ChipJS

This is a description of what ChipJS is and various notes on its development.

CHIP-8 is an old game development language from the 1970s, based entirely on hexadecimal instructions known as "opcodes" to manipulate data in memory. ChipJS is an implementation of a CHIP-8-like machine and interpreter, kept entirely separate from any sort of opinion on graphics rendering (libraries, etc.).

Components

A typical CHIP-8 machine has memory structure as such:

[...] a 64x32 pixel monochrome display, a little less than 4k of shared program/data space (some of the VIP's low memory is reserved for the interpreter), a 16-level return stack, 16 general-purpose 8-bit registers (the 16th is used as a status flag by some instructions), a 16-button hex keypad (with a really goofy layout) and a "buzzer" which can make some unspecified noise.

The CHIP-8 language also has particular behavior:

The thirty-one instructions comprising the original CHIP-8 instruction set provide utilities for primitive audio and monochrome video output, user input, and data manipulation. CHIP-8 enjoyed relative success during the late 1970s and early 1980s as a popular language for the development of simple video games, and even spawned a multitude of dialects providing additional features.

ChipJS will emulate a CHIP-8 machine and the CHIP-8 language via state and behavior, which implies object-oriented programming. Therefore, it is necessary to elaborate what ChipJS is and what ChipJS does.

• ChipJS has:
  • 4096 bytes of RAM
    • in classic machines, addresses 0x000 through 0x1FF are taken up by the language interpreter, but that will not be the case in ChipJS
    • regardless, ChipJS will assume all programs begin at 0x200 in memory
  • 16 8-bit registers, V0 through VF
    • hold numbers between 0 and 255, or 0x00 and 0xFF
    • VF is commonly used as a "status" flag
  • a 12-bit address register, I
    • used for keeping track of an address in memory
    • e.g. 0x200, 0x3FF
  • a 16-level return stack
    • essentially a stack of 12-bit addresses like I
    • allows for subroutines
  • a program counter
    • keeps track of where in memory the program is currently executing
  • a pixel display, implemented as a 2D array
    • 64x32 pixels, potentially other sizes as well
  • an 8-bit sound timer register
  • an 8-bit delay timer register
• ChipJS does:
  • 35 different commands known as opcodes

Opcode Table (via Mastering CHIP-8)

| Opcode | Operation | | ------ | --------- | | 0NNN | Execute machine language subroutine at address NNN (more or less deprecated) | | 00E0 | Clear the screen | | 00EE | Return from a subroutine | | 1NNN | Jump to address NNN | | 2NNN | Execute subroutine starting at address NNN | | 3XNN | Skip the following instruction if the value of register VX equals NN | | 4XNN | Skip the following instruction if the value of register VX is not equal to NN | | 5XY0 | Skip the following instruction if the value of register VX is equal to the value of register VY | | 6XNN | Store number NN in register VX | | 7XNN | Add the value NN to register VX | | 8XY0 | Store the value of register VY in register VX | | 8XY1 | Set VX to VX OR VY | | 8XY2 | Set VX to VX AND VY | | 8XY3 | Set VX to VX XOR VY | | 8XY4 | Add the value of register VY to register VX; Set VF to 01 if a carry occurs; Set VF to 00 if a carry does not occur | | 8XY5 | Subtract the value of register VY from register VX; Set VF to 00 if a borrow occurs; Set VF to 01 if a borrow does not occur | | 8XY6 | Store the value of register VY shifted right one bit in register VX; Set register VF to the least significant bit prior to the shift | | 8XY7 | Set register VX to the value of VY minus VX; Set VF to 00 if a borrow occurs; Set VF to 01 if a borrow does not occur | | 8XYE | Store the value of register VY shifted left one bit in register VX; Set register VF to the most significant bit prior to the shift | | 9XY0 | Skip the following instruction if the value of register VX is not equal to the value of register VY | | ANNN | Store memory address NNN in register I | | BNNN | Jump to address NNN + V0 | | CXNN | Set VX to a random number with a mask of NN | | DXYN | Draw a sprite at position VX, VY with N bytes of sprite data starting at the address stored in I; Set VF to 01 if any set pixels are changed to unset, and 00 otherwise | | EX9E | Skip the following instruction if the key corresponding to the hex value currently stored in register VX is pressed | | EXA1 | Skip the following instruction if the key corresponding to the hex value currently stored in register VX is not pressed | | FX07 | Store the current value of the delay timer in register VX | | FX0A | Wait for a keypress and store the result in register VX | | FX15 | Set the delay timer to the value of register VX | | FX18 | Set the sound timer to the value of register VX | | FX1E | Add the value stored in register VX to register I | | FX29 | Set I to the memory address of the sprite data corresponding to the hexadecimal digit stored in register VX | | FX33 | Store the binary-coded decimal equivalent of the value stored in register VX at addresses I, I+1, and I+2 | | FX55 | Store the values of registers V0 to VX inclusive in memory starting at address I; I is set to I + X + 1 after operation | | FX65 | Fill registers V0 to VX inclusive with the values stored in memory starting at address I; I is set to I + X + 1 after operation |

Resources

• Mastering CHIP-8
• How to write a CHIP-8 interpreter