Re: 40x24 Resolution Question

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

Bryan Parkoff <BParkoff@satx.rr.com> wrote:

> 40x24 Resolution Question
> 
>     It is very interesting.  I believe that all type of computers like
> Intel, Motorola, PowerPC, DEC, Alpha, etc ALWAYS show text screen that is
> built by graphic.  I do not talk about emulator, but real.
>     I think that 9x13 each character probably show 320x200 resolution for
> text only.  All Apple I,II,II+ and later have no way to work with text that
> looks like 320x200 resolution.

The monochrome resolution of the Apple II video output is 280x192.  In
text mode, each character is formed from a grid which is 7 pixels wide
and 8 pixels high.  280/7=40, and 192/8=24, so text mode is 40x24.

In text mode, the data in RAM consists of an ASCII character which is
mapped to the appropriate dot pattern through the character generator
ROM.  If you want to change the appearance of the text characters, you
have to replace the ROM.

Lo-res graphics mode (40x48) operates on a similar principle, with each
byte in the text/lo-res screen being used to look up two colour patterns
in the ROM, corresponding to the top and bottom half of each text
character.  You can generate 16 colours in lo-res mode.

In hi-res graphics mode (280x192), software has direct control over each
pixel on the screen, but only when a monochrome monitor is used.  If a
colour monitor is used, certain combinations of pixels within a line
cause colours to appear, which effectively halves the horizontal
resolution to 140.  Any pair of pixels can display four colours: black,
white, and either green/violet or orange/blue, depending on the "colour
shift" bit (bit 7 of each byte).  Strange effects can occur on byte
boundaries when the colour shift is not the same.

If you have a monochrome monitor connected to an Apple II, it is
possible to operate the machine in hi-res graphics mode showing a
simulation of the text screen, with the ability to customize the
character images.  This is a lot slower than the real text screen, since
the computer is having to process eight times as much data to generate
and update the screen image.

The Apple IIe added "double resolution" modes, which increase the
horizontal resolution to 560 pixels.  In text mode, this allows display
of 80 column text (the characters are still 7x8, with half the width and
the same height).  In double lo-res mode, the available resolution is
80x48.  (This mode isn't supported by the IIe firmware, but is on the
IIgs.)  In double hi-res mode, the available resolution is 560x192
(monochrome) or 140x192 (colour - 16 colours are available, with limits
on placement).

The IIgs adds a "super hi-res graphics" mode, which supports 200 rows
and either 320 or 640 pixels per line (settable independently for each
line).  It has a palette scheme for colour selection, allowing each line
to display up to 16 colours from a palette of 4096 (16 shades each for
red, green and blue, mulitplied together).

In 320 mode, each pixel can select from any of the 16 colours for the
line, but in 640 mode, each pixel can only select from 4 of the colours
for the line.  This led to the use of "dithering" techniques, which
effectively increase the number of colours to 16 per line while in 640
mode, but at the cost of halving the resolution (and limiting the choice
of colour combinations), while retaining the full resolution for black
and white (e.g. text).

-- 
David Empson
dempson@actrix.gen.nz