cpu & register
cycle:
pc:
instruction:
sp:
v:
stack:
I:
CHIP-8 was most commonly implemented on 4K systems, such as the Cosmac VIP and the Telmac 1800. These machines had 4096 (0x1000) memory locations, all of which are 8 bits (a byte) which is where the term CHIP-8 originated. However, the CHIP-8 interpreter itself occupies the first 512 bytes of the memory space on these machines. For this reason, most programs written for the original system begin at memory location 512 (0x200) and do not access any of the memory below the location 512 (0x200). The uppermost 256 bytes (0xF00-0xFFF) are reserved for display refresh, and the 96 bytes below that (0xEA0-0xEFF) were reserved for the call stack, internal use, and other variables.
| Header |
|---|
| Data |
| Opcode | Type | C Pseudo | Explanation |
|---|---|---|---|
| 0NNN | Call | Calls machine code routine (RCA 1802 for COSMAC VIP) at address NNN. Not necessary for most ROMs. | |
| 00E0 | Display | disp_clear() | Clears the screen. |
| 00EE | Flow | return; | Returns from a subroutine. |
| 1NNN | Flow | goto NNN; | Jumps to address NNN. |
| 2NNN | Flow | *(0xNNN)() | Calls subroutine at NNN. |
| 3XNN | Cond | if(Vx==NN) | Skips the next instruction if VX equals NN. (Usually the next instruction is a jump to skip a code block) |
| 4XNN | Cond | if(Vx!=NN) | Skips the next instruction if VX doesn't equal NN. (Usually the next instruction is a jump to skip a code block) |
| 5XY0 | Cond | if(Vx==Vy) | Skips the next instruction if VX equals VY. (Usually the next instruction is a jump to skip a code block) |
| 6XNN | Const | Vx = NN | Sets VX to NN. |
| 7XNN | Const | Vx += NN | Adds NN to VX. (Carry flag is not changed) |
| 8XY0 | Assign | Vx=Vy | Sets VX to the value of VY. |
| 8XY1 | BitOp | Vx=Vx|Vy | Sets VX to VX or VY. (Bitwise OR operation) |
| 8XY2 | BitOp | Vx=Vx&Vy | Sets VX to VX and VY. (Bitwise AND operation) |
| 8XY3[a] | BitOp | Vx=Vx^Vy | Sets VX to VX xor VY. |
| 8XY4 | Math | Vx += Vy | Adds VY to VX. VF is set to 1 when there's a carry, and to 0 when there isn't. |
| 8XY5 | Math | Vx -= Vy | VY is subtracted from VX. VF is set to 0 when there's a borrow, and 1 when there isn't. |
| 8XY6[a] | BitOp | Vx>>=1 | Stores the least significant bit of VX in VF and then shifts VX to the right by 1.[b] |
| 8XY7[a] | Math | Vx=Vy-Vx | Sets VX to VY minus VX. VF is set to 0 when there's a borrow, and 1 when there isn't. |
| 8XYE[a] | BitOp | Vx<<=1 | Stores the most significant bit of VX in VF and then shifts VX to the left by 1.[b] |
| 9XY0 | Cond | if(Vx!=Vy) | Skips the next instruction if VX doesn't equal VY. (Usually the next instruction is a jump to skip a code block) |
| ANNN | MEM | I = NNN | Sets I to the address NNN. |
| BNNN | Flow | PC=V0+NNN | Jumps to the address NNN plus V0. |
| CXNN | Rand | Vx=rand()&NN | Sets VX to the result of a bitwise and operation on a random number (Typically: 0 to 255) and NN. |
| DXYN | Disp | draw(Vx,Vy,N) | Draws a sprite at coordinate (VX, VY) that has a width of 8 pixels and a height of N+1 pixels. Each row of 8 pixels is read as bit-coded 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 | KeyOp | if(key()==Vx) | Skips the next instruction if the key stored in VX is pressed. (Usually the next instruction is a jump to skip a code block) |
| EXA1 | KeyOp | if(key()!=Vx) | Skips the next instruction if the key stored in VX isn't pressed. (Usually the next instruction is a jump to skip a code block) |
| FX07 | Timer | Vx = get_delay() | Sets VX to the value of the delay timer. |
| FX0A | KeyOp | Vx = get_key() | A key press is awaited, and then stored in VX. (Blocking Operation. All instruction halted until next key event) |
| FX15 | Timer | delay_timer(Vx) | Sets the delay timer to VX. |
| FX18 | Sound | sound_timer(Vx) | Sets the sound timer to VX. |
| FX1E | MEM | I +=Vx | Adds VX to I. VF is not affected.[c] |
| FX29 | MEM | I=sprite_addr[Vx] | 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 | BCD | set_BCD(Vx);
*(I+0)=BCD(3); *(I+1)=BCD(2); *(I+2)=BCD(1); |
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 | MEM | reg_dump(Vx,&I) | Stores V0 to VX (including VX) in memory starting at address I. The offset from I is increased by 1 for each value written, but I itself is left unmodified.[d] |
| FX65 | MEM | reg_load(Vx,&I) | Fills V0 to VX (including VX) with values from memory starting at address I. The offset from I is increased by 1 for each value written, but I itself is left unmodified.[d] |