Chip-8, 8X and ETI
Definitions
| aaa | 12 bit address |
| kk | 8 bit constant |
| n | 4 bit constant |
| x | Register number Vx (x = 0 to F) |
| y | Register number Vy (y = 0 to F) |
| I | Register I (12 bit) |
| [I] | Memory value of address in register I |
| NH | Bit 0-3 Horizontal Location Bit 4-7, Horizontal width |
| NV | Bit 0-3 Vertical Location Bit 4-7, Vertical height |
| 1..9, A..F | Hexadecimal digits |
Chip-8 Syntax
| Opcode | Mnemonic | Parameter | Definition |
|---|---|---|---|
| Fx1E | ADD | I, Vx | I = I + Vx |
| 7xkk | ADD | Vx, kk | Vx = Vx + kk |
| 8xy4 | ADD | Vx, Vy | Vx = Vx + Vy, VF = carry |
| 8xy2 | AND | Vx, Vy | Vx = Vx AND Vy, VF undefined |
| 2aaa | CALL | aaa | Call subroutine at aaa (16 levels) |
| 00E0 | CLS | Clear display | |
| Dxyn | DRW | Vx, Vy, n | Draw n byte sprite stored at [I] at Vx, Vy. Set VF = collision |
| 1aaa | JP | aaa | Jump to address aaa |
| Baaa | JP | V0, aaa | Jump to address aaa + V0 |
| Fx33 | LD | B, Vx | Store BCD value of Vx in [I], [I+1], [I+2] |
| Fx15 | LD | DT, Vx | Delaytimer = Vx |
| Fx29 | LD | F, Vx | Point I to 5 byte numeric sprite for value in Vx |
| Aaaa | LD | I, aaa | I = aaa |
| Fx18 | LD | ST, Vx | Soundtimer = Vx |
| Fx07 | LD | Vx, DT | Vx = Delaytimer |
| Fx0A | LD | Vx, K | Vx = key, wait for keypress |
| 6xkk | LD | Vx, kk | Vx = kk |
| 8xy0 | LD | Vx, Vy | Vx = Vy, VF undefined |
| Fx65 | LD | Vx, [I] | Read V0 .. Vx from [I] .. [I+x] |
| Fx55 | LD | [I], Vx | Store V0 .. Vx in [I] .. [I+x] |
| 8xy1 | OR | Vx, Vy | Vx = Vx OR Vy, VF undefined |
| 00EE | RETURN | Return from subroutine (16 levels) | |
| Cxkk | RND | Vx , kk | Vx = random AND kk |
| 3xkk | SE | Vx , kk | Skip next instruction if Vx == kk |
| 5xy0 | SE | Vx , Vy | Skip next instruction if Vx == Vy |
| 8xxE | SHL | Vx | Vx = Vx << 1, VF = carry |
| 8xyE | SHL | Vx , Vy | Vx = Vy << 1, VF = carry |
| 8xx6 | SHR | Vx | Vx = Vx >> 1, VF = carry |
| 8xy6 | SHR | Vx , Vy | Vx = Vy >> 1, VF = carry |
| Ex9E | SKP | Vx | Skip next instruction if key Vx down |
| ExA1 | SKNP | Vx | Skip next instruction if key Vx up |
| 4xkk | SNE | Vx, kk | Skip next instruction if Vx != kk |
| 9xy0 | SNE | Vx, Vy | Skip next instruction if Vx != Vy |
| 8xy5 | SUB | Vx, Vy | Vx = Vx - Vy, VF = !borrow |
| 8xy7 | SUBN | Vx, Vy | Vx = Vy - Vx, VF = !borrow |
| 0aaa | SYS | aaa | Call CDP1802 code at aaa |
| 8xy3 | XOR | Vx, Vy | Vx = Vx XOR Vy, VF undefined |
Chip-8X Syntax
All of the instruction from Chip-8, except 'Baaa: JP V0, aaa' are valid for Chip-8X. Chip-8X offers the following additional instructions:
| Opcode | Mnemonic | Parameter | Definition |
|---|---|---|---|
| 5xy1 | ADD8 | Vx, Vy | Vx = Vx + Vy (hex digits 00 to 77, mainly for manipulating the NH and NV parameters) |
| 02A0 | CLR | B BACK |
Step background 1 colour (-> blue, -> black, -> green, -> red ->) |
| Bxy0 | CLR CLRL |
Vx, Vy Vx/Vz, Vy |
Set Vy colour @ Vx (NH), Vz (Vz has to be V(x+1)) (NV) |
| Bxyn | CLR CLRH |
Vx, Vy, n Vx/Vz, Vy, n |
Set Vy colour @ Vx, Vz (Vz has to be V(x+1)) byte, n bytes vertically, n != 0 |
| FxFB | INP1 | Vx | Input port 1 to Vx (waits for EF4=1) |
| FxF8 | OUT3 | Vx | Vx to output port 3 (used for simple sound) |
| ExF2 | SKP2 | Vx | Skip next instruction if key Vx on keypad 2 is down |
| ExF5 | SKNP2 | Vx | Skip next instruction if key Vx on keypad 2 is up |
ETI-660 Chip-8 Syntax
All of the instruction from Chip-8 are valid for the TI-660 Chip-8 variant. ETI-660 Chip-8 offers the following additional instruction:
| Opcode | Mnemonic | Parameter | Definition |
|---|---|---|---|
| Fx00 | LD | P, Vx | Set the pitch (frequency) of the tone generator (beeper) to Vx |