Free memory

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Free memory

Postby deathshadow60 » Sat Feb 08, 2014 9:54 am

I'm working on something for early 128k machines, and have a question about the Jr. Since it shares RAM from video, how much memory is free once DOS 2 (or whatever's appropriate to the Jr) is loaded on a 128k Jr. I'd like to use 60% to 75% of that as my minimum target (wiggle room), since I figure that's going to be even lower than a 128k 5150.

Note, I'll not be setting any graphics modes if that makes a difference -- strictly 80x25 text mode since the original MDA is also on my target sheet.

The way I figure it I'm going to need at LEAST 48k to pull this off, and that's gonna be tight.
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Re: Free memory

Postby Brutman » Sat Feb 08, 2014 11:02 am

Straight DOS 2.1 with no device drivers reports 114688 bytes total memory, 90000 bytes free.

The 114688 is exactly 128K - 16KB. So resident DOS is consuming about 24KB.

You have plenty of room. If you are really only using one page of the text mode then you can gain back another 12KB by using the other pages.
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Re: Free memory

Postby Trixter » Thu Feb 27, 2014 11:32 pm

Brutman wrote:You have plenty of room. If you are really only using one page of the text mode then you can gain back another 12KB by using the other pages.


And you don't have to worry about any speed hit reading/writing those pages because the entire 128K is made up of "video ram" and has the same speed penalty. So while it seems like heresy to use screen RAM as general-purpose RAM, it's not silly at all on the PCjr.
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