In DOS, in real mode, the framebuffer was just mapped directly into memory. If you wanted to put something on the screen, you wrote to that memory address (often segment A000). For text modes, you would write ASCII characters straight into the buffer. For bitmap modes, your bytes would be interpreted as colors (or shades of gray, depending on the mode.) It was very exciting when I figured out how to use extended memory to draw high-resolution (640x480) graphics.