Re: Apple ///?

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <5igsr7$s05$1@darla.visi.com> nathan@visi.com (Nathan Mates) writes:
>In article <5igjah$teu@uni.library.ucla.edu>,  <pubpc1@library.ucla.edu> wrote:
>>How does the Apple III access more than 64K without bank
>>switching?  It uses a mere 6502B with a 16 bit memory
>>address.
>
>   It apparently doesn't switch full banks but lets you swap out
>part(s?)  of main memory for some section of the extended memory.
>Basically a far more intelligent, intuitive, easier to use, and
>intelligent design than the 'bank switching' hacks of the //c and //e.

Actually, it uses two different schemes for addressing beyond 64K.  The
first scheme is a traditional bank-switching scheme--the 32K hunk of memory
from $2000 to $9FFF is bank-switchable.

The second scheme is rather bizarre--the III has hardware that sits between
the 6502 and the memory, watching for zero-page indirect memory accesses.
When one occurs, the final memory access goes to a location in one of the
bank-switched banks, ignoring the normal memory mapping.

If that made no sense, you might want to try pointing your Web browser at
http://ground.isca.uiowa.edu/apple2/MiscInfo/III/appleiii.mem...it
explains Apple III memory mapping in greater (and more technical)
detail.

            - Neil Parker
-- 
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker
     "I was going to be a neo-deconstructivist but Mom wouldn't let me."
                                     -- Calvin and Hobbes, 7/13/1995
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