'===========================================================================
' Subject: WHY DOUBLE BUFFER?                 Date: 08-14-98 (18:00)       
'  Author: Sten Daniel Soersdal               Code: Text                   
'  Origin: prinz@mailexcite.com             Packet: FAQS.ABC
'===========================================================================

        Why Double buffer ?
      -======================-

        Copyright Sten Daniel S›rsdal, 1998


  Some programmers out there have asked me the question 'Why double buffer?'
  And the answer is quite simple. 
  Of course there are alot of advantages with double buffering techniques
  versus non-double buffering.
  Alot of people tend to think that writing to the screen directly
  is faster, because they dont have to copy the buffer to the
  screen afterwards. 
  This is somewhat true, but it has some major disadvantages;

  1. Speed, its slower to manipulate the screen directly, the DRAM refresh
     takes alot of time. This means the onboard screen DRAM
     has to refresh very often. And the DRAM is 9 out of 10 times
     ALOT slower than the RAM you have in your computer.
     On my own computer my DRAM has an access time (yours will vary)
     of about 3.500 kb/second. And my RAM has about 65.000 kb/second
     access time. (Dell 486dx66.)
     This makes every graphics operation alot slower, for example when
     i smooth the screen or do alot of sprite operations.
     Even just drawing directly is slower, because during
     write/read the graphics card has to refresh, and the operation
     stalls. And in an average game or graphics application
     you do alot of reading and writing (mostly writing) to the
     screen memory. So unless you do less than pasting one single
     sprite to the screen, it _will_ be much more efficient to
     draw these in a virtual screen instead.
  
  2. Flicker, because your hardware has to refresh its memory banks
     the user will often see the screen being redrawn.
     And of course the refresh of your monitor, makes slow operations
     look like they where done halfway before they are refresh yet again.
     On a really fast computer this will still be a problem, the time
     wasted isnt your CPU doing slow calculations, its the CPU time wasted
     while accessing screen memory.

  3. Annoying, its much more calming to see a finished result planted
     on your screen instead of seeing it be done.
     I get all nerves when i see a game/routine that i can see
     updates the screen. Its much more calming to the general user
     to see the results instead of the process, this is in the back of
     your mind.


 Disadvantages, it take abit more memory to implement double buffering.
 For instance the memory it requires to have an equally large virtual
 screen memory bank.
 Thats about the only drawback.

  So a little summing up here.
    Create_a_virtual_screen
    Enter_correct_screen_mode
    DO
      Do_your_calculations
      Draw_your_calculations
      Wait_for_vertical_blank
      Copy_virtual_screen_to_real_screen
      Erase_virtual_screen              ; Not always nessessary
    OD

  This pseudo code example will guarantee flickerless results, and
  a speed increase.
  Though most the of the gained speed from drawing to a virtual
  screen will be used to copy the virtual screen to the real screen,
  it _will_ give you a speed increase (unless your copy routine sucks)

Regards

Sten Daniel S›rsdal
  prinz@mailexcite.com

Check out my PB13h library, it has _ultra_ fast mode 13h routines
based around double-buffering (though not nessessary).
The assembly code has been optimized to the fullest, cutting
alot of corners, yet retaining full userfriendlyness. (PB3.2+)


---------------8<----[ TIMINGS.LST ]---->8------------------

Sten Daniel S›rsdal
pRiNz software
xxx-xxx-xxxxxx-xxxx       * edited *

First Meg / Timings 

                         First Meg
                              Speed  PC/XT
                Memory Area   K/Sec  Index
                Memory Cache  32248  62.3
                0000 - 1CFF   18910  36.5
                1D00 - 1DFF   29829  57.6
                1E00 - 9EFF   18881  36.4
                9F00 - 9FFF   23627  45.6
                A000 - AFFF    3442   6.6
                B000 - B7FF   18864  36.4
                B800 - BAFF    2903   5.6
                BB00 - BBFF    3375   6.5
                BC00 - BFFF    2907   5.6
                C000 - C7FF   18808  36.3
                C800 - CFFF    1147   2.2
                D000 - FFFF   18877  36.4
                    HMA       23863  46.1