CHIP-8-emu

My first attempt with emulation. It was focused in the CHIP-8 Virtual Machine, a 8-bit CPU, originally developed in the 70's by Joseph Weisbecker.

Prerequisites

• installed OpenGL Library.

Building

~$ git clone https://github.com/tonisidneimc/CHIP-8-emu
~$ cd CHIP-8-emu
~$ make

Running

usage: make run [scale] [delay] [ROM]

The default (recommended) values for the scale and delay are 10 and 17, respectively.

Example

~$ make run 10 10 tests/pong2.ch8

CHIP-8 Virtual Machine specs

Memory Map

​ The original CHIP-8 machine has 4096 (0x1000) 8-bit memory locations, of which:

• [0x000 - 0x1FF] - reserved space for the CHIP-8 interpreter, or to store font data, in most modern CHIP-8 implementations.
• [0x200 - 0xE9F] - available memory space to be used by programs.
• [0xEA0 - 0xEFF] - reserved for the call stack, internal use and other variables.
• [0xF00 - 0xFFF] - reserved for display refresh.

Registers

​ CHIP-8 has 16 8-bit data registers (V0 - VF). The VF register is used as a flag for some instructions (carry flag for addition, "no borrow" flag for subtraction and pixel collision flag for the draw instruction). In addition to these 16 registers, the CHIP-8 has an address register, named I, that is 16-bits wide, and is used with several instructions that involve memory operations.

The stack

​ The stack is only used to store return address when subroutines are called. The original CHIP-8 machine uses 48 bytes for up to 12 levels of nesting. In this emulator, it uses 32 bytes for at most 16 levels of nesting.

Timers

​ CHIP-8 has 2 timers. They both count down at 60Hz, until they reach 0.

• delay timer: is used for timing the events of games.
• sound timer: as the name says, it is used for sound effects.

Input

​ Input is done with a hex keyboard that maps 16 keys, ranging from 0 to F. The '8', '4', '6' and '2' keys are typically used for directional input. Three opcodes are used to detect input, waiting for a key press, or not, and then storing it value in one of the data registers.

Graphics

​ Original CHIP-8 display is monochrome and has a resolution of 64x32 pixels. Each sprite drawing to the screen is 8 bits 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.

Opcodes

​ CHIP-8 has 35 opcodes, wich are all two bytes long and stored big-endian. The chip8 opcode table can be consulted at the web.