RE: LZ-Adv: vid_mem ques.

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RE: LZ-Adv: vid_mem ques.

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How do you directly access the video memory to create graphics?


  VIDEO_MEM is just an array of 1024 bytes.  Each byte represents 8 =
pixels -- each bit in the byte represents one pixel.  To turn on one =
pixel, you would have to load the byte of the pixel that will be drawn =
into a register (so all the other pixels in that byte won't be =
overwritten).  The specific byte can be attained by zeoring the least =
significant bit of the Y value, then shifting that to the left (which is =
equivilant to dividing by 8 (bits) then multiplying by 16 (bytes on =
every row)), then adding the X value of the pixel to the Y, and adding =
that finally to VIDEO_MEM. This sounds kinda compilcated, but in =
mathematical terms, it is this:


VIDEO_MEM + (Y/8)*16 + X


Once you have this byte, you need to turn on or off the pixel you want =
to.  The easiest way to do this is with a RES or SET instruction, but in =
some cases there's faster ways...  Anyway, you put this changed byte =
back into the above location, and Voila -- the pixel has been changed!!
This seems kind of difficult just to turn on one pixel, but it is =
precisely what the pixel routines do.  There are many ways to take =
shortcuts when you are not turning on individual pixels.  For example, =
if you want to zero the whole screen, just do a loop 1024 times to load =
0 into the bytes at VIDEO_MEM.  This is one of the main reasons why =
assembly programs can be as fast as they are -- shortcuts are taken!
	(Take a look at Daedalus -- heh heh heh  BTW, a new version of this =
should be released soon, with faster speed, higher resolution walls, =
grayscale, and possibly Collision detection.  After this, I will finally =
begin working on the Badguys.)
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