Frame Buffer Memory

PC video systems are memory-mapped, which means that the color of every pixel on your monitor gets stored in a memory location that your PC's microprocessor can directly alter the same way it writes data into memory. Your PC holds a complete image frame in memory. Your video system scans the memory, address by address, to draw an image frame on your monitor screen. The memory that holds a complete image frame is termed a frame buffer.

Because your PC's microprocessor needs direct access to the frame buffer to change the data (and hence pixels or dots) in the image, the memory used by the frame buffer must fit within the addressing range of your PC's microprocessor. In the early years of PCs, IBM reserved several address ranges in upper memory for the frame buffers used by the different video standards it developed. The frame buffer of the VGA system begins immediately after the 640K top boundary of lower memory. The memory assigned to the original monochrome display system and still used in VGA text modes starts 64K higher.

Video systems more recent than VGA often place frame buffers in the protected mode addressing range. Even these still use the VGA frame buffer range for compatibility purposes.

The physical memory of the frame buffer is usually separate and distinct from the main memory of your PC. In most PCs, the frame buffer is part of the video board installed in an expansion slot. Even PCs that incorporate their video circuitry on the motherboard separate the frame buffer from main memory (although this design is changing in Unified Memory Architecture systems, noted later in this chapter). Because of this separation and because it cannot be used for running programs, the amount of memory in the frame buffer is usually not counted in totaling up the amount of RAM installed in a PC.