CHIP-8

Image:PONG CHIP8.png|framed|Screenshot of Pong implemented in CHIP-8
CHIP-8 is an interpreted programming language, developed by Joseph Weisbecker on his 1802 microprocessor. It was initially used on the COSMAC VIP and Telmac 1800, which were 8-bit microcomputers made in the mid-1970s.
CHIP-8 was designed to be easy to program for and to use less memory than other programming languages like BASIC.
Interpreters have been made for many devices, such as home computers, microcomputers, graphing calculators, mobile phones, and video game consoles.

Community

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 ran a series of articles on building a computer, the ETI-660, which was also very similar to the VIP. 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 memory locations, all of which are 8 bits 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 and do not access any of the memory below the location 512. The uppermost 256 bytes are reserved for display refresh, and the 96 bytes below that 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, 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 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 ;
VN: One of the 16 available variables. N may be 0 to F ;

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, as that is the most commonly encountered extension set today. Footnotes denote incompatibilities with the original CHIP-8 instruction set from 1978.

Opcode	Type	C Pseudocode	Explanation
			Calls machine code routine at address NNN. Not necessary for most ROMs.
			Clears the screen.
			Returns from a subroutine.
			Jumps to address NNN.
			Calls subroutine at NNN.
			Skips the next instruction if VX equals NN.
			Skips the next instruction if VX does not equal NN.
			Skips the next instruction if VX equals VY.
			Sets VX to NN.
			Adds NN to VX.
			Sets VX to the value of VY.
		Vx \|= Vy	Sets VX to VX or VY..
			Sets VX to VX and VY..
			Sets VX to VX xor VY.
			Adds VY to VX. VF is set to 1 when there's an overflow, and to 0 when there is not.
			VY is subtracted from VX. VF is set to 0 when there's an underflow, and 1 when there is not..
			Shifts VX to the right by 1, then stores the least significant bit of VX prior to the shift into VF.
			Sets VX to VY minus VX. VF is set to 0 when there's an underflow, and 1 when there is not..
			Shifts VX to the left by 1, then sets VF to 1 if the most significant bit of VX prior to that shift was set, or to 0 if it was unset.
			Skips the next instruction if VX does not equal VY..
			Sets I to the address NNN.
			Jumps to the address NNN plus V0.
			Sets VX to the result of a bitwise and operation on a random number and NN.
			Draws a sprite at coordinate that has a width of 8 pixels and a height of N pixels. Each row of 8 pixels is read as bit-coded starting from memory location I; I value does not change after the execution of this instruction. As described above, VF is set to 1 if any screen pixels are flipped from set to unset when the sprite is drawn, and to 0 if that does not happen.
			Skips the next instruction if the key stored in VX is pressed.
			Skips the next instruction if the key stored in VX is not pressed.
			Sets VX to the value of the delay timer.
			A key press is awaited, and then stored in VX.
			Sets the delay timer to VX.
			Sets the sound timer to VX.
			Adds VX to I. VF is not affected.
			Sets I to the location of the sprite for the character in VX. Characters 0-F are represented by a 4x5 font.
		set_BCD = BCD; = BCD; = BCD;	Stores the binary-coded decimal representation of VX, with the hundreds digit in memory at location in I, the tens digit at location I+1, and the ones digit at location I+2.
			Stores from V0 to VX in memory, starting at address I. The offset from I is increased by 1 for each value written, but I itself is left unmodified.
			Fills from V0 to VX with values from memory, starting at address I. The offset from I is increased by 1 for each value read, but I itself is left unmodified.