Chip 8 instruction set

(all values in Hexadecimal unless stated)

The Chip-8 instruction set runs in 4k of memory (addresses 000 - FFF). Programs start at 200, memory before that containing the chip-8 interpreter on a real 1802 based machine. The screen is 64 x 32 (128 x 64 on superchip) and is monochrome. Their is a sound buzzer

There are 16 primary registers, called V0 - VF. VF is used for carries and borrows and shouldn't really be used as a general purpose register. There is a 12 bit index register called I. There is a program counter and stack pointer, but neither of these are accessible from program code.

There are 2 counters, the sound timer and the delay timer. Both count down at about 60Hz (on Chip8 they count down in threes using the PC's 18.2Hz Clock). When the sound timer is non-zero the buzzer sounds.

This is the Chip-8 Instruction set as I understand it....

NNN is an address
KK is an 8 bit constant
X and Y are two 4 bit constants
Code Assembler Description Notes
00CN scdown N scroll the screen down N lines SuperChip only
00E0 cls clear the screen
00EE rts return from subroutine call
00FB scright scroll screen 4 pixels right SuperChip only
00FC scleft scroll screen 4 pixels left SuperChip only
00FE low disable extended screen mode SuperChip only
00FF high enable extended screen mode (128 x 64) SuperChip only 
1NNN jmp nnn jump to address NNN
2NNN jsr nnn jump to subroutine at address NNN 16 levels maximum
3XRR skeq vx,rr skip next istruction if register VX == constant RR
4XRR skne vx,rr skip next intruction if register VX != constant RR
5XY0 skeq vx,vy skip next instruction if register VX == register VY
6XRR mov vx,rr move constant RR to register VX
7XRR add vx,rr add constant RR to register VX No carry generated
8XY0 mov vx,vy move register VY into VX
8XY1 or vx,vy or register VY with register VX, store result into register VX
8XY2 and vx,vy and register VY with register VX, store result into register VX
8XY3 xor vx,vy exclusive or register VY with register VX, store result into register VX
8XY4 add vx,vy add register VY to VX, store result in register VX, carry stored in register VF
8XY5 sub vx,vy subtract register VY from VX, borrow stored in register VF register VF set to 1 if borrows
8X06 shr vx shift register VX right, bit 0 goes into register VF
8XY7 rsb vx,vy subtract register VX from register VY, result stored in register VX register F set to 1 if borrows
8X0E shl vx shift register VX left, bit 7 stored into register VF
9XY0 skne vx,vy skip next instruction if register VX != register VY
ANNN mvi nnn Load index register (I) with constant NNN
BNNN jmi nnn Jump to address NNN + register V0
CXKK rand vx,kk    register VX = random number AND KK
DXYN sprite vx,vy,n Draw sprite at screen location (register VX,register VY) height N Sprites stored in memory at location in index register (I), maximum 8bits wide. Wraps around the screen. If when drawn, clears a pixel, register VF is set to 1 otherwise it is zero. All drawing is XOR drawing (e.g. it toggles the screen pixels)
dry0 xsprite rx,ry Draws extended sprite at screen location rx,ry Superchip only: As above,but sprite is always 16 x 16.
ek9e skpr k skip if key (register rk) pressed The key is a key number, see the chip-8 documentation
eka1 skup k skip if key (register rk) not pressed
fr07 gdelay vr get delay timer into vr
fr0a key vr wait for for keypress,put key in register vr
fr15 sdelay vr set the delay timer to vr
fr18 ssound vr set the sound timer to vr
fr1e adi vr add register vr to the index register
fr29 font vr point I to the sprite for hexadecimal character in vr Sprite is 5 bytes high
fr30 xfont vr point I to the sprite for hexadecimal character in vr Superchip only: Sprite is 10 bytes high.
fr33 bcd vr store the bcd representation of register vr at location I,I+1,I+2 Doesn't change I
fr55 str v0-vr store registers v0-vr at location I onwards I is incremented to point to the next location on. e.g. I = I + r + 1
fx65 ldr v0-vr load registers v0-vr from location I onwards as above.

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