why not disc area at &7c00 ?

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duikkie
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Joined: Fri Feb 07, 2014 3:28 pm

why not disc area at &7c00 ?

Postby duikkie » Tue Mar 14, 2017 7:23 pm

i know that this area &7c00 is for mode 7 in beeb but

is there software that move or can work with &d00-&1100 moved to &7c00-&8000

and why was it not in the first place there.

the hole &d00-&&1100 is mostly in the way , in every game :shock: with m-code.

was it not much easyer to have the disc area at top then you can load in from &400 till &7c00 without a problem.

i know there is allso sideway dfs, but i you don't have it

you are stuck with &d00-&1100.

RobC
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Joined: Sat Sep 01, 2007 9:41 pm

Re: why not disc area at &7c00 ?

Postby RobC » Tue Mar 14, 2017 8:10 pm

Maybe it made the screen wraparound easier to have the end of screen memory at 0x7FFF?

dominicbeesley
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Re: why not disc area at &7c00 ?

Postby dominicbeesley » Thu Mar 16, 2017 11:27 am

Yes, screen hardware relies on the counters rolling over to 0 at &8000 then an adder kicks in and adds the screen start back to the address -8000, sounds complicated but requires only a couple of 'ls chips to implement fast(ish) scrolling.

D

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sweh
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Re: why not disc area at &7c00 ?

Postby sweh » Fri Mar 17, 2017 1:28 am

If disk memory was stuck at &7C00 and screen memory moved downward as a result then we'd just get different problems. A game written to run off TAPE and writes directly to the screen would stop working when DFS/ADFS/ANFS etc were installed because the screen has now moved. We'd end up writing disk loads that'd disable DFS and relocate the screen so those games would work.

You'd just be trading one set of problems with another set.
Rgds
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