Re: High Resolution Question

["Roger Johnstone" <rojaws@mac.com>]

In article <1fataa2.vvxj7rhq548wN%dempson@actrix.gen.nz>, 
dempson@actrix.gen.nz (David Empson) wrote:

> Does anyone have any further information on this "dynamic shift
> register" arrangement used by the Apple I's screen?
>
> My recollection (probably from reading something in a magazine in the
> order of fifteen to twenty years ago) is that the Apple I screen
> operated pretty much like a parallel connection to a dumb terminal: data
> to be output to the screen was stored in a single memory location, and
> the output position was updated automatically in hardware.  There was
> presumably some way of doing a "new line" operation (outputting
> sufficient spaces would have done the trick if there wasn't an easier
> method), and clearing the screen.  I expect it implemented the cursor in
> hardware as well.
>
> The only feature this has in common with the Apple II text screen is its
> resolution.
>
> Even if I have the details wrong, I'm sure that it was a lot simpler
> than the Apple II text screen, and it wasn't able to support random
> access to arbitrary positions on the screen.

According to Woz himself at http://www.woz.org/letters/general/95.html the
Apple I was based on a terminal he had built earlier, which used shift
registers to hold the display characters.

http://emulation.net/apple1/ has an Apple I emulator for the Mac and an HTML
version of the Apple I manual. From reading that I see the display memory
was completely seperate from the main memory. As you correctly recalled the
display was written to via a single I/O location. The only thing you could
read back was a busy flag, you had to wait after sending each character for
the display circuit to process it.

--
Roger Johnstone, Invercargill, New Zealand

Apple II - FutureCop:LAPD - iMac Game Wizard
http://homepage.mac.com/rojaws
________________________________________________________________________
"Computers in the future may weigh no more than 1.5 tons"

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