DrvCopyBits be called 10000 times and picture is separate into point

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printing

My driver is a virtual printer driver.It used a device managed surface and
implemented DrvCopyBits and other painting functions.Here ,The DrvCopyBits
needs to be paid attention..
The DrvCopyBits function have six intake parameters,thereinto,The fifths
parameter prclDest, the destination rectangle that defines the area to be
modified.The second parameter psoSrc , the source surface,the sizlBitmap of
the SURFOBJ structure default the size of the surface.

The problem that,
In the application ,the follow code is used:
pDC->BitBlt(400,0,200,306,&dcmem,0,0,SRCCOPY);
and in the driver,the drvCopyBits is called.

In the ideal situation,the parameters of DrvCopyBits ,prclDest should be
(400, 0)--(600,306),and sizlBitmap of the psoSrc is (200,306).
But in the fact,receives the parameters is that,prclDest is
(400,0)--(401,1),and sizlBitmap of the psoSrc is (1,1)

Why would like this ?Anybody can help me?
thanks