The NES uses a custom-made Picture Processing Unit (PPU) developed by Ricoh. All variations of the PPU feature 2 kB of video RAM, 256 bytes of on-die "object attribute memory" (OAM) to store the positions, colors, and tile indices of up to 64 sprites on the screen, and 28 bytes of on-die palette RAM to allow selection of background and sprite colors. The console's 2 kB of onboard RAM may be used for tile maps and attributes on the NES board and 8 kB of tile pattern ROM or RAM may be included on a cartridge. The system has an available color palette of 48 colors and 6 grays. Up to 25 simultaneous colors may be used without writing new values mid-frame: a background color, four sets of three tile colors and four sets of three sprite colors. The NES palette is based on NTSC rather than RGB values. A total of 64 sprites may be displayed onscreen at a given time without reloading sprites mid-screen. The standard display resolution of the NES is 256 horizontal pixels by 240 vertical pixels.