# 5. AVR Assembly Instruction Set

### Operand Notation

Before diving into the instructions, here are the common operand symbols used:

| Symbol | Description |
| :--- | :--- |
| **Rd** | Destination register (R0-R31). The result of the operation is stored here. |
| **Rr** | Source register (R0-R31). Used as input for the operation. |
| **K** | Constant/Immediate value (8-bit: 0-255 or 0x00-0xFF). |
| **k** | Address constant for SRAM or program memory. |
| **A** | I/O register address (0-63 for IN/OUT, 0-31 for SBI/CBI). |
| **b** | Bit number (0-7) within a register or I/O address. |
| **X, Y, Z** | Pointer registers (X=R27:R26, Y=R29:R28, Z=R31:R30). |

**Note:** Some instructions only work with upper registers (R16-R31), such as `LDI`, `ANDI`, `ORI`, `SUBI`, `SBCI`, and `CPI`.

### A. Data Transfer Instructions

Used to move data between registers or between registers and memory/I/O.

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **LDI** | Rd, K | Load Immediate | `LDI R16, 0xFF` | Loads 8-bit constant K into register Rd (R16-R31 only) |
| **MOV** | Rd, Rr | Move/Copy Register | `MOV R0, R1` | Copies contents of register Rr to Rd |
| **IN** | Rd, A | Input from I/O | `IN R16, PINB` | Reads data from I/O port A to register Rd |
| **OUT** | A, Rr | Output to I/O | `OUT PORTB, R16` | Sends data from register Rr to I/O port A |
| **LDS** | Rd, k | Load from SRAM | `LDS R16, 0x0100` | Loads data from SRAM address k to register Rd |
| **STS** | k, Rr | Store to SRAM | `STS 0x0100, R16` | Stores register Rr contents to SRAM address k |
| **LD** | Rd, X/Y/Z | Load Indirect | `LD R16, X` | Loads data from address pointed by pointer X/Y/Z |
| **ST** | X/Y/Z, Rr | Store Indirect | `ST X, R16` | Stores data to address pointed by pointer X/Y/Z |
| **PUSH** | Rr | Push to Stack | `PUSH R16` | Saves register to stack |
| **POP** | Rd | Pop from Stack | `POP R16` | Retrieves data from stack to register |

### B. Bit Manipulation Instructions (I/O Specific)

These instructions operate on the lower 32 I/O addresses ($00-$1F). Very efficient for changing one bit without affecting other bits.

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **SBI** | A, b | Set Bit in I/O | `SBI DDRB, 5` | Sets bit b in I/O register A to 1 |
| **CBI** | A, b | Clear Bit in I/O | `CBI PORTB, 5` | Clears bit b in I/O register A to 0 |
| **BST** | Rr, b | Bit Store to T | `BST R16, 3` | Copies bit b from register Rr to T flag |
| **BLD** | Rd, b | Bit Load from T | `BLD R17, 5` | Copies T flag to bit b of register Rd |

### C. Arithmetic Instructions

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **ADD** | Rd, Rr | Add | `ADD R1, R2` | Rd = Rd + Rr |
| **ADC** | Rd, Rr | Add with Carry | `ADC R1, R2` | Rd = Rd + Rr + C (Carry flag) |
| **SUB** | Rd, Rr | Subtract | `SUB R16, R17` | Rd = Rd - Rr |
| **SBC** | Rd, Rr | Subtract with Carry | `SBC R16, R17` | Rd = Rd - Rr - C |
| **SUBI** | Rd, K | Subtract Immediate | `SUBI R16, 10` | Rd = Rd - K (R16-R31 only) |
| **SBCI** | Rd, K | Subtract Immediate with Carry | `SBCI R17, 0` | Rd = Rd - K - C |
| **INC** | Rd | Increment | `INC R16` | Rd = Rd + 1 |
| **DEC** | Rd | Decrement | `DEC R16` | Rd = Rd - 1 |
| **MUL** | Rd, Rr | Multiply Unsigned | `MUL R16, R17` | R1:R0 = Rd × Rr (16-bit result) |
| **MULS** | Rd, Rr | Multiply Signed | `MULS R16, R17` | R1:R0 = Rd × Rr (signed) |
| **NEG** | Rd | Negate (Two's Complement) | `NEG R16` | Rd = 0x00 - Rd |

