Atmel AVR instruction set explained

See main article: article and Atmel AVR.

The Atmel AVR instruction set is the machine language for the Atmel AVR, a modified Harvard architecture 8-bit RISC single chip microcontroller which was developed by Atmel in 1996. The AVR was one of the first microcontroller families to use on-chip flash memory for program storage.

Processor registers

+ Atmel AVR registers
21201918171615141312111009080706050403020100(bit position)
General purpose registers
 R1R0
 R3R2
 R5R4
 R7R6
 R9R8
 R11R10
 R13R12
 R15R14
 R17R16
 R19R18
 R21R20
 R23R22
 R25R24
 R27R26X (pointer)
 R29R28Y (pointer)
 R31R30Z (pointer)
Stack pointer
 SPHSPLStack Pointer
Program counter
PCProgram Counter
Extended memory
 RAMPDExtended direct
 RAMPXExtended X
 RAMPYExtended Y
 RAMPZExtended Z
 EINDExtended indirect
Status register
 ITHSVNZCSREG
There are 32 general-purpose 8-bit registers, R0–R31. All arithmetic and logic operations operate on those registers; only load and store instructions access RAM.

A limited number of instructions operate on 16-bit register pairs. The lower-numbered register of the pair holds the least significant bits and must be even-numbered. The last three register pairs are used as pointer registers for memory addressing. They are known as X (R27:R26), Y (R29:R28) and Z (R31:R30). Postincrement and predecrement addressing modes are supported on all three. Y and Z also support a six-bit positive displacement.

Instructions which allow an immediate value are limited to registers R16–R31 (8-bit operations) or to register pairs R25:R24–R31:R30 (16-bit operations ADIW and SBIW). Some variants of the MUL operation are limited to eight registers, R16 through R23.

Special purpose registers

In addition to these 32 general-purpose registers, the CPU has a few special-purpose registers:

Status register

The status register bits are:

  1. C Carry flag. This is a borrow flag on subtracts.
  2. Z Zero flag. Set when an arithmetic result is zero, and cleared when it is non-zero.
  3. N Negative flag. Set to a copy of the most significant bit of an arithmetic result.
  4. V Overflow flag. Set in case of two's complement overflow.
  5. S Sign flag. Unique to AVR, this is always N⊕V, and shows the true sign of a comparison.
  6. H Half-carry flag. This is an internal carry from additions and is used to support BCD arithmetic.
  7. T Bit copy. Special bit load and bit store instructions use this bit.
  8. I Interrupt flag. Set when interrupts are enabled.

There are two special cases which exist to facilitate multi-byte arithmetic:

Addressing

The following address spaces are available:

The first 64 I/O registers are accessible through both the I/O and the data address space. They have therefore two different addresses. These are usually written as " " through " ", where the first item is the I/O address and the second, in parentheses, the data address.

The special-purpose CPU registers, with the exception of PC, can be accessed as I/O registers. Some registers (RAMPX, RAMPY) may not be present on machines with less than 64 KiB of addressable memory.

Register I/O address Data address
SREG
SP
EIND
RAMPZ
RAMPY
RAMPX
RAMPD

A typical ATmega memory map may look like:

Data address I/O address Contents
Registers R0 – R31
I/O registers (bit-addressable)
I/O registers (not bit-addressable)
Extended I/O registers (memory-mapped I/O only)
Internal SRAM

where RAMEND is the last RAM address. In parts lacking extended I/O the RAM would start at .

Instruction timing

Arithmetic operations work on registers R0–R31 but not directly on RAM and take one clock cycle, except for multiplication and word-wide addition (ADIW and SBIW) which take two cycles.

RAM and I/O space can be accessed only by copying to or from registers. Indirect access (including optional postincrement, predecrement or constant displacement) is possible through registers X, Y, and Z. All accesses to RAM takes two clock cycles. Moving between registers and I/O is one cycle. Moving eight or sixteen bit data between registers or constant to register is also one cycle. Reading program memory (LPM) takes three cycles.

Instruction list

Instructions are one 16-bit long word, save for those including a 16-bit or 22-bit address, which take two words.

There are two types of conditional branches: jumps to address and skips. Conditional branches (BRxx) can test an ALU flag and jump to specified address. Skips (SBxx) test an arbitrary bit in a register or I/O and skip the next instruction if the test was true.

