CHIP8 Emulator/Interpreter

Yet another CHIP8 emulator/interpreter. Written in C++ using SDL2 for graphics and input.

Graphical User Interface

How to run

Linux/Mac

1. Install Conan
2. Install CMake
3. Run sh release.sh
4. Run ./build/bin/chip8 passing the ROM file as argument
   1. Example: ./build/bin/chip8 'roms/invaders.bin'

Features

Frame interpolation

Some games can present flickery graphics, this is due to the Graphic system on CHIP8 consistently XORing the video buffer + no synchronization support. To smooth out this effect the emulator provides double frame interpolation, which can be toggled by pressing SPACE.

Regular CHIP8 flickering vs Frame interpolated CHIP8

Disassembler

The emulator provides a disassembler, which can be accessed by passing --disassemble as argument. This will print the disassembled code to the console and exit.

LOG_VERBOSE mode

You can #define LOG_VERBOSE for extra debug information. Which includes all the opcodes, their arguments and an explanation of what they do in real time, performance impact is negligible.

Architecture

Memory map

+-----------------------+
| 0x0050 - 0x01FF       | - Reserved memory (contains font set in emu)
+-----------------------+
| 0x0200 - 0x1000       | - Program ROM and work RAM
+-----------------------+

Input map

The CHIP-8 has a HEX based keypad (0x0-0xF), which maps to the following key-matrix:

╔═══╦═══╦═══╦═══╗                        ╔═══╦═══╦═══╦═══╗
║ 1 ║ 2 ║ 3 ║ C ║                        ║ 1 ║ 2 ║ 3 ║ 4 ║
╠═══╬═══╬═══╬═══╣                        ╠═══╬═══╬═══╬═══╣
║ 4 ║ 5 ║ 6 ║ D ║                        ║ q ║ w ║ e ║ r ║
╠═══╬═══╬═══╬═══╣  which translates to:  ╠═══╬═══╬═══╬═══╣
║ 7 ║ 8 ║ 9 ║ E ║   (in the keyboard)    ║ a ║ s ║ d ║ f ║
╠═══╬═══╬═══╬═══╣                        ╠═══╬═══╬═══╬═══╣
║ A ║ 0 ║ B ║ F ║                        ║ z ║ x ║ c ║ v ║
╚═══╩═══╩═══╩═══╝                        ╚═══╩═══╩═══╩═══╝

Audio

This emulator uses SDL2 for audio. Generating a sine wave with a frequency of 440Hz for 1/60th of a second. This is the same frequency as the original CHIP-8.

Instruction set

Instruction	Description
0NNN	Not supported
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 VX¹. Set register VF to the least significant bit prior to the shift. VY is unchanged
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 VX¹. Set register VF to the most significant bit prior to the shift. VY is unchanged
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

Roadmap

For when I have spare time:

• Disassembler
• Create a Graphical User Interface using Dear ImGui
  • Load ROM files within the GUI
  • Implement a register view within the GUI
  • Implement a stack view within the GUI
  • Implement a dissasembler view within the GUI
  • Implement a memory view within the GUI
  • Implement a keypad view within the GUI
  • Implement a screen view within the GUI
  • Implement a debug view within the GUI
  • Implement a settings view within the GUI
    • Define a CLOCK hz
    • Define a Frame Rate hz
    • Define the screen size
    • Define the screen scale
    • Define the screen color
    • Define the audio frequency

References

Name	Description
Corax89's - CHIP8 Test room	Used for testing
Zophar's domain - CHIP8 Game room pack	Used for rooms
Austin Morlan's - CHIP8 Article	Used as reference
Matt Mikolay's - CHIP8 Reference	Used as reference
Wikipedia - CHIP8 Page	Used as reference
faizilham's - CHIP8 Article	Inspiration for the Frame interpolation feature
fluentcpp.com - Warning article	Used to disable useless warnings due to macro conditions