ST Demos (1)

See it? See that huge pile of  old knackered disks lying in the corner of your room? Just over there near old those expensive boxed games you played through after a week?  I  bet  you  a  fortune that if you ever owned an ST, you kept all those old DS disks - because they have demos on them (or Maggie). No-one ever seems to format (good) old demo disks - because they were free, unlike  all those games we played. Probably. So here comes Maggie to  answer  all  those useless questions: How the hell did they do that on an ST?

Eight megahertz was never enough to really do all those things you saw in demos: they were all merely illusions... you *thought* you saw huge amounts of  screen  data  being  shifted,  but  most  times  it  was a combination of clever tricks  and  fiddling with little-known hardware tricks. Parallax scrolling in the  Cuddly  demos?  An illusion! So sit back, read on, and see how they did it...

N.B. Anyone wishing to know how a  specific effect or demo worked, let us here at Maggie know (demo name/  screen name) and we'll publish the secrets (and source code  if  possible)  in  a  further article. Amiga demos don't count: their hardware was designed for cheating!

Part One

(Borders and Scrolling, ColourShocks)

1.1. Borders

The staple of demos was the ability  to do what nearly all games could not. Therefore,  opening  borders  was  the  staple  diet  of  a demo. Unfortunately this process was  nowhere  near  as  easy  as it sounds: therefore we'll need to introduce you to some theory as we go along... you have been warned!

1.1.1. The Bottom Border

The bottom was the first  to  go:  the  first  demo  I remember with a removed bottom border was The Exceptions' BIG Demo (although I believe they did a slideshow  demo  before  this)  The  method is surprisingly simple...

Unlike the imfamous reply to  a  reader's  question  in ST Format, the method of opening borders is NOT to  switch the whole system into 60Hz NTSC mode to get a bigger screen (!!) In fact, the principle of border removal is the same in all  cases:  at  certain points, the video chip would perform certain checks,  a  bit  like  a  piece  of software, to determine whether phases of its operation should change (i.e. starting or stopping the screen display.)

At the bottom of a normal screen, after the final line becomes border, before the next line appears, the  video  processor checks that the ST was set to 50Hz (PAL) mode.  If  it  was, then the processor output no more screen data. By quickly flicking  into  60Hz while no screen data is displayed, then switching  back  to  50Hz,  the processor is fooled into displaying more screen data until  it reached the point where all display is automatically cut off (38 lines later, I seem to remember.)

Voila! No bottom border. To see how programmers determined exactly how to write a program that switched  50Hz/60Hz at exactly the right time, see "Screen Synchronisation."

1.1.2. The Top Border

This is slightly  more  difficult,  due  to  the  way that processor interrupts and video display  was  set  up,  but  the principle is the same: switch into 60Hz very briefly,  then back into 50Hz (once only). Early attempts  at  this  were  not  very  reliable  (anyone  see  the Whattaheck Demos?) but more of this later.

1.1.3. Side Borders

Yuk! Ever wonder why the  Lost  Boys, probably the best "pure" coders of early ST days,  hardly ever did fullscreens? Because it was a total  nightmare,  that's  why.  People went to great lengths to give the  illusion  of  fullscreen effects by using rasters (see later) - I remember a  Level 16 demo as being the best example of this, and the  ICE STe demo reviewed in Maggie 20 also uses a similar trick,  nearly  8 years later. "So Des,
how did they do that?"

Knowing the way  that  a  monitor  displays  graphics  (i.e.  left to right, then next line, etc.)  you  should  be  able  to work out that you need to  repeat  the  process  of  opening  borders  EVERY SCREEN LINE.  Not  only  that,  but   the   precise    times  at  which  the switches  needed  to  be  made   were   much   more exact  than  the processes  for  top  and  bottom   border   opening.  In  fact,  they really needed a  precision  of  4  processor  cycles  -  the speed of execution of the fastest 68000 instruction possible!

The process then involved  synchronising the processor exactly to the screen  display  (see  "Screen Synchronisation" again), then analysing your  code  to  add  up  exactly  how long each instruction takes... therefore programs using conditions (i.e. IF...THEN  type  statements)  were impossible,  and  to  make matters worse, the original execution speed lists published by Motorola were incorrect. Typical.

Side border  removal  was  notoriously   unreliable.   No  sooner  had groups  managed  to  make  one   type  of  removal  possible,  foreign machines  would  fail  to  be  compatible,   and  when  the  STe  came out...  everyone  started  again.  Reliable  border  removal  code was a closely-guarded  secret.  Good  job  we  all  had  ripper  programs, eh?

