CHIP-8

Systems memory map

• 0x000 - 0x1FF: CHIP-8 interpreter (contains font set in emu)
• 0x200 - 0xFFF: Program ROM and work RAM

Details:

• 0x050 - 0x0A0: Used for the built in 4x5 pixel font set (0-F)
• 0xEA0 - 0xEFF: Reserved for the call stack, internal use, and other variables
• 0xF00 - 0xFFF: Reserved for display refresh

Memory

• Most commonly implemented on 4K systems (f.e. Cosmac VIP, Telmac 1800)
• These machines had 4096 (0x1000) memory locations
• Each location with 8 bits (a byte) --> 4096 * 8 = 32768 bits = 4096 byte =
• The CHIP-8 interpreter itself occupies the first 512 bytes of the memory space
• Most programs written for the original system begin at memory location 512 (0x200)
• The uppermost 256 bytes (0xF00-0xFFF) are reserved for display refresh
• 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.

Registers

• 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.

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

• 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 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 (= 2048) and color is monochrome (= black and white)
• 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.

Opcode table

• 35 / each two bytes long and stored big-endian

• Big-endian is an order in which the "big end" (most significant value in the sequence) is stored first (at the lowest storage address).

• Use on of these types: unsigned short unsigned short int uint16_t u_int16_t --> 0 to 65,535

• 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)