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. . _ * .
* / \ *
. . * CHAPTER 2 : PIXELPUSHING .
. * \_/ . .
This part of the book is all about the basics of Atari graphics. To put
something on the screen (a pixel for instance), you need to put your
CPU in a special mode called SuPeRViSoRMoDe.. This allows you machine
(and program) to access all kinds of chips that normally can't be
approached. If you should try to access those nasty chips in the normal
mode, you'll get two bombs on screen. So, here's how you set the Super-
visormode:
clr.l -(sp)
move.w #32,-(sp)
trap #1
addq.l #6,sp
Now a longword that is very important is put in register D0 by the system.
You should back this up somewhere in the memory, because you need it to
return safely to the desktop. It's called the User-stackpointer so you
should put it under a label called something like 'oldsp' or 'old_stack'
or something. Here we go:
move.l d0,oldsp
If you want to exit your program you should use the 'oldsp' value again
and do like this before calling you termination routine:
move.l oldsp,-(sp)
move.w #32,-(sp)
trap #1
addq #6,sp
You should reserve a LONGWORD for 'oldsp' like this:
oldsp: ds.l 1
You probably know that you need something called a screenaddress. This is
simply a 24 bits long number that points to the first pixel on screen. In
order to get the screenaddress you can use a systemroutine. Just type in
this:
move #2,-(sp)
trap #14
addq #2,sp
The screenaddress is returned in d0, so you can now put in an
addressregister with something like:
move.l d0,a0
Now it's time to put a pixel on screen.
move.w #$ffff,(a0)
This makes the first pixel in the screen white. Simply moving to the next
pixel is done with:
move.w #$ffff,(a0)+
But this only goes for the Falcon truecolor mode. This mode is far more
easy to use than the normal ST resolutions. The ST uses a type of screen-
layout known as BITPLANES.. (aaarrrggghh!) You might also know that the
ST works with a pallette of 16 colors. This makes 4 bits for every color.
(If you didn't know that please don't continue to read this..)
A complete ST-LOW screen of 320*200 4bit pixels makes up a total of
320*200*4 bits = 256000 bits = 32000 bytes. Each screenline has 320*4 =
1280 bits = 160 bytes.
Each of those 4-bit values points to a piece of color information the video-
chip can use. Now you probably assume that the screenbuffer just consists of
a bucketload of 4-bit values just put next to each other, like this:
1st pix 2nd pix 3rd pix 4th pix 5th pix 6th pix
| 0110 | 0101 | 0000 | 1100 | 0110 | 1001 | -> etc ->
1st byte 2nd byte 3rd byte
I'm very sorry but it just isn't as logical as this.... :( It would have
been much nicer if Atari had done like the above method, because it's much
handier for 3d and easier to understand for beginners. Here's how the
situation really is:
4 bitplane mode (ST LOW):
1 word
/ \
------- ------- ------- ------- ------- ------- ------- -------
|plane 0|plane 1|plane 2|plane 3|plane 0|plane 1|plane 2|plane 3| etc.
------- ------- ------- ------- ------- ------- ------- -------
A bundle of 4 consequent words make up 16 pixels in the screen. There are
20 of such bundles (or chunks or whatever you want to call them) in one
screenline (20*16 = 320 pixels).
value of 1st pixel
+------------------+-------0110-------+------------------+
| | | |
| 0101101011100100 | 1011100101101110 | 1100001100011010 | 0101010101110101
1st word 2nd word 3rd word 4th word
(This value is 0110 so that's > color 6 < in plain English)
Yikes! That is indeed a whole lot more complicated!! You see the actual
value of each consequent pixel is got out of the consequent bits from
4 consequent words.. Sounds difficult? In order to code well on an ST you
should at least have figured out how this works. I'm gonna give you some
more examples:
value of 2nd pixel
+------------------+-------1101-------+------------------+
| | | |
| 0101101011100100 | 1011100101101110 | 1100001100011010 | 0101010101110101
1st word 2nd word 3rd word 4th word
( > color 13 < )
For the next pixel you just take the bits that are positioned right from
the current bit in each word. Putting all those bits toghether in one 4-bit
number gives the actual code.
When you reach the seventeenth pixel you need to move on to the next 4
words. This is because each of those 4 word blocks hold 16 pixels. Get the
value of the 17th pixel like this:
value of 17th pixel
+------------------+-------0111-------+------------------+
| | | |
| 1101010101010100 | 1000110101110010 | 1011011100101010 | 0111010110100010
5th word! 6th word! 7th word! 8th word!
( > color 7 < )
Now let's skip this confusing theory and go on with some more practical
approach to these bitplanes...
Let's assume that we got our screenaddress in a0 again! In order to make
the first pixel on screen color 0 you do this:
* Clear most right bit of 1st word.
andi.w #%0111111111111111,(a0)+
* Clear most right bit of 2nd word.
andi.w #%0111111111111111,(a0)+
* Clear most right bit of 3rd word.
andi.w #%0111111111111111,(a0)+
* Clear most right bit if 4th word.
andi.w #%0111111111111111,(a0)+
So now, 4 cleared bits make 0000 which happens to be color 0, hurrah!!
