One of the first three strobe addresses above is
           placed on the RGA bus during the first refresh time
           slot of every other line, to identify lines with
           long counts (228- NTSC, HTOTAL+2- VARBEAMEN=1
           hires chips only).There are 4 refresh time slots
           and any not used for strobes will leave a null
           (1FE) address on the RGA bus.
Interesting links: