and its clock speed or frequency in hertz. For some CPUs, the clock speed can be variable in response to software needs. In general, larger word sizes and faster clock sizes indicate better performance, but other factors will impact the actual speed. Another distinguishing feature for a console's CPU is the instruction set architecture. The instruction set defines low-level machine code to be sent to the CPU to achieve specific results on the chip. Differences in the instruction set architecture of CPU of consoles of a given generation can make for difficulty in software portability. This had been used by manufacturers to keep software titles exclusive to their platform as one means to compete with others. Consoles prior to the sixth generation typically used chips that the hardware and software developers were most familiar with, but as personal computers stabilized on the x86 architecture, console manufacturers followed suit as to help easily port games between computer and console. Newer CPUs may also feature multiple processing cores, which are also identified in their specification. Multi-core CPUs allow for multithreading and parallel computing in modern games, such as one thread for managing the game's rendering engine, one for the game's physics engine, and another for evaluating the player's input. Graphical processing unit (GPU) The processing unit that performs rendering of data from the CPU to the video output of the console. In the earlier console generations, this was generally limited to simple graphic processing routines, such as bitmapped graphics and manipulation of sprites, all otherwise involving integer mathematics while minimizing the amount of required memory needed to complete these routines, as memo. For example, the Atari 2600 used its own Television Interface Adaptor that handled video and audio, while the Nintendo Entertainment System used the Picture Processing Unit. For consoles, these GPUs were also designed to send the signal in the proper analog formation to a cathode ray television, NTSC (used in Japan and North America) or PAL (mostly used in Europe). These two formats differed by their refresh rates, 60 versus 50 Hertz, and consoles and games that were manufactured for PAL markets used the CPU and GPU at lower frequencies. The introduction of real-time polygonal 3D graphics rendering in the early 1990s—not just an innovation in video games for consoles but in arcade and personal computer games—led to the development of GPUs that were capable of performing the floating-point calculations