1.2 Screen Synchronisation and Rasters

Ok, time for a little diagram, then some explanations:

VBL     _________________________________
        |       Non-Visible Time        |  ^
        |ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ|  |
        |       Top Border Area         |  |    (Not to scale,
        |ÿÿÿÿÿÿÿ|ÿÿÿÿÿÿÿÿÿÿÿÿÿÿ|ÿÿÿÿÿÿÿÿ|  |    obviously!)
        |       |/Normal\Screen|        |  |
        |       |\/\/\/\/\/\/\/|        |  |313 scanlines
        |ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ|  |
        |       Bottom Border Area      |  |
        |ÿÿÿÿÿÿÿÿÿÿNon-Vis Timeÿÿÿÿÿÿÿÿÿ|  V

This diagram shows more precisely how  the video system operated. Most readers will have heard  of  the  VBL  (Vertical BLank) interrupt: the processor will stop each time the  video processor restarted sending a frame of screen data from the top  again  (50  times a second on a PAL system),  jump to an address defined  by software, and do whatever the programs in ROM or RAM  want.  Then  it  returns  to what it was doing beforehand. That's what an interrupt  is.  Interrupts are also used to process keyboard/Midi data, disk access,  or internal clocks. In demos such other interrupts were switched  off  because they stole time from raster effects or complex  calculations.  Leaving a keyboard interrupt enabled with rasters, then  moving  the  mouse,  caused the rasters to wobble up and down - a source of much hilarity.

Similar to the  VBL  is  the  HBL  interrupt  (Horizontal BLank) which occurs every time the  video  processor  restarts processing a screen-line (even when in the borders or  past the top/bottom of the screen - video processors don't take a  rest.)  This  was usually switched off: interrupting  the  main  processor  every   line  slows  the  ST  down significantly.

1.2.1. Rasters

 In addition, there was another way  to cause an interrupt, called the Timer B interrupt, which only occurred  when the screen display itself was on (i.e. not  in  the  top  and  bottom  borders.) By changing the background (or other) colours with this interrupt, it was possible for raster effects: more than 16 colours  on screen, for example. This was a bit pass‚ though: but you got  a  hell  of a buzz the first time you got it to work. From then on  ("Wow!  I've bent the limits of my ST!") most people who did it were hooked.

1.2.2. Syncing

Now, back to the theory:  The  time-relation  between the start of the VBL interrupt and the point in time  needed to open borders was fixed, but not accurately enough for side  borders.  A more precise method of being accurately able to know where  the  video processor was when you changed a colour  was  needed...  hence  "syncing."  This  relied on a stunning  coincidence  (not  really):   every   4   pixels  on  screen corresponded (almost) exactly to 4 processor  cycles of the main 68000 processor. In addtion, the 68000  could  read exactly which section of screen data was being  processed  at  any  specific moment... So... By using a piece of code which code  vary how long it waited depending on where the screen was up to (either a Shift-instruction or jumping into a long list of dummy instructions at a variable point) the coder could change a colour at EXACTLY the same  point every frame, or reliably do the necessary switches  to  remove  side  borders.  The  left one went first, the right one later (it needed two switches!)

1.2.3. ColourShock and Plasma Effects

The ability to change background  colours  gave  an added bonus. If we did nothing but continually change  the  background colour (as long as we switched off any interruptions to it) we get a repeating pattern of colour, that we could  overlay  the  other  15  colours over. Later we could distort  the  pattern  by  jumps  of  4  pixels  left/right,  by inserting short paused in  the  code  (the distorting plasma effects.) Later single colours in a picture  e.g. the insides of lettering, were also changed in this way. See  "Plasma"  in  a later article for other variations on this.

1.2.4. Sync Scrolling

A real buzz-word of demos in  the  days  of the ST. Basically this was the ability to hardware scroll  to  an  accuracy  of  2 bytes. But the normal ST only allowed screens laid on  boundaries of 256 bytes - just over one normal screen-line,  which  required  160  bytes. 256*5 bytes came to 1,280 bytes, which was  exactly  8  screen lines, so you could hardware scroll up and down 8 lines  at a time, but this wasn't really good enough, and sideways scrolling was impossible... or was it? Enter Sync-scrolling.

The great Swedish crews Omega  and  The  Carebears take the credit for inventing the new technique. They realised that a screen line with the side borders opened took 230  bytes  (70  bytes  more), while one with only the right border removed took 192 (32 bytes more.) By opening the top border and playing with these numbers (multiples of 256,70 and 32) they managed,

- by opening both borders for 'x'-many lines.... by opening both borders for 'x
          then the right borders for 'y' many.... then the right borders for 'y

.... then adjusting the base screen address  by 256*'z' bytes, you could scroll the screen to multiples of 8 bytes!

Later, more sophisticated forms  of  the  sync-scroller appeared using more types of  border  opening  (e.g.  left  border  only, leaving the processor in 60Hz for a tiny  bit  longer)  that reduced the number of screenlines needed to do this effect  to  around 9-10 lines, and to an accuracy of 2 bytes. Stupid amounts of effort were expended to squeeze that extra free line  of  processor  time,  but  we  were all very sad people then...

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