In the following:

AVR instruction set
ArithmeticBit & OthersTransferJump
valign=top rowspan=5valign=top rowspan=5valign=top rowspan=5valign=top
Call
valign=top
Branch
valign=top

Instruction set inheritance

Not all instructions are implemented in all Atmel AVR controllers. This is the case of the instructions performing multiplications, extended loads/jumps/calls, long jumps, and power control.

The optional instructions may be grouped into three categories:

While higher-end processors tend to have both more capable cores and more memory, the presence of one does not guarantee the presence of the other.

Core CPU instructions

Beginning with the original "classic" core, enhancements are organized into the following levels, each of which includes all the preceding:

  1. The "Classic" core has only the zero-operand form of the LPM instruction, which is equivalent to LPM r0,Z.
  2. "Classic plus" adds the MOVW instruction for moving register pairs, and the more general form of the LPM instruction (LPM Rd,Z and LPM Rd,Z+) which permit an arbitrary destination register and auto-increment of the Z pointer.
  3. "Enhanced" cores add the multiply instructions.
  4. The XMEGA cores do not add new instructions per se, but make some significant changes:
    • The memory map is reorganized, eliminating memory-mapping of the processor register file (so I/O ports begin at RAM address 0) and expanding the I/O port range. Now the first 4K is special function registers, the second 4K is data flash, and normal RAM begins at 8K.
    • It is not necessary to explicitly disable interrupts before adjusting the stack pointer registers (SPL and SPH); any write to SPL automatically disables interrupts for 4 clock cycles to give time for SPH to be updated.
    • Other multi-byte registers are provided with shadow registers to enable atomic read and write. When the lowest-order byte is read, the higher-order bytes are copied to the shadow registers, so reading them later produces a snapshot of the register at the time of the first read. Writes to low-order bytes are buffered until the highest-order byte is written, upon which the entire multi-byte register is updated atomically.
  5. Later XMEGA cores (specifically, the B, C, and AU models such as the ATxmega16A4U, but not the earlier A, D and E models such as the ATxmega16D4) add four atomic read-modify-write instructions: exchange (XCH), load-and-set, load-and-clear, and load-and-toggle. These help coordinate with direct memory access peripherals, notably a USB controller.

Less capable than the "classic" CPU cores are two subsets: the "AVR1" core, and the "AVR tiny". Confusingly, "ATtiny" branded processors have a variety of cores, including AVR1 (ATtiny11, ATtiny28), classic (ATtiny22, ATtiny26), classic+ (ATtiny24) and AVRtiny (ATtiny20, ATtiny40).

The AVR1 subset was not popular and no new models have been introduced since 2000. It omits all RAM except for the 32 registers mapped at address 0–31 and the I/O ports at addresses 32–95. The stack is replaced by a 3-level hardware stack, and the PUSH and POP instructions are deleted. All 16-bit operations are deleted, as are IJMP, ICALL, and all load and store addressing modes except indirect via Z.

A second, more successful attempt to subset the AVR instruction set is the "AVR tiny" core.

The most significant change is that the AVRtiny core omits registers R0–R15. The registers are also not memory-mapped, with I/O ports from 0–63 and general-purpose RAM beginning at address 64. The 16-bit arithmetic operations (ADIW, SBIW) are omitted, as are the load/store with displacement addressing modes (Y+d, Z+d), but the predecrement and postincrement addressing modes are retained. The LPM instruction is omitted; instead program ROM is mapped to the data address space and may be accessed with normal load instructions.

Finally, the AVRtiny core deletes the 2-word LDS and STS instructions for direct RAM addressing, and instead uses the opcode space previously assigned to the load/store with displacement instructions for new 1-word LDS and STS instructions which can access the first 128 locations of general-purpose RAM, addresses 0x40 to 0xBF. (The IN and OUT instructions provide direct access to I/O space from 0 to 0x3F.)

Memory addressing instructions

The smallest cores have ≤256 bytes of data address space (meaning ≤128 bytes of RAM after I/O ports and other reserved addresses are removed) and ≤8192 bytes (8 KiB) of program ROM. These have only an 8-bit stack pointer (in SPL), and only support the 12-bit relative jump/call instructions RJMP/RCALL. (Because the AVR program counter counts 16-bit words, not bytes, a 12-bit offset is sufficient to address 213 bytes of ROM.)