But now we want to give the pixel at the coordinates (165, 45) the color
11. Keep in mind that all the screenlines are directly behind eachother.
The upper screenline starts at offset 0, the second at 160, the third at
320, the fourth at 480, etc, etc.
* First adjust the address to the right couple of words.
* Adjust it down 45 lines. (45 times the number of bytes in each line)
adda.l #45*160,a0
* Adjust it to the right 4 word block. 165pixels/16pixels per block=10
* So the pixel is in the 10th block on the line and each block is 8 bytes
* large so, you need to add 10*8.
adda.l #10*8,a0
* Now which pixel to address in this block? If you calculate the remainder
* of the 165/16 division, you've got the answer.
* 165 - (165/10)*10 = 165 - 16*10 = 165 - 160 = 5, or:
* 165 MOD 16 = 5 as some people like to put it.
* Set the 5th right bit in the 1st word.
ori.w #%000010000000000,(a0)+
* Set the 5th right bit in the 2nd word.
ori.w #%000010000000000,(a0)+
* Clear the 5th right bit in the 3rd word.
andi.w #%111101111111111,(a0)+
* Set the 5th right bit in the 4th word.
ori.w #%000010000000000,(a0)+
So, as you can see this is a question of clearing and setting the
appropriate bits in the corresponding couple of words. OK, that's that for
those bits at the moment.
Now, suppose you want to reconfigure all the colors in the pallette so you
don't have those ugly bright 16 desktop colors, but a more atmospheric
pallette.. For this you really need SUPERVISOR-MODE again. Make sure you
put that supervisor routine at the beginning of all your programs!
OK, let's show something about those colorvalues. Everytime the Video-chip
converts 4-bits into a color to put on screen, it looks up the pallette
registers. These contain all sixteen red, green and blue brightnesses for
the colors.
A colorregister is one word with bits for red green and blue.
ST(e) Colorregister:
+----+----+----+----+
|xxxx|Rrrr|Gggg|Bbbb|
+----+----+----+----+
The three r's represent the brightness of red with a value that can range
from 0 to 7. The g's and b's do the same for the colors green and blue.
The x's do nothing and you don't need to mess around with these.
The Capital letters are for STe/Falcon/TT only. They give twice the
precision of brightness for each color. I won't use 'em in here, because
they are kinda confusing.
The registers are located in high memory. At $ffff8240 to $ffff8260 to be
exact. (These are hexa-decimal numbers. If you don't know what they are,
please don't go on!)
Now you want to make all the colors (0 to 15) bright yellow. You just set
the maximum of red and green(green+red makes yellow!). The value of a
register would look like this:
xxxx Rrrr Gggg Bbbb
+----+----+----+----+
|0000|0111|0111|0000|
+----+----+----+----+
If you want to do this with all the registers you do this:
move.l #$ffff8240,a0 * Put address of first color in a0.
move.w #16-1,d7 * Prepare for looping 16 times.
loop: move.w #%0000011101110000,(a0)+ * Make register bright yellow.
dbra d7,loop * Loop 16 times.
Make sure you set supervisor mode first! Now you have a totally yellow
screen. You now see how easy it is to change the color in the 16 color mode.
If you wanted to do this in Falcon True color it would be very easy to code
as well. The principles are a bit different, but it's not a bit harder. It's
only takes the CPU much longer to execute! Since you have no pallette in
True-color you need to adjust the color for every pixel!
But OK, let's explain how a Falcon-16 bit pixel is put together. It's a bit
like the pallette register of the ST(e):
Falcon 16-bit truecolor pixel:
+-----+------+-----+
|rrrrr|gggggg|bbbbb|
+-----+------+-----+
The r's give the value of red from 0 to 31. The g's give the value of green
from 0 to 63. The b's give the value of blue from 0 to 31.
Now for the routine (the screenaddress is in a0):
move.w #64000-1,d7 * Prepare for looping 64000 times.
loop: move.w #%1111111111100000,(a0)+ * Make a pixel yellow.
dbra d7,loop * Loop 64000 times.
Simple, but very SLOW, because you need to do the loop 64000 times for
320*200 resolution!!
Phew! Now that that stuff is done you know some stuff about giving the
screen all kinds of colors for both the Falcon and the ST. With certain
looping structures and combinations of putting pixels you can make all sorts
of graphical effects. But more about that in Chapter 6.
What we're worried about now is the flickering of the screen when you change
something on screen. Let's say you want to change the 16 colors of the ST
when you're displaying a picture. When you do this without synchronisation
you'll get some ugly flickering on screen. This is because when the video-
chip is still transfering picture-signals to your monitor/TV the colors
change in the middle of the screen. Sounds difficult? I'll explain here:
Let's assume your whole screenbuffer is filled with color 3. In the curremt
situation this is blue. You want to change it to red....
Picture 1:
Color 3 is still blue. The Video-chip is halfway through reading the buffer
and so the monitor is halfway through refreshing the picture on screen.
+------------+
|""""""""""""|
|""""""""""""|
|"""""" |
| |
+------------+
" = blue
The current position of where the screen is refreshing is where the "'s end.
