This implementation of the Chip8 virtual machine currently requires Saphrion's GUI Rebol 3 (Windows only--for now) (http://development.saphirion.com/downloads/) to run. It also needs an internet connection to download the current R3-Gui.r3 file if the file does not exist in the current directory.
- Currently tracing a conversion bug between Rebol's 1-based indexing and Chip8's 0-based indexing
- Draw engine
- Game loading from Games/ Directory
- Scaling
- Opcodes
- Controls (partial implementation - working on a bug in the FX0A instruction to get them fully working)
- Sound (unimplemented - Will implement a Rebol3 extension for SDL through a Red/System extension)
- When new timer system is complete in Rebol 3 switch from current R3-Gui hack
- When final opcodSes are implemented, use vprint-esque optional debugging output
- test
The specs of the Chip8 virtual machine follow (copied and adapted from Wikipedia):
CHIP-8's memory addresses range from 200h to FFFh, making for 3,584 bytes.
The reason for the memory starting at 200h is that on the Cosmac VIP and Telmac 1800, the first 512 bytes are reserved for the interpreter. On those machines, the uppermost 256 bytes (F00h-FFFh on a 4K machine) were reserved for display refresh, and the 96 bytes below that (EA0h-EFFh) were reserved for the call stack, internal use, and the variables.
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.
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.
As previously described, a beeping sound is played when the value of the sound timer is nonzero.
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.
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
Opcode Explanation
0NNN Calls RCA 1802 program at address NNN.
00E0 Clears the screen.
00EE Returns from a subroutine.
1NNN Jumps to address NNN.
2NNN Calls subroutine at NNN.
3XNN Skips the next instruction if VX equals NN.
4XNN Skips the next instruction if VX doesn't equal NN.
5XY0 Skips the next instruction if VX equals VY.
6XNN Sets VX to NN.
7XNN Adds NN to VX.
8XY0 Sets VX to the value of VY.
8XY1 Sets VX to VX or VY.
8XY2 Sets VX to VX and VY.
8XY3 Sets VX to VX xor VY.
8XY4 Adds VY to VX. VF is set to 1 when there's a carry, and to 0 when there isn't.
8XY5 VY is subtracted from VX. VF is set to 0 when there's a borrow, and 1 when there isn't.
8XY6 Shifts VX right by one. VF is set to the value of the least significant bit of VX before the shift.[2]
8XY7 Sets VX to VY minus VX. VF is set to 0 when there's a borrow, and 1 when there isn't.
8XYE Shifts VX left by one. VF is set to the value of the most significant bit of VX before the shift.[2]
9XY0 Skips the next instruction if VX doesn't equal VY.
ANNN Sets I to the address NNN.
BNNN Jumps to the address NNN plus V0.
CXNN Sets VX to a random number and NN.
DXYN Draws a sprite at coordinate (VX, VY) that has a width of 8 pixels and a height of N pixels. Each row of 8 pixels is read as bit-coded (with the most significant bit of each byte displayed on the left) starting from memory location I; I value doesn't change after the execution of this instruction. As described above, VF is set to 1 if any screen pixels are flipped from set to unset when the sprite is drawn, and to 0 if that doesn't happen.
EX9E Skips the next instruction if the key stored in VX is pressed.
EXA1 Skips the next instruction if the key stored in VX isn't pressed.
FX07 Sets VX to the value of the delay timer.
FX0A A key press is awaited, and then stored in VX.
FX15 Sets the delay timer to VX.
FX18 Sets the sound timer to VX.
FX1E Adds VX to I.[3]
FX29 Sets I to the location of the sprite for the character in VX. Characters 0-F (in hexadecimal) are represented by a 4x5 font.
FX33 Stores the Binary-coded decimal representation of VX, with the most significant of three digits at the address in I, the middle digit at I plus 1, and the least significant digit at I plus 2. (In other words, take the decimal representation of VX, place the hundreds digit in memory at location in I, the tens digit at location I+1, and the ones digit at location I+2.)
FX55 Stores V0 to VX in memory starting at address I.[4]
FX65 Fills V0 to VX with values from memory starting at address I.[4]