Additional memory addressing capabilities are present as required to access available resources:

  1. Models with >256 bytes of data address space (≥256 bytes of RAM) have a 16-bit stack pointer, with the high half in the SPH register.
  2. Models with >8 KiB of ROM add the 2-word (22-bit) JUMP and CALL instructions. (Some early models suffer an erratum if a skip instruction is followed by a 2-word instruction.)
  3. Models with >64 KiB of ROM add the ELPM instruction and corresponding RAMPZ register. LPM instructions zero-extend the ROM address in Z; ELPM instructions prepend the RAMPZ register for high bits. This is not the same thing as the more general LPM instruction; there exist "classic" models with only the zero-operand form of ELPM (ATmega103 and at43usb320). When auto-increment is available (most models), it updates the entire 24-bit address including RAMPZ.
  4. (Rare) models with >128 KiB of ROM have a 3-byte program counter. Subroutine calls and returns use an additional byte of stack space, there is a new EIND register to provide additional high bits for indirect jumps and calls, and there are new extended instructions EIJMP and EICALL which use EIND:Z as the destination address. (The previous IJMP and ICALL instructions use zero-extended Z.)
  5. (Rare) models with >64 KiB of RAM address space extend the 16-bit RAM addressing limits with RAMPX, RAMPY, RAMPZ and RAMPD registers. These provide additional high bits for addressing modes which use the X, Y, or Z register pairs, respectively, or the direct addressing instructions LDS/STS. Unlike ROM access, there are no distinct "extended" instructions; instead the RAMP registers are used unconditionally.

Optional feature instructions

Three instructions are present only on models which have the corresponding hardware facility

Architectures other than AVR1 are named according to avr-libc conventions.[2]

FamilyMembersArithmeticBranchesTransfersBit-Wise
scope=row Minimal AVR1 Core
scope=row Classic Core up to 8K Program Space ("AVR2")new instructions: new instructions: new instructions:
scope=row AVR2, with MOVW and LPM instructions ("AVR2.5")
  • ATa5272
  • ATtiny13/a
  • ATtiny2313/a
  • ATtiny24/a
  • ATtiny25
  • ATtiny261/a
  • ATtiny4313
  • ATtiny43u
  • ATtiny44/a
  • ATtiny45
  • ATtiny461/a
  • ATtiny48
  • ATtiny828
  • ATtiny84/a
  • ATtiny85
  • ATtiny861/a
  • ATtiny87
  • ATtiny88
new instructions:
    scope=row Classic Core with up to 128K ("AVR3")new instructions:new instructions:
    scope=row Enhanced Core with up to 8K ("AVR4")new instructions:new instructions:
    scope=row Enhanced Core with up to 128K ("AVR5", "AVR5.1")new instruction: new instructions:
    scope=row Enhanced Core with up to 4M ("AVR5" and "AVR6")new instructions:
    scope=row XMEGA Core ("avrxmega" 2-6)ATxmega seriesnew instructions:new instructions (from second revision silicon - AU,B,C parts)
    scope=row Reduced AVRtiny Core ("avrtiny10")(Identical to minimal core, except for reduced CPU register set)(Identical to classic core with up to 8K, except for reduced CPU register set)Identical to classic core with up to 8K, with the following exceptions:(Identical to enhanced core with up to 128K, except for reduced CPU register set)
    Reduced register set is limited to R16 through R31.[1]

    Instruction encoding

    Bit assignments:

    The Atmel AVR uses many split fields, where bits are not contiguous in the instruction word. The load/store with offset instructions are the most extreme example where a 6-bit offset is broken into three pieces.

    Atmel AVR instruction set overview
    width=15pt15 !width=15pt14 !width=15pt13 !width=15pt12width=15pt11 !width=15pt10 !width=15pt9 !width=15pt8width=15pt7 !width=15pt6 !width=15pt5 !width=15pt4width=15pt3 !width=15pt2 !width=15pt1 !width=15pt0 !Instruction-->1
    5
    1
    4
    1
    3
    1
    2
    1
    1
    1
    0

