CHIP-8

CHIP-8 has been used on a wide range of devices over time; the first community to use CHIP-8 started in the 1970s on microcomputers. They shared extensions and programs in newsletters such as ARESCO's VIPER for COSMAC VIP users or DREAMER for DREAM 6800 users. In the VIPER newsletter, the first three issues detailed the machine code for the CHIP-8 interpreter for the VIP. CHIP-8 applications include original games, demos, as well as recreations of popular games from other systems. Some CHIP-8 applications have been released into the public domain using licenses like the Creative Commons Zero license. ==CHIP-8 extensions and variations==

During the 1970s and 1980s, CHIP-8 users shared CHIP-8 programs, but also changes and extensions to the CHIP-8 interpreter, like in the VIPER magazine for COSMAC VIP. These extensions included CHIP-10 and Hi-Res CHIP-8, which introduced a higher resolution than the standard 64x32, and CHIP-8C and CHIP-8X, which extended the monochrome display capabilities to support limited color, among other features. These extensions were mostly backwards compatible, as they were based on the original interpreter, although some repurposed rarely used opcodes for new instructions. In 1979, Electronics Australia ran a series of articles on building a kit computer similar to the COSMAC VIP, based on the Motorola 6800 architecture. This computer, the DREAM 6800, came with its own version of CHIP-8. A newsletter similar to VIPER, called DREAMER, was used to share CHIP-8 games for this interpreter. In 1981, Electronics Today International (ETI) ran a series of articles on building a computer, the ETI-660, which was also very similar to the VIP (and used the same microprocessor). ETI ran regular ETI-660 and general CHIP-8 columns until 1985. In 1990, a CHIP-8 interpreter called CHIP-48 was made by Andreas Gustafsson for HP-48 graphing calculators. Erik Bryntse later created another interpreter based on CHIP-48, titled "SUPER-CHIP", often shortened to SCHIP or S-CHIP. SCHIP extended the CHIP-8 language with a larger resolution and several additional opcodes meant to make programming easier. David Winter's interpreter, disassembler, and extended technical documentation popularized CHIP-8/SCHIP on other platforms. It laid out a complete list of undocumented opcodes and features and was distributed across hobbyist forums. Many interpreters used these works as a starting point. However, CHIP-48 subtly changed the semantics of a few of the opcodes, and SCHIP continued to use those new semantics in addition to changing other opcodes. Many online resources about CHIP-8 propagate these new semantics, so many modern CHIP-8 games are not backwards compatible with the original CHIP-8 interpreter for the COSMAC VIP, even if they do not specifically use the new SCHIP extensions. Some extensions take opcodes or behavior from multiple extensions, like XO-CHIP which takes some from behavior from SCHIP and CHIP-8E. ==Virtual machine description==

Memory CHIP-8 was most commonly implemented on 4K systems, such as the Cosmac VIP and the Telmac 1800. These machines had 4096 (0x1000) memory locations, all of which are 8 bits (a byte) which is where the term CHIP-8 originated. However, the CHIP-8 interpreter itself occupies the first 512 bytes of the memory space on these machines. For this reason, most programs written for the original system begin at memory location 512 (0x200) and do not access any of the memory below the location 512 (0x200). The uppermost 256 bytes (0xF00-0xFFF) are reserved for display refresh, and the 96 bytes below that (0xEA0-0xEFF) were reserved for the call stack, internal use, and other variables. In modern CHIP-8 implementations, where the interpreter is running natively outside the 4K memory space, there is no need to avoid the lower 512 bytes of memory (0x000-0x1FF), and it is common to store font data there. Registers CHIP-8 has 16 8-bit data registers named V0 to VF. The VF register doubles as a flag for some instructions; thus, it should be avoided. In an addition operation, VF is the carry flag, while in subtraction, it is the "no borrow" flag. In the draw instruction 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. Its value can only be set. 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 The original CHIP-8 display resolution is 64×32 pixels, and its 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 : 12bit 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 based on the SUPER-CHIP specification from 1991 (but without the additional opcodes that provide extended functionality), as that is the most commonly encountered extension set today. Footnotes denote incompatibilities with the original CHIP-8 instruction set from 1978. ==See also==