In Japan, on March 21, 2001, Nintendo released a significant upgrade to the Game Boy line. The Game Boy Advance (also referred to as GBA) featured a 32 bit 16.8 MHz ARM. It included a Z80 processor and a switch activated by inserting a Game Boy or Game Boy Color game into the slot for backward compatibility, and had a larger, higher resolution screen. Controls were slightly modified with the addition of "L" and "R" shoulder buttons. The system was technically likened to the SNES and showed its power with successful ports of SNES titles such as Super Mario World and Super Mario World 2: Yoshi's Island. There were also new titles in popular SNES series, such as Mario Kart Super Circuit and F-Zero: Maximum Velocity, and a variety of original titles. A widely-criticized drawback of the Game Boy Advance is that the screen is not backlit, making viewing difficult in some conditions. The Game Paks for the GBA are roughly half the length of original Game Boy cartridges, and so older Game Paks would stick out of the top of the unit. When playing older games, the GBA provides the option to play the game at the standard equal square resolution of the original screen or the option to stretch it over the wider GBA screen.