FLICKTTY : 80 COLUMNS VIC SCREEN DRIVER FOR LUNIX
-------------------------------------------------
Usage: sh0.exe ! flicktty [-0..7]
Using 80 columns with a C64 has many difficulties. Since we have the
VIC resolution of 320x200, we are limited to 4 pixels wide characters
to be shown in the 80 columns text screen. This difficulty still exists
with the 'flicktty' but not the memory and speed disadvantages, which
conventional 80-columns drivers has.
Current implementation of 'flicktty' for lunix has the throughput of
5250 characters per second. This speed is practical characteristic speed.
It is measured and calculated concerning the bad raster lines and smooth
scroll (one line up per one scroll). Using a rather optimized tricks can
make it possible to go up to 8000 cps theoretically. This would make the
connections at 57600bps flawless.
VIC memory requirement of 'flicktty' is only 3328 bytes. Two kilobytes
are used to hold partial screen data, and one kilobytes hold the font
definitions. One page of memory is for the sprites. Only one sprite is
used for the moment, which is the cursor. But the driver routine eats
seven pages of system memory, I was so lazy that I didn't deallocate the
initialization routines, which are take approximately two pages.
The idea behind the 'flicktty' is simple: Illusion. A raster interrupt
routine switches and shifts the text screen memories at each screen
refresh. This make two half screens to complement each other by using a
feature of our brains and CRTs. If your CRT has slow phosphorus and if
you have chose low contrast and intense color with proper color patterns.
Your brain cannot sense the flickker. Here the colors make important
contributions to this illusion. Our eyes have mainly two groups of sensors.
One group senses only the brightness and brain use these sensors for
shape detection. Other group gives the color information, and they are
not as fast as the brightness sensors, especially if you are lack of
vitamin A.
Technique of 'flicktty' is simple either. For the LUnix implementation
I used $a300-$afff area. With defining the first screen as the one at the
$a400-$a7e7 and the second screen at $ac00-$afe7. I initalized the screens
by using these instructions:
lda$dd00
and#$fe
sta$dd00 ; selection of $8000-$bfff for VIC display
lda#$10
sta$d011 ; 24 rows
lda#$9a
sta$d018 ; screen 1
lda#$c8
sta$d016 ; no shift
And at each raster interrupt I used these instructions to swap the screens:
lda$d018
eor#$20
sta$d018 ; display other screen
lda$d016
eor#$04
sta$d016 ; shift 4 pixels right or left
Now, together with the font, the idea is capable of showing 80 columns
wide screen. The font consists of 128 character definitions. Characters
are defined only in the most significant nybles (left four bits of a
byte).
To write something on the screen we interleave the text properly. For
example, printing 'hello world' has this representation:
odd numbered characters go to the first screen:
$a400 : 08 0c 0f 17 12 04 20 20 : hllwrd
and even numbered ones go to the second screen:
$ac00 : 05 0c 20 0f 0c 20 20 20 : el ol
Then at each 1/50 seconds one of the screens is displayed with proper
shifting:
------------------
1/50 : Ih l l w r d I
I I
I I
------------------
------------------
2/50 : I e l o l I
I I
I I
------------------
.
.....and so on,
Of course this swapping cause a flicker on the screen which is somehow
annoying. But it can be reduced by several ways:
1. Mount a quality screen filter to your monitor.
2. Keep the contrast adjust of your monitor as low as possible.
3. Keep the color intensity as high as possible.
4. Adjust the brightness inbetween low and middle.
5. Choose proper colors (you have only 8 choices with 'flicktty')
6. Wear sunglasses (in that case increase the contrast adjust)
7. Take no more vitamin A ;-)
As for the usage and implementation of 'flicktty', I can say that it
is simple and limited. It's simple because it only directs its input
to the screen. For example, these usages are possible:
sh0.exe ! flicktty
cat <unix-text-file> ! unix2cbm ! flicktty
ed ! flicktty
The implementation is limited because the screen driver mimics only the
tty device. That is, no movement up or down, no color changes, and no more
reverse characters.
But it has some special character codes to refresh the color ram.
For instance pressing control-1 refreshs it.
Upon exit (which is the situation of no input) it recovers the old
screen.
Current implementation for LUnix has also other drawbacks. For example
there are screen flushes when there is a access to disk-drive. Also if
an application has long atomic processes, it also cause flushes. The
scroller within the 'flicktty' has also a long atomic area. which reduces
the multitasking a little bit. But this is not a big problem because it
can scroll 95 lines in a second. And finally it gives you only 24 rows
of text. Also note that when you scroll long texts you cannot follow it.
This is caused from the idea, not the code. You simple see the unscrolled
part for the 1/50th of a second.
Most of these drawbacks can be avoided for later releases of LUnix.
They are caused from the resource conflicts, therefore they are easy to
be corrected. But as for the last one, simply there is no solution for it.
Actually there is one similar to FLI routine, but eats 63% of CPU time.
For the color patterns you only have 8 different selections. You can
select one of them like this:
sh0.exe ! flicktty -3
instead of 3 you can enter a number 0 to 7, default is 0.
the numbers has these color patterns:
number font color background color
------- --------------- ----------------
0 yellow light-blue
1 light-green light-blue
2 yellow purple
3 light-green purple
4 yellow green
5 light-blue blue
6 green blue
7 cyan blue
Note that border color same as the background color. Furthermore, the font
colors are not the same as I give above, they are mixed colors caused by
the flickering.
In conclusion it can be said that the use of 'flicktty' is the only
viable solution to 80 columns problem, for limited memory and slow C64
machines.
Ilker Ficicilar
filker-at-newton.physics.metu.edu.tr
-----------end of the flicktty documentation------------
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--------end of uuencoded flicktty--------------