   With D3DXMATRIX  does it need to be NULLed or cleared after use for reuse?
  With D3DXMATRIX  does it need to be NULLed or cleared after being used for reuse?by used i mean transformed.also whats difference between D3DXMATRIX   and D3DXMATRIX16, like which one should i use generally?thanks, 
  I''m not sure what you mean, but in general, no you don''t have to zero the matrix out to ''reuse'' it.D3DXMATRIXA16 is a 16-byte aligned matrix, which is faster for a lot of math operations.  That''s in the doc, by the way.   I like pie. ;  No, you don''t have to NULL it to re-use it again.  Just pass it to the function that will fill it again, for example D3DXMatrixRotationX().As for D3DXMATRIXA16, it is 16-byte aligned.  This means, when it''s used by D3DX functions, it has improved performance, especially on Pentium 4''s. 