Implement the Apple II floating bus

[TomHarte]

Cf. http://rich12345.tripod.com/aiivideo/softalk.html

... a complete sixty-five-cycle scan line consists of sixty-five consecutive bytes of display buffer memory that starts twenty-five bytes prior to the actual data to be displayed.

During VBL the data acts just as if it were starting a whole new frame from the beginning, but it never finishes this pseudo-frame. After getting one third of the way through the frame (to scan line $3F), it suddenly repeats the previous six scan lines ($3A through $3F) before aborting to begin the next true frame

[TomHarte]

Also, if implementing the IIe ever becomes a priority, per Understanding the Apple IIE, page I-4:

HBL scanned addresses in TEXT/LORES scanning in the Apple II are $1000 higher than HBL' scanned addresses.

[TomHarte]

https://groups.google.com/forum/#!topic/comp.sys.apple2.programmer/1G4Fs91g_w0 also seems to suggest some good test material.