Bitmap Storage in RAM

The BIOS Font Services use a dedicated 16-KB buffer (Bitmap buffer) for the loaded Bitmap data. This buffer is not mapped into the CPU address space. Another resource used is an 7-level, 10-byte wide stack used to maintain information about the loaded bitmap data. This stack resides in the BIOS Extended Data Segment (9000h:BMSTK). A Stack pointer (BMSP) is also maintained in the same area. The map below assumes a standard 1M RAM memory consisting of 2-512 KB chips.

          ------------------

                ....
 
          ------------------
             Bitmap Buffer
          ------------------
             Frame Buffer 
          ------------------
                Bank 9
          ------------------
                Bank 8
          ------------------
                NRAS[1]

In text modes, the bitmap data for the font required by the Display Controller is loaded starting on a 256-word page in the Bitmap buffer. The BIOS video service Int 10h, Fcn 00h loads the required font(s) when the display mode is changed to a text mode. Generally the buffer will contain three font sizes (8x8, 10x11, and 16x12) to support fast zooming.

In graphics modes, the full 16K Byte Bitmap buffer is available for bitmap and special symbol table storage. Any bitmap data present from a prior text mode may also be used.