The PC Vision plus is a framegrabber card.

  base+0  (R/W): Control register
  bit   0  Zoom. If set pixels are doubled when displayed.
	1  Gain Adjust Enable.
	   If clear the Gain will be incremented or decremented
	   (depending on the DIR flag in bit 3) by writes to the Gain and Offset
	   Counter (base+4), if set the Gain is fixed.
	2  Offset Adjust Enable.
	   If clear the Offset Voltage will be incremented or decremented
	   (depending on the DIR flag in bit 3) by writes to the Gain and Offset
	   Counter (base+4), if set the Offset Voltage is fixed.
	3  Adjust Direction Control.
	   If set writes to the Gain and Offset Counter (base+4)
	   will increment the Gain and Offset counters if the appropriate bits
	   (bit 1 and 2) are clear.
	4  Board Select. If set the frame buffer is mapped into PC memory.
      5-7  Page select. Selects which of 8 64k pages are mapped in.    


  base+1  (R/W): Lut Control Register
  bit 0-1  Look-Up Table Mode. Selects one of 4 Look Up Table banks
	   for manipulation.
	     0= Red LUT, 1= Green LUT, 2= Blue LUT, 3= Input LUT.
      2-4  Input Lut Select. Selects one of 8 Input LUTs
      5-7  Output Lut Select. Selects one of 8 LUTs for each color.

  base+2  (R/W): Lut Address Register
  bit 0-7  Offset into the LUT selected by the Lut Control Register (base+1).
	   Reads and writes to the LUT Data Port (base+3) will go to this entry.

  base+3  (R/W): Lut Data Port
  bit 0-7  This port directly accesses the offset selected by the Lut Address
	   Register (base+2) in the LUT selected by the Lut Control Register
	   (base+1).

  base+4  (W): Gain and Offset Counter
  bot 0-7  Each write to this register will increment or decrement (depending on
	   the Direction flag in bit 3 of base+4) the Gain and Offset Voltage
	   Counters if Adjust is enabled (base+4 bit 1 and 2).

  base+5  (R/W): Acquire and Status register
  bit   0  Video Input Select. If set video input is taken from Channel 1,
	   else from Channel 0.
	1  Clock Source Select. If set clock source is a Phase=Locked Loop, 
	   else a Crystal Oscillator.
	2  (R) Odd/Even Status. Odd field displayed if set.
	3  (R) Last Field Status. If clear the PCVISION plus is acquiring
	   the last field of a grab.
	4  (R) Vertical Blank Status.
	       Vertical Blank active if clear (1.4ms).
	5  (R) Horizontal Blank Status.
	       Horizontal blank active if clear (12.7us).
      6-7  Acquire Mode.
	     0: Freeze. Terminate operation. 
	     1: Clear.  Clear video memory.
	     2: Snap.   Acquire a single image.  
	     3: Grab.   Continously acquire images.  
	   When read 0 indicates the system is not busy.

  base+6  (R/W): Pan Register
  base+7  (R/W): Scroll Register
  base+8  (R/W): Memory Access Control Register
  base+9  (R/W): Host Mask Register
  bit 0-7  Each bit set prevents the corresponding bit plane from changes
	   during host access and clear operations.     

  base+Ah (R/W): Video Mask Register
  bit 0-7  Each bit set prevents the corresponding bit plane from changes
	   during acquire operations.     

  base+Bh (R/W): Pixel Buffer Register 