Picture 2:
The palette is now changed and color 3 becomes red. The screen is now
updates a bit further and you can see blue bits are still on screen!!
+------------+
|""""""""""""|
|""""""""""""|
|""""""!! |
| |
+------------+
" = blue ! = red
The current position of refreshing is where the !'s end.
Picture 3:
The video-chip finishes reading the screenbuffer and the monitor finishes
refreshing. The upper half of the screen is now blue and the lower half is
red!!
+------------+
|""""""""""""|
|""""""""""""|
|""""""!!!!!!|
|!!!!!!!!!!!!|
+------------+
" = blue ! = red
Now the monitor and video-chip have stopped reading/refreshing for a moment
and went into Vertical Blank. This is a moment when the monitor adjusts the
Cathode Ray Tube to start in the top-left corner again. In this so called
Vertical Blank or VBL nothing is put on screen... Conclusion: if you want
flickerless colorchanging you need to adjust the colors in the VBL!!
OK, you say, but how do I do this in assembly code Mr. Smartypants? Well,
you can easily do this with a systemroutine:
move.w #37,-(sp)
trap #14
addq #2,sp
This simply waits and executes no other instructions untill the VBL is
reached. If you simply put this in your code before you start kicking around
some pixels, you will have no flickering! Yeh!!
This VBL-syncing stuff works fair enough with only a few pixels on screen,
but still you need one extra trick if the drawing of graphics costs quite
alot of time. You remember the funky ASCII representations of our screen
=) ? Well, even if you start drawing when a VBL occurs, the CRT can catch up
with your drawing in the screenbuffer and again you've got flickering.
What to do about this now? '!|: Double bufferinG :|!'
Always draw your pixeldata on a screenbuffer that isn't read by the
videochip. When you've finished drawing, wait until the VBL occurs and then
give the screenbuffer-address to the videochip. When drawing the next frame
for your animation you draw to another screenbuffers (yes, you have TWO
screenbuffers, hence the word "double" in double buffering =)).
The code will now look like this:
*=/\_ Funky '!|Double buffeR|!' examplE _/\====================================
* 0) Initialize screenaddresses.
* The basic ST(fm) must always have it's screenaddress aligned on a 256 byte
* boundary. Keep this in mind when coding for a basic ST!
move.l #screenbuffer+255,d0 * Get bufferaddress+256 in d0.
sub.b d0,d0 * Make it 256 byte aligned.
move.l d0,physical_screen * Store first address.
addi.l #160*200,d0 * Move to next screenaddress.
move.l d0,logical_screen * Store second address.
drawing_loop:
* 1) Draw some stuff on the logical screen.
;
;
* 2) Swap the screenaddresses. i.e. swap the logical and physical addresses.
move.l logical_screen,d0
move.l physical_screen,logical_screen
move.l d0,physical_screen
* 3) Kick in the new physical screen.
lsr.w #8,d0 * Get mid byte in low byte.
move.l d0,$ffff8200.w * Kick in new screenaddress.
* 4) Wait for the VBL...
move.w #37,-(sp)
trap #14
addq #2,sp
bra drawing_loop * And loop once more...
******** DATA MEMORY SECTION ********
DATA
physical_screen:
DS.L 1 * Address of visible screen.
logical_screen:
DS.L 1 * Address of invisible screen.
******** RESERVED MEMORY SECTION ********
BSS
screenbuffer:
DS.B 256 * For 256 byte aligning!
DS.B 160*200 * Reserve one ST-LOW screen.
DS.B 160*200 * Reserve one ST-LOW screen.
*==============================================================================
Well, that's about all for this chapter. I hope you now know the basics of
how you can put something on screen. Here's a summary of what you have seen
in this chapter. I hope you remember them:
Bitplanes: A couple of words(16-bits) in the screenbuffer that
contain the bits of 16 pixels in a very illogical order.
Cathode Ray Tube: The tube in a TV or monitor that spits out electrons
on the phospor-screen so that you can see colors.
Double buffering: Technique used to avoid flickering and stripes on the
screen. Without this, no flickerless animation! Used in
combination with VBL-syncing.
DS: Assembler command used for reserving memory space at an
address (label).
Flickering: Flickering of the picture on screen.
Logical screen: The actual invisible screen.
Pallette-registers: The 16 hardware registers that contain red, green and
blue values for each value that is read from the
bitplanes. They are used for the ST 16 color mode.
Physical screen: The actual visible screen.
Screenaddress: The address of the first pixel or bitplane in the
screenmemory. In the Atari the screenmemory is held
in the main RAM.
Supervisor-mode: A special mode your 680x0 needs to be in access
pallette-registers and other nasty bits.
User-stackpointer: A value that needs to be saved when you called the
supervisor-setting routine. It needs to be given to
the routine that exits supervisor-mode.
Vertical Blank or VBL: A period when the monitor and video-chip have stopped
reading/refreshing and nothing is put on screen.
Video-chip: A chip in your Atari that reads from the screen-
memory/pallette register and converts the digital
color-values into a analog signal that your monitor
understands.
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