Re: Apple II Interger ROMs

[pausch@saaf.se (Paul Schlyter)]

In article <mrudnV3DFqHiilXfRVn-qw@comcast.com>,
Michael J. Mahon <mjmahon@aol.com> wrote:
> Paul Schlyter wrote:
> 
> <snip>
> 
>> IMO a more straightforward way to create the INTBASIC file would be to
>> boot Apple DOS on a machine with Integer Basic in ROM, select INT if
>> needed, and then just BSAVE $D000 to $FFFF to a binary file.  If no
>> ROM is present at $D800-$DFFF, that memory area ought to read as
>> 00 00 FF FF 00 00 FF FF 00 00 FF FF ......
> 
> Actually, if an address maps to an uninstalled ROM, nothing
> will drive the bus during the read, and the processor will
> read whatever "phantom" data is left on the bus by the last
> video refresh read, generally blanks ($A0) or other high ASCII.
> 
> The same behavior occurs when reading any output that does
> not drive the bus when reading, for example, many softswitches.
> This behavior is what makes the "vaporlock" synchronization
> of a program with the video refresh possible on any Apple II.
> 
> The 00 00 FF FF pattern is a typical pattern present in
> an Apple II's uninitialized DRAM.
> 
> -michael

You're right - I got a bit confused here.  Thanks for straightening
this up.

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