homemade computer for fun and experience...

From: Allison J Parent <allisonp_at_world.std.com>
Date: Sun Apr 4 12:40:25 1999

<memory mapping would be a mite silly, wouldn't it, given that the HERC use
<64K of refresh RAM. I'd say the way to do it is to use a strategy put fort

No, you partition in to 16k quads and your off.

<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 th
<chip and then send a stream of characters, kind of like the cursor
<addressing on a terminal.

Run a kaypro some time... slower than sludge video.

<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, whic
<works as a window in the display RAM. The refresh of the display would kee
<the entire video memory refreshed, while the banking scheme could minimize
<the memory consumption of the video device.

Full circle. Hence the need for a MMU. Me I'd never use the 6845/herc in
graphic modes. For text the Herc cards only need 4k.

<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

Volume will do that.

<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 vi
<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.

That's why things are buffered. the 8255 will drive a few feet of ribbon
but generally thngs like ground loops and other anomolies make a buffered
interface more robust. At the time of the 8255 heyday the output current
was limited by the Nmos process of the time to only a few mA. Now, the
CMOS processes can easily do 30-70 mA but... you can still fry them.

Allison
Received on Sun Apr 04 1999 - 12:40:25 BST

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