DW      BITMAPUTIL      ; Bitmap Utilities Function Table

  Input:
     Nothing

  Return:
     ES:BX      --> BitMap Utility Table.


The BITMAPUTIL vector loads a pointer into ES:BX, which is the base of a
table defining special case-entry points used for pixel manipulation.
These functions are currently only called by the ellipse emulation routines
that are in the BGI Kernel. If the device driver does not use emulation
for ellipses, this entry does not need to be implemented. This entry was
provided because some hardware requires additional commands to enter and
exit pixel mode, thus adding overhead to the GETPIXEL and SETPIXEL vectors.
This overhead affected the drawing speed of the ellipse emulation routines.
These entry points are provided so that the ellipse emulation routines can
enter pixel mode, and remain in pixel mode for the duration of the ellipse-
rendering process.

The format of the BITMAPUTIL table is as follows:

  DW    offset  GOTOGRAPHIC     ; Enter pixel mode on the graphics hardware
  DW    offset  EXITGRAPHIC     ; Leave pixel mode on the graphics hardware
  DW    offset  PUTPIXEL        ; Write a pixel to the graphics hardware
  DW    offset  GETPIXEL        ; Read a pixel from the graphics hardware
  DW    offset  GETPIXBYTE      ; Return a word containing the pixel depth
  DW    offset  SET_DRAW_PAGE   ; Select page in which to draw primitives
  DW    offset  SET_VISUAL_PAGE ; Set the page to be displayed
  DW    offset  SET_WRITE_MODE  ; XOR Line Drawing Control

The parameters of these functions are as follows:

        GOTOGRAPHIC     ; Enter pixel mode on the graphics hardware
        This function is used to enter the special Pixel Graphics mode.

        EXITGRAPHIC     ; Leave pixel mode on the graphics hardware
        This function is used to leave the special Pixel Graphics mode.

        PUTPIXEL        ; Write a pixel to the graphics hardware
        This function has the same format as the PUTPIXEL entry described
        above.

        GETPIXEL        ; Read a pixel from the graphics hardware
        This function has the same format as the GETPIXEL entry described
        above.

        GETPIXBYTE      ; Return a word containing the pixel depth
        This function returns the number of bits per pixel (color depth) of
        the graphics hardware in the AX register.

        SET_DRAW_PAGE   ; Select alternate output graphics pages (if any)
        This function take the desired page number in the AL register and
        selects alternate graphics pages for output of graphics primitives.

        SET_VISUAL_PAGE ; Select the visible alternate graphics pages (if any)
        This function take the desired page number in the AL register and
        selects alternate graphics for displaying on the screen.

        SET_WRITE_MODE  ; XOR Line drawing mode control. XOR Mode is selected
        if the value in AX is one, and disabled if the value in AX is zero.