Custom Char Set and Memory

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plbyrd
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Custom Char Set and Memory

Postby plbyrd » Mon Aug 21, 2017 9:24 am

Is it possible to tuck a custom char set under the char rom ala C64? Or is that a special feature of the 6510?

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tokra
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Re: Custom Char Set and Memory

Postby tokra » Mon Aug 21, 2017 10:06 am

There is no RAM under ROM on the VIC-20. That is a special feature of the 6510. On the VIC-20 you can only put custom chars (and the video-RAM) in the RAM-area of the unexpanded VIC ($0000-$03ff and $1000-$1fff). The first 1K-area at $0000-$03ff does not make a lot of sense for custom chars in most cases but is possible anyway.

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plbyrd
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Re: Custom Char Set and Memory

Postby plbyrd » Mon Aug 21, 2017 10:16 am

OK, thanks. I'm going to use a modified version of the C128 character set. I need to use some special glyphs for the display and the C128 font looks awesome on the VIC20.

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Mike
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Re: Custom Char Set and Memory

Postby Mike » Tue Aug 22, 2017 1:09 am

To be exact, what the VIC-II chip 'sees' in the C64 has nothing to do with the 6510 (port) at all.

What is addressed by VIC-II is selected by the PAL IC, independent on the 6510's port settings; VIC-II only sees the C64 RAM with the exception of two mirror copies of the character ROM at $1000..$1FFF and $9000..$9FFF (unless there's Ultimax-Mode active). The 6510 has its own copy of the character ROM selected at $D000..$DFFF with /CHAREN=0 and at least one of /LORAM or /HIRAM =1.


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