CHIP-8 is an interpreted programming language, developed by Joseph Weisbecker. It was initially used on the COSMAC VIP and Telmac 1800 8-bit microcomputers in the mid-1970s. CHIP-8 programs are run on a CHIP-8 virtual machine. It was made to allow video games to be more easily programmed for said computers.
Roughly twenty years after CHIP-8 was introduced, derived interpreters appeared for some models of graphing calculators (from the late 1980s onward, these handheld devices in many ways have more computing power than most mid-1970s microcomputers for hobbyists).
An active community of users and developers existed in the late 1970s, beginning with ARESCO's "VIPer" newsletter whose first three issues revealed the machine code behind the CHIP-8 interpreter.
There are a number of classic video games ported to CHIP-8, such as Pong, Space Invaders, Tetris, and Pac-Man. There's also a random maze generator available. These programs are reportedly placed in the public domain, and can be easily found on the Internet.
There is a CHIP-8 implementation for almost every platform imaginable, as well as some development tools. Despite this, there are only a small number of games for the CHIP-8.
CHIP-8 has a descendant called SCHIP (Super Chip), introduced by Erik Bryntse. In 1990, a CHIP-8 interpreter called CHIP-48 was made for HP-48 graphing calculators so that games could be programmed more easily. Its extensions to CHIP-8 are what became known as SCHIP. It features a larger resolution and several additional opcodes which make programming easier. If it were not for the development of the CHIP-48 interpreter, CHIP-8 would not be as well known today.
The next most influential developments (which popularized S/CHIP-8 on many other platforms) were David Winter's emulator, disassembler, and extended technical documentation. It laid out a complete list of undocumented opcodes and features, and was distributed across many hobbyist forums. Many of the emulators listed below had these works as a starting point.
Virtual machine description
CHIP-8's memory addresses range from 200h to FFFh, making for 3,584 bytes. The reason for the memory starting at 200h is that on the Cosmac VIP and Telmac 1800, the first 512 bytes are reserved for the interpreter. On those machines, the uppermost 256 bytes (F00h-FFFh on a 4K machine) were reserved for display refresh, and the 96 bytes below that (EA0h-EFFh) were reserved for the call stack, internal use, and the variables.
The address register, which is named I, is 16 bits wide and is used with several opcodes that involve memory operations.
The stack is only used to store return addresses when subroutines are called. The original 1802 version allocated 48 bytes for up to 12 levels of nesting; modern implementations normally have at least 16 levels.
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 is done with a hex keyboard that has 16 keys which range from 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
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 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.
As previously described, a beeping sound is played when the value of the sound timer is nonzero.
CHIP-8 has 35 opcodes, which are all two bytes long. The most significant byte is stored first. 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
|0NNN||Calls RCA 1802 program at address NNN.|
|00E0||Clears the screen.|
|00EE||Returns from a subroutine.|
|1NNN||Jumps to address NNN.|
|2NNN||Calls subroutine at NNN.|
|3XNN||Skips the next instruction if VX equals NN.|
|4XNN||Skips the next instruction if VX doesn't equal NN.|
|5XY0||Skips the next instruction if VX equals VY.|
|6XNN||Sets VX to NN.|
|7XNN||Adds NN to VX.|
|8XY0||Sets VX to the value of VY.|
|8XY1||Sets VX to VX or VY.|
|8XY2||Sets VX to VX and VY.|
|8XY3||Sets VX to VX xor VY.|
|8XY4||Adds VY to VX. VF is set to 1 when there's a carry, and to 0 when there isn't.|
|8XY5||VY is subtracted from VX. VF is set to 0 when there's a borrow, and 1 when there isn't.|
|8XY6||Shifts VX right by one. VF is set to the value of the least significant bit of VX before the shift.|
|8XY7||Sets VX to VY minus VX. VF is set to 0 when there's a borrow, and 1 when there isn't.|
|8XYE||Shifts VX left by one. VF is set to the value of the most significant bit of VX before the shift.|
|9XY0||Skips the next instruction if VX doesn't equal VY.|
|ANNN||Sets I to the address NNN.|
|BNNN||Jumps to the address NNN plus V0.|
|CXNN||Sets VX to a random number and NN.|
|DXYN||Sprites stored in memory at location in index register (I), maximum 8bits wide. Wraps around the screen. If when drawn, clears a pixel, register VF is set to 1 otherwise it is zero. All drawing is XOR drawing (e.g. it toggles the screen pixels)|
|EX9E||Skips the next instruction if the key stored in VX is pressed.|
|EXA1||Skips the next instruction if the key stored in VX isn't pressed.|
|FX07||Sets VX to the value of the delay timer.|
|FX0A||A key press is awaited, and then stored in VX.|
|FX15||Sets the delay timer to VX.|
|FX18||Sets the sound timer to VX.|
|FX1E||Adds VX to I.|
|FX29||Sets I to the location of the sprite for the character in VX. Characters 0-F (in hexadecimal) are represented by a 4x5 font.|
|FX33||Stores the Binary-coded decimal representation of VX, with the most significant of three digits at the address in I, the middle digit at I plus 1, and the least significant digit at I plus 2. (In other words, take the decimal representation of VX, place the hundreds digit in memory at location in I, the tens digit at location I+1, and the ones digit at location I+2.)|
|FX55||Stores V0 to VX in memory starting at address I.|
|FX65||Fills V0 to VX with values from memory starting at address I.|
- "VIPER for RCA VIP owner". Intelligent Machines Journal (InfoWorld) (InfoWorld Media Group). 1978-12-11. p. 9. Retrieved 2010-01-30.
- On the original interpreter, the value of VY is shifted, and the result is stored into VX. On current implementations, Y is ignored.
- VF is set to 1 when range overflow (I+VX>0xFFF), and 0 when there isn't. This is undocumented feature of the Chip-8 and used by Spacefight 2019! game.
- On the original interpreter, when the operation is done, I=I+X+1. On current implementations, I is left unchanged.
- "RCA COSMAC VIP CDP18S711 Instruction Manual," RCA Solid State Division, Somerville, NJ 08776, February 1978. Part VIP-311. pp. 13–18, 35-37.
- BYTE magazine, December 1978, pp. 108–122. "An Easy Programming System," by Joseph Weisbecker. Describes CHIP-8 with specific example of a rocketship and UFO shooting-gallery game.
- Archive of Chip8.com Website dedicated to Chip-8 and related systems. Maintains the most complete collection of Chip-8 programs on the net.
- David Winter's Chip-8 Emulator, utilities and games.
- Let's Emu : Chip-8 Emulator – A list of CHIP-8 and SCHIP emulators.
- BytePusher A minimalist virtual machine inspired by the CHIP-8.
- RCA COSMAC group on Yahoo, with authorized scans of the VIPER magazine.
- OChip8 A CHIP-8 emulator in a browser