Re: (no subject given)

[dempson@actrix.gen.nz (David Empson)]

Jeff Blakeney <CUTjefbla@bconnex.net> wrote:

> On 18 Jun 1998 16:03:48 GMT, mab22@po.CWRU.Edu (Michael A. Balfour)
> wrote:
> 
> >This might sound like a silly question, but I really don't understand:
> >how does Flashing text work in the Apple II series of computers.  I've
> >been looking through Apple IIe schematics, and I don't see any video
> >invert signals tied to the output of the character generator ROM.
> 
> As far as I know, the flashing text that is generated by Applesoft's
> FLASH command is handled completely in software in ROM.

Nope.  It is handled completely in hardware.  In the Apple ][ and ][+,
there are some discrete components involved in the flash generation
circuitry, but in the IIe/IIc/IIgs it is all buried inside the IOU chip
(Mega II in the case of the IIgs), possibly still using an external
resistor and/or capacitor to determine the timing (I haven't looked at
this for a while).

Just talking about the IIe design for the moment (the ][/][+ is
simpler): The flashing is triggered by a particular range of characters
($40-$7F), but only in the primary character set.  The IOU is constantly
generating a toggling signal which affects the address output to the
video ROM should any FLASH-range characters happen to be displayed.

If I remmeber right, the video ROM gets told to display alternating
between the $00-$3F and $80-$BF ranges (inverse/normal upper case and
symbols).
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand