OpenEuphoria

Re: Different copying methods

• Posted by Pete Eberlein <xseal at HARBORSIDE.COM> Sep 08, 1997
• 758 views

Ralf Nieuwenhuijsen wrote:
>
> Why the difference...??
> In the ASM, i think it should ask something like:
>     Is the value of the source memory a zero? yes-don't copy no-copy
>             or    (source see-through effect above)(dest s.t. eff.
> beneath)
>     Is the value of the dest memory a zero? yes-copy no-don't copy

Here's what (I think) the code uses:
Normal copy:
  MOVSB  -- move a byte from source index to dest index

See through copy:
  INSB  -- get a byte from the source index
  if the byte read is zero then,
    OUTSB -- send the byte to dest index
  else
    INC EDI -- don't send it, but skip over that spot

Destination see through:
  CMP BYTE PTR [EDI], 0  --compare the byte at dest index to zero
  if it was 0 then
    INC EDI  -- skip over both
    INC ESI
  else
    MOVSB  -- move a byte from source index to dest index

As you can see, the Dest see thru copy has to do an extra check before
it can do anything else.  It has to read the destination memory and then
test it, and that value isn't used again and effectively wasted, unlike
the see through copy where the byte to be tested can be tested and then
used again.  And the normal copy is the fastest, it's just one
instruction and doesn't have to do any tests.

> Is it me not knowing ASM at all... or could this be overcome be an
> optimization ??
> Please Micheal Bolin, tell me why...
> An other request which would make stuff a *bit* faster would be to a new
> command (4) to switch to the bank given in the next byte.
> And what about a command (5) which would activate a wait_retrace?
>
> And as last...
> the wait_retrace code by Pete Eberlein first waits for a wait retrace to
> begin.. and ends.... it doesn't sound very logically to me... can't we
> just only wait until it begins..

The only way to detect if a retrace is occurring by testing a bit read
from a video register (#3DA).  If bit 4 is on, then a the monitor is in
the process of being refreshed from video memory, (a vertical retrace).
If the bit is off, then the monitor is not doing anything, and it is
okay to write to video memory.  There is no easy way to tell where the
monitor is on a retrace.  The only way to know for sure where what the
monitor is doing is to wait for the point where the bit changes from on
to off.  If we only test to see if the bit is off, the monitor could
start a refresh at any time during a write to video memory.  To make
sure that a full length of time is available between refreshes, we must
do the extra check.

The wait_retrace code does this:
-- a retrace could be in any state at this point:
--  occurring, so we just wait for it to stop
--  not ocurring, we don't know when it will start again
  while a retrace is not occurring
  end while
-- a retrace is occurring, or just started
  while a retrace is occuring
  end while
-- a retrace has just ended
-- from here on, we have the full length of time
-- between retraces to write to video memory

See also my post "Re: Graphics mode 19 update rate" on 1 Sept 97.

> we aren't gonna catch up with the video-refresh...

You are right, we can't keep up with the video refreshes,
we just have to make sure that we stay in between them.  You will
probably have to have a second timer so that the number of retraces
missed stays constant.  Otherwise your display rate might vary,
depending on how much processing had to be done between frames, and this
will affect the overall game speed.

> Maybe some1 knows a way to handle the video refreshes manually (just
> start one after we finished drawing :)

There is a way to disable and disable video logic, sortof simulating a
retrace.  But I'm not sure you could do it fast enough to keep the
screen from badly flickering.
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