21 votes
Accepted

Is it possible to detect a CGA card on an IBM PC 5150 by write/reading the Motorola 6845?

You can, in fact, relatively reliably and safe, detect a 6845 in your system by trying to write to a 6845 register and see whether you can read back the same value. I have successfully done that with ...
tofro's user avatar
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18 votes
Accepted

How did the Commodore PET 80 column display work?

As Tommy wrote in the comment: There are two different RAM chips, one for even columns and one for odd columns. Both are read and latched in one cycle (ESD and OSD bus), and the latched output of each ...
dirkt's user avatar
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11 votes

Is it possible to detect a CGA card on an IBM PC 5150 by write/reading the Motorola 6845?

If all the other adapters have already been detected to be absent from the system, the possible options are that there are no MDA or CGA adapter at all, there is either MDA or CGA adapter installed, ...
Justme's user avatar
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11 votes

Is it possible to detect a CGA card on an IBM PC 5150 by write/reading the Motorola 6845?

The Paku Paku game source code doesn't directly detect it. The strategy is to detect VGA, EGA, Tandy, PCJr, etc etc, then if none of those "hit", assume it is CGA. Does it, or would MDA ...
Raffzahn's user avatar
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9 votes
Accepted

How does the Motorola MC6845 in the Amstrad CPC 464 behave on start-up?

The short answer is that the contents of the CRTC control registers are indeterminate after power-up, and that the CRTC will nevertheless operate according to their contents immediately after /RES is ...
Chromatix's user avatar
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4 votes

Which addresses does the Motorola 6845 CRTC output during retrace?

Short answer from memory: At the beginning of a frame the address is set to zero The address is incremented over a line for R0 (total chars aka horizontal timing) character access cycles. Only R1 (...
Raffzahn's user avatar
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1 vote

Is it possible to detect a CGA card on an IBM PC 5150 by write/reading the Motorola 6845?

To answer the specific question: What happens if you write to an I/O-port that doesn't exist in the hardware? Does it crash? That doesn't happen with any Intel or Zilog microprocessor I'm aware of. ...
Toby Speight's user avatar
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1 vote

Which addresses does the Motorola 6845 CRTC output during retrace?

The horizontal counter increments continuously with the CRTC clock, and the vertical counter (divided into a pixel row counter and a text line counter) increments continuously with the horizontal sync ...
Chromatix's user avatar
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