Unofficial CGA port mirroring? (Prohibition). \ VOGONS

Unofficial CGA port mirroring? (Prohibition).

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First post, by jmk

Posted on 2008-04-14, 10:30

jmk Offline

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I decided to have a peek at Prohibition and it appears to be writing to the CRTC at 0x3d0 and 0x3d1 to do the "scrolling" (writes to regs C and D to set the start address).

I tried compiling with the crtc ports mirrored at 0x3d0/0x3d1 (VGA_SetupOther) and the game does seem to work. I don't know how safe it is to mirror these ports, perhaps for CGA, TANDY and PCJR but not VGA?

Reply 1 of 5, by wd

Posted on 2008-04-14, 11:47

wd Offline

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http://www.seasip.info/AmstradXT/ppctech/section1.html
Might not be present on real cga systems??

Reply 2 of 5, by jmk

Posted on 2008-04-15, 12:39

jmk Offline

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Aha... according to this list:
http://bochs.sourceforge.net/techspec/PORTS.LST

There are indeed mirrored ports at 03D0!

Reply 3 of 5, by wd

Posted on 2008-04-15, 12:45

wd Offline

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Though
http://www.seasip.info/AmstradXT/ppctech/section1.html
is way more explicit about WHY they are duplicated, but well.

Reply 4 of 5, by jmk

Posted on 2008-04-15, 12:59

jmk Offline

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Oops, didn't read all the way through that one. 😊

But, at least we know that it happens on non-Amstrad systems as well.

Reply 5 of 5, by wd

Posted on 2008-04-15, 13:19

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Think the machine=cga is fine for that, the game seems to be playable with it.

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