homemade computer for fun and experience...

memory mapping would be a mite silly, wouldn't it, given that the HERC uses 64K of refresh RAM. I'd say the way to do it is to use a strategy put forth by the guys who designed the 6545, the '65xx' version of the 6845. It had an update register which essentially allowed you to write an address to the chip and then send a stream of characters, kind of like the cursor addressing on a terminal. Some scheme like that would work even in the limited I/O space of a Z-80. Another option might be to bank the BIOS such that when it's writing to or reading from the video page, it has a space equal to the disk buffers, which works as a window in the display RAM. The refresh of the display would keep the entire video memory refreshed, while the banking scheme could minimize the memory consumption of the video device. The PC certainly has made for cheaper serial/parallel interfaces than we ever saw for the S-100. It's too bad there were never any truly general purpose parallel ports use commonly enough on the PC to make them cheap. It's only since the adoption of the 1284 standard that bidirectional I/O via the printer port has been practical. It's too bad there aren't i8255-equivalents which can drive something. The problem with my IMSAI PIO-6 is that it won't even drive an LED directly. Dick -----Original Message----- From: Allison J Parent <allisonp(a)world.std.com> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Saturday, April 03, 1999 8:54 PM Subject: Re: homemade computer for fun and experience... MH