MITS 8800B CPU Board

From: "Tom Jennings" <tomj at wps.com> Sent: Wednesday, June 15, 2005 2:51 PM <snip> I understood, I was referring to the trigger for phantom. It does not matter how the flip flop gets set as long as it does not interfere with the initialization of the monitor. I was agreeing that it is best to use phantom to access ROM rather than address decoding that leaves a ROM window. In the early days of S100 systems mass storage was not the norm, so having a permanent ROM monitor (some complete OS's with assemblers, etc) made sense. Another point is there were no 64K RAM chips so address decoding had a different standard to what anyone would build today. Today I prefer 512K RAM's so that address decoding is replaced with MMU. The methods of releasing phantom that I'm most familiar with are either a bit in an I/O port so the ROM can be brought back in the address space if needed or a particular addressing bit that shows the ROM is no longer needed. Tarbell used address bit 5 to reset phantom, the boot ROM just falls through to disable the boot ROM. Tarbell's boot ROM reads a sector into RAM and when execution hits it phantom is disabled. There are lots of ways to reset phantom depending on needs. With Flash ROM's I prefer I/O port access so the ROM can be re-programmed ala SuperIO. Randy www.s100-manuals.com