### D. Logic Instructions

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **AND** | Rd, Rr | Logical AND | `AND R1, R2` | Rd = Rd AND Rr |
| **ANDI** | Rd, K | AND Immediate | `ANDI R16, 0x0F` | Rd = Rd AND K (masking) |
| **OR** | Rd, Rr | Logical OR | `OR R1, R2` | Rd = Rd OR Rr |
| **ORI** | Rd, K | OR Immediate | `ORI R16, 0x80` | Rd = Rd OR K |
| **EOR** | Rd, Rr | Exclusive OR | `EOR R16, R17` | Rd = Rd XOR Rr |
| **COM** | Rd | One's Complement | `COM R16` | Rd = 0xFF - Rd (inverts all bits) |
| **CLR** | Rd | Clear Register | `CLR R16` | Rd = 0 (same as EOR Rd, Rd) |
| **SER** | Rd | Set Register | `SER R16` | Rd = 0xFF (R16-R31 only) |

### E. Shift & Rotate Instructions

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **LSL** | Rd | Logical Shift Left | `LSL R16` | Shift left, bit 0 = 0, bit 7 → Carry |
| **LSR** | Rd | Logical Shift Right | `LSR R16` | Shift right, bit 7 = 0, bit 0 → Carry |
| **ROL** | Rd | Rotate Left through Carry | `ROL R16` | Rotate left through Carry flag |
| **ROR** | Rd | Rotate Right through Carry | `ROR R16` | Rotate right through Carry flag |
| **ASR** | Rd | Arithmetic Shift Right | `ASR R16` | Shift right, bit 7 remains (preserve sign) |
| **SWAP** | Rd | Swap Nibbles | `SWAP R16` | Swaps upper and lower 4-bits in register |

### F. Branch & Control Flow Instructions

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **RJMP** | k | Relative Jump | `RJMP loop` | Jump to label k (±2K words) |
| **JMP** | k | Jump | `JMP far_label` | Jump to 22-bit address (all memory) |
| **RCALL** | k | Relative Call | `RCALL delay` | Call subroutine relative to PC |
| **CALL** | k | Call | `CALL far_sub` | Call subroutine at 22-bit address |
| **RET** | - | Return | `RET` | Return from subroutine |
| **RETI** | - | Return from Interrupt | `RETI` | Return from interrupt handler |
| **CP** | Rd, Rr | Compare | `CP R16, R17` | Compare Rd with Rr (updates flags) |
| **CPI** | Rd, K | Compare Immediate | `CPI R16, 5` | Compare Rd with constant K |
| **CPC** | Rd, Rr | Compare with Carry | `CPC R17, R19` | For multi-byte comparison |
| **BREQ** | k | Branch if Equal | `BREQ target` | Jump if Z flag = 1 (result equal) |
| **BRNE** | k | Branch if Not Equal | `BRNE loop` | Jump if Z flag = 0 (result not equal) |
| **BRLO** | k | Branch if Lower | `BRLO less` | Jump if C flag = 1 (unsigned <) |
| **BRSH** | k | Branch if Same or Higher | `BRSH greater` | Jump if C flag = 0 (unsigned ≥) |
| **BRLT** | k | Branch if Less Than | `BRLT neg` | Jump if S flag = 1 (signed <) |
| **BRGE** | k | Branch if Greater or Equal | `BRGE pos` | Jump if S flag = 0 (signed ≥) |

### G. Skip Instructions

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **SBIS** | A, b | Skip if Bit in I/O Set | `SBIS PINB, 0` | Skip next instruction if bit = 1 |
| **SBIC** | A, b | Skip if Bit in I/O Cleared | `SBIC PIND, 2` | Skip next instruction if bit = 0 |
| **SBRS** | Rr, b | Skip if Bit in Register Set | `SBRS R16, 7` | Skip if bit b in register = 1 |
| **SBRC** | Rr, b | Skip if Bit in Register Cleared | `SBRC R16, 0` | Skip if bit b in register = 0 |

### H. Other Instructions

| Mnemonic | Operand | Description | Example | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **NOP** | - | No Operation | `NOP` | Does nothing (1 clock cycle) |
| **SLEEP** | - | Sleep | `SLEEP` | Enters sleep mode (power saving) |
| **WDR** | - | Watchdog Reset | `WDR` | Resets watchdog timer |
| **SBIW** | Rd, K | Subtract Immediate from Word | `SBIW R24, 1` | Subtract K from 16-bit value (R25:R24) |
| **ADIW** | Rd, K | Add Immediate to Word | `ADIW R24, 1` | Add K to 16-bit value |