CHIP-8 was most commonly implemented on 4K systems, such as the Cosmac VIP and the Telemac 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 call stack, internal use, and other variables.
In modern CHIP-8 implementations, there is no need for any of the memory space to be used, but it is common to store font data in the lower 512 bytes (0x000-0x200).
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 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 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.
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
1 2 3 C 4 5 6 D 7 8 9 E A 0 B F
There are a number of differences between the various emulators. No doubt this could all be resolved if anyone had the sources to the original (circa 1977) interpreter. Add Aligned Opcodes Some emulators accept opcodes at odd addresses, and some don’t. Defensive programming should use even-aligned opcodes. (This distribution demands even-aligned opcodes on emulation, and gives a fatal error for odd-aligned opcodes on assembly, but accepts odd-aligned 16 chip8as(5) chip8as(5) opcodes for disassembly.) The I Register The value of the I register after save and restore opcodes is not well defined. Some emulator leaves the I register pointing past the last location referenced, other emulators leave it pointing at the first location referenced. Defensive programming should assume the I register is not meaningful after these opcodes. (This distribution is consistent with Gustafsson’s emulator, which moves the I register past the last location referenced.) The Borrow Flag Many emulators calculate the borrow after the sub and dif opcodes incorrectly. Defensive programming should assume the v15 register is not meaningful after these opcodes. (This distribution is consistent with Gustafsson’s emulator, which performs the operation in more than 8 bits, and sets the v15 register to 1 if any of the high bits of the result are non-zero, otherwise sets it to 0)