Re: A few technical q's from an apple newbie

["josephoswaldgg@hotmail.com" <josephoswaldgg@hotmail.com>]

The Apple II,II+,IIe do not have a periodic IRQ. The IIc has IRQs
associated with the built-in serial ports, the mouse port, the
keyboard, and a periodic VBL interrupt. I don't know about IIgs.

The flashing cursor is usually generated using the flashing character
set in the text mode. There is an internal counter in the video
circuitry that inverts the video of these characters for 32 out of each
64 vertical scans.

VBLANK on the IIe causes the high bit of $C019 to be zero during the
vertical blanking interval, and one otherwise (during the field). The
IIc uses $C019 to indicate the state of the VBL interrupt, and to reset
it.
The Apple II/II+ had no built-in access to the VBL signal. I'm sure
some software did use this, but I don't know of any commercially
released examples myself. There were other tricks to sync to video as
well: in many Apples, the video data stayed on the data bus long enough
to show up on reads of I/O address locations that were otherwise meant
for writing only. That was not an officially supported technique.

The memory jumpers were only present on the original II/II+. They
affected the memory decode logic. Software could only detect memory
size by testing whether addresses were fully decoded, or whether the
same RAM showed up at multiple addresses (i.e. for < 48K ram, the
decoding was incomplete, and the same RAM location could have multiple
addresses.)