CHIP-8

CHIP-8

CHIP-8 is an interpreted programming language, developed by the late 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).

CHIP-8 applications

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.

CHIP-8 today

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.

In 2007, another CHIP8 extension has been introduced, called Mega Chip (or MCHIP). Much like the SCHIP, it allows for a larger resolution (256x192), as well as the use of color graphics and sound. Like Super Chip, the MegaChip extension only adds a handful of new opcodes and is 100% backwards compatible with CHIP-8 and Super Chip. An emulator for Chip8/SuperChip/MegaChip, as well as an updated developers kit is available at the Revival Studios website. Various MegaChip games and demos (including an update of the infamous Blinky game) are available.

Virtual machine description

Memory

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.

Registers

CHIP-8 has 16 8-bit data registers named from V0 to VF. The VF register doubles as a carry flag.

The address register, which is named I, is 16 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 1802 version allocated 48 bytes for up to 12 levels of nesting; modern implementations normally have at least 16 levels.

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 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.

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. They are listed below, in hexadecimal and with the following symbols:

  • NNN: address
  • NN: 8-bit constant
  • N: 4-bit constant
  • X and Y: registers

Opcode Explanation
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 Draws a sprite at coordinate (VX, VY) that has a width of 8 pixels and a height of N pixels. 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 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 at the addresses I, I plus 1, and I plus 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.

Additional Resources

  • "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.
  • http://www.pong-story.com/chip8 David Winter's Chip-8 Emulator, utilities and games.
  • Let's Emu : Chip-8 Emulator – A list of CHIP-8 and SCHIP emulators.

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