VF is set upon pixel collision. The address register, which is named I, is 12 bits wide and is used with several opcodes that involve memory operations. The stack The stack is only used to store return addresses when subroutines are called. The original RCA 1802 version allocated 48 bytes for up to 12 levels of nesting; modern implementations usually have more. Timers CHIP-8 has two timers. They both count down at 60 hertz, until they reach 0. Delay timer: This timer is intended to be used for timing the events of games. Its value can be set and read. Sound timer: This timer is used for sound effects. When its value is nonzero, a beeping sound is made. Input Input is done with a hex keyboard that has 16 keys ranging 0 to F. The '8', '4', '6', and '2' keys are typically used for directional input. Three opcodes are used to detect input. One skips an instruction if a specific key is pressed, while another does the same if a specific key is not pressed. The third waits for a key press, and then stores it in one of the data registers. Graphics and sound Original CHIP-8 Display resolution is 64×32 pixels, and color is monochrome. Graphics are drawn to the screen solely by drawing sprites, which are 8 pixels wide and may be from 1 to 15 pixels in height. Sprite pixels are XOR'd with corresponding screen pixels. In other words, sprite pixels that are set flip the color of the corresponding screen pixel, while unset sprite pixels do nothing. The carry flag (VF) is set to 1 if any screen pixels are flipped from set to unset when a sprite is drawn and set to 0 otherwise. This is used for collision detection. As previously described, a beeping sound is played when the value of the sound timer is nonzero. Opcode table CHIP-8 has 35 opcodes, which are all two bytes long and stored big-endian. The opcodes are listed below, in hexadecimal and with the following symbols: NNN: address NN: 8-bit constant N: 4-bit constant X and Y: 4-bit register identifier PC : Program Counter I : 16bit register (For memory address) (Similar to void pointer); VN: One of the 16 available variables. N may be 0 to F (hexadecimal); There have been many implementations of the CHIP-8 instruction set since 1978. The following specification is