors in this way; it's possible that on other monitors, not everything would have been displayed. That's also probably the reason Commodore didn't configure it by default.

I'd also assume that VICE doesn't simulate analog monitors and/or the VDC to the degree necessary to see the full effect.

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  • A lot of monitors had an accessible vertical height adjustment on the back, and could thus easily be made to work with the taller characters even if the factory-default adjustment would clip the borders.
    – supercat
    May 5, 2021 at 19:58

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