Re: Which video processor have a Apple ][ ?

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

Landoq@landoq.com <landoq@landoq.com> wrote:

> Which video processor chip have a Apple ][ ? Motorola 6845 ?

None. In an original Apple ][ or ][+, the video is generated using
simple logic circuitry such as shift registers and timing circuits,
combined with a ROM to get characters for text mode.

The data for the display is read directly out of main RAM. The CPU and
video circuitry do alternating memory accesses during each CPU clock
cycle. The video accesses have the side effect of refreshing the RAM,
avoiding the need for explicit refresh cycles.

There are separate RAM areas reserved for lo-res and hi-res graphics
modes, with the lo-res area also used for text display.

In the Apple //e and //c, the same basic design was incorporated into an
ASIC (Apple's "IOU", or "Input/Output Unit"), with a few extra features
(e.g. to support 80-column text and double resolution graphics modes).

In the Apple IIgs, Apple merged yet more circuitry into a larger ASIC
called the "Mega II", which generates all the video modes supported on
8-bit Apple II models. There is also a custom chip called the VGC (Video
Graphics Controller) which implements an additional super hi-res
graphics mode which is unique to the IIgs. This mode also gets its video
data from main RAM.

-- 
David Empson
dempson@actrix.gen.nz