Scroll register
A hardware register in old graphics chips that slides the visible window over a larger background, so the picture scrolls smoothly without redrawing a single tile.
Early consoles could not afford to redraw the screen every frame. Instead the graphics chip held a background larger than the screen plus a register — the NES PPU's $2005, the C64's VIC-II, the Mega Drive's VDP — pointing at which slice to show. Bumping that register one pixel per frame gave smooth side-scrolling in hardware, almost for free. It was often reset mid-frame by a raster interrupt to hold a status bar still while the playfield scrolled. This register is what made Super Mario Bros. (1985) possible — and the demoscene pushed it into fluid effects far beyond anything the designers intended.