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 these 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, 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 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 meant to 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 emulators used these works as a starting point.
Virtual machine description
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 (0x000-0x200), and it is common to store font data there.
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 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 24 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 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.
- 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)
|0NNN||Call||Calls RCA 1802 program at address NNN. Not necessary for most ROMs.|
|00E0||Display||disp_clear()||Clears the screen.|
|00EE||Flow||return;||Returns from a subroutine.|
|1NNN||Flow||goto NNN;||Jumps to address NNN.|
|2NNN||Flow||*(0xNNN)()||Calls subroutine at NNN.|
|3XNN||Cond||if(Vx==NN)||Skips the next instruction if VX equals NN. (Usually the next instruction is a jump to skip a code block)|
|4XNN||Cond||if(Vx!=NN)||Skips the next instruction if VX doesn't equal NN. (Usually the next instruction is a jump to skip a code block)|
|5XY0||Cond||if(Vx==Vy)||Skips the next instruction if VX equals VY. (Usually the next instruction is a jump to skip a code block)|
|6XNN||Const||Vx = NN||Sets VX to NN.|
|7XNN||Const||Vx += NN||Adds NN to VX. (Carry flag is not changed)|
|8XY0||Assign||Vx=Vy||Sets VX to the value of VY.|
|8XY1||BitOp||Vx=Vx|Vy||Sets VX to VX or VY. (Bitwise OR operation)|
|8XY2||BitOp||Vx=Vx&Vy||Sets VX to VX and VY. (Bitwise AND operation)|
|8XY3||BitOp||Vx=Vx^Vy||Sets VX to VX xor VY.|
|8XY4||Math||Vx += Vy||Adds VY to VX. VF is set to 1 when there's a carry, and to 0 when there isn't.|
|8XY5||Math||Vx -= Vy||VY is subtracted from VX. VF is set to 0 when there's a borrow, and 1 when there isn't.|
|8XY6||BitOp||Vx>>=1||Stores the least significant bit of VX in VF and then shifts VX to the right by 1.|
|8XY7||Math||Vx=Vy-Vx||Sets VX to VY minus VX. VF is set to 0 when there's a borrow, and 1 when there isn't.|
|8XYE||BitOp||Vx<<=1||Stores the most significant bit of VX in VF and then shifts VX to the left by 1.|
|9XY0||Cond||if(Vx!=Vy)||Skips the next instruction if VX doesn't equal VY. (Usually the next instruction is a jump to skip a code block)|
|ANNN||MEM||I = NNN||Sets I to the address NNN.|
|BNNN||Flow||PC=V0+NNN||Jumps to the address NNN plus V0.|
|CXNN||Rand||Vx=rand()&NN||Sets VX to the result of a bitwise and operation on a random number (Typically: 0 to 255) and NN.|
|DXYN||Disp||draw(Vx,Vy,N)||Draws a sprite at coordinate (VX, VY) 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 doesn’t 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 doesn’t happen|
|EX9E||KeyOp||if(key()==Vx)||Skips the next instruction if the key stored in VX is pressed. (Usually the next instruction is a jump to skip a code block)|
|EXA1||KeyOp||if(key()!=Vx)||Skips the next instruction if the key stored in VX isn't pressed. (Usually the next instruction is a jump to skip a code block)|
|FX07||Timer||Vx = get_delay()||Sets VX to the value of the delay timer.|
|FX0A||KeyOp||Vx = get_key()||A key press is awaited, and then stored in VX. (Blocking Operation. All instruction halted until next key event)|
|FX15||Timer||delay_timer(Vx)||Sets the delay timer to VX.|
|FX18||Sound||sound_timer(Vx)||Sets the sound timer to VX.|
|FX1E||MEM||I +=Vx||Adds VX to I.|
|FX29||MEM||I=sprite_addr[Vx]||Sets I to the location of the sprite for the character in VX. Characters 0-F (in hexadecimal) are represented by a 4x5 font.|
|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||MEM||reg_dump(Vx,&I)||Stores V0 to VX (including 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.|
|FX65||MEM||reg_load(Vx,&I)||Fills V0 to VX (including VX) with values from memory starting at address I. The offset from I is increased by 1 for each value written, but I itself is left unmodified.|
- "VIPER for RCA VIP owner". Intelligent Machines Journal (InfoWorld). InfoWorld Media Group. 1978-12-11. p. 9. Retrieved 2010-01-30.
- "Cowgod's Chip-8 Technical Reference". devernay.free.fr. Retrieved 2018-08-07.
- "Cowgod's Chip-8 Technical Reference". devernay.free.fr. Retrieved 2018-08-07.
- VF is set to 1 when there is a range overflow (I+VX>0xFFF), and to 0 when there isn't. This is an undocumented feature of the CHIP-8 and used by the Spacefight 2091! game.
- "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.
- 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
- Dream 6800 The popular Dream 6800 Microcomputer featured in Electronics Australia in 1979 ran CHIP-8.
- FPGA SuperChip A Verilog implementation of the SCHIP specification.
- Octo is an Online CHIP-8 IDE, Development System, Compiler/Assembler and Emulator, with a proprietary scripting language
- Cowgod's Chip-8 Technical Reference
- Matt Mikolay CHIP-8 Extensions Reference
- CHIP-8.com CHIP-8 Classic Computer Manual