Re: Apple II Interger ROMs

["Michael J. Mahon" <mjmahon@aol.com>]

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

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