    9

    8

    7

    6

    5

    4

    3

    2

    1

    0
    Instruction
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 NOP
    0 0 0 0 0 0 0 1 D D D D R R R R MOVW Rd,Rr Move register pair
    0 0 0 0 0 0 1 0 d d d d r r r r MULS Rd,Rr
    0 0 0 0 0 0 1 1 0 d d d 0 r r r MULSU Rd,Rr
    0 0 0 0 0 0 1 1 0 d d d 1 r r r FMUL Rd,Rr
    0 0 0 0 0 0 1 1 1 d d d u r r r FMULS(U) Rd,Rr
    0 0 opcode r d d d d d r r r r 2-operand instructions
    0 0 0 c̅y̅ 0 1 r d d d d d r r r r CPC/CP Rd,Rr
    0 0 0 c̅y̅ 1 0 r d d d d d r r r r SBC/SUB Rd,Rr
    0 0 0 cy 1 1 r d d d d d r r r r ADD/ADC Rd,Rr (LSL/ROL Rd when Rd=Rr)
    0 0 0 1 0 0 r d d d d d r r r r CPSE Rd,Rr
    0 0 1 0 0 0 r d d d d d r r r r AND Rd,Rr
    0 0 1 0 0 1 r d d d d d r r r r EOR Rd,Rr
    0 0 1 0 1 0 r d d d d d r r r r OR Rd,Rr
    0 0 1 0 1 1 r d d d d d r r r r MOV Rd,Rr
    0 0 1 1 K K K K d d d d K K K K CPI Rd,K
    0 1 opc K K K K d d d d K K K K Register-immediate operations
    0 1 0 c̅y̅ K K K K d d d d K K K K SBCI/SUBI Rd,K
    0 1 1 0 K K K K d d d d K K K K ORI Rd,K
    SBR Rd,K
    0 1 1 1 K K K K d d d d K K K K ANDI Rd,K
    CBR Rd,K
    1 0 k 0 k k s d d d d d y k k k LDD/STD Rd through Z+k or Y+k
    1 0 0 1 0 0 s d d d d d opcode Load/store operations
    1 0 0 1 0 0 s d d d d d 0 0 0 0 align=left rowspan=2LDS rd,i/STS i,rd
    16-Bit immediate SRAM address i
    1 0 0 1 0 0 s d d d d d y 0 0 1 LD/ST Rd through Z+/Y+
    1 0 0 1 0 0 s d d d d d y 0 1 0 LD/ST Rd through -Z/-Y
    1 0 0 1 0 0 0 d d d d d 0 1 q 0 LPM/ELPM Rd,Z
    1 0 0 1 0 0 0 d d d d d 0 1 q 1 LPM/ELPM Rd,Z+
    1 0 0 1 0 0 1 d d d d d 0 1 0 0 XCH Z,Rd
    1 0 0 1 0 0 1 d d d d d 0 1 0 1 LAS Z,Rd
    1 0 0 1 0 0 1 d d d d d 0 1 1 0 LAC Z,Rd
    1 0 0 1 0 0 1 d d d d d 0 1 1 1 LAT Z,Rd
    1 0 0 1 0 0 s d d d d d 1 1 0 0 LD/ST Rd through X
    1 0 0 1 0 0 s d d d d d 1 1 0 1 LD/ST Rd through X+
    1 0 0 1 0 0 s d d d d d 1 1 1 0 LD/ST Rd through -X
    1 0 0 1 0 0 s d d d d d 1 1 1 1 POP/PUSH Rd
    1 0 0 1 0 1 0 d d d d d 0 opcode One-operand instructions:
    1 0 0 1 0 1 0 d d d d d 0 0 0 0 COM Rd
    1 0 0 1 0 1 0 d d d d d 0 0 0 1 NEG Rd
    1 0 0 1 0 1 0 d d d d d 0 0 1 0 SWAP Rd
    1 0 0 1 0 1 0 d d d d d 0 0 1 1 INC Rd
    1 0 0 1 0 1 0 d d d d d 0 1 0 0 (reserved)
    1 0 0 1 0 1 0 d d d d d 0 1 0 1 ASR Rd
    1 0 0 1 0 1 0 d d d d d 0 1 1 0 LSR Rd
    1 0 0 1 0 1 0 d d d d d 0 1 1 1 ROR Rd
    1 0 0 1 0 1 0 0 b b b 1 0 0 0 SEx/CLx Status register clear/set bit
    1 0 0 1 0 1 0 1 opcode 1 0 0 0 Zero-operand instructions
    1 0 0 1 0 1 0 1 0 0 0 0 1 0 0 0 RET
    1 0 0 1 0 1 0 1 0 0 0 1 1 0 0 0 RETI
    1 0 0 1 0 1 0 1 0 0 1 x 1 0 0 0 (reserved)
    1 0 0 1 0 1 0 1 0 1 x x 1 0 0 0 (reserved)
    1 0 0 1 0 1 0 1 1 0 0 0 1 0 0 0 SLEEP
    1 0 0 1 0 1 0 1 1 0 0 1 1 0 0 0 BREAK
    1 0 0 1 0 1 0 1 1 0 1 0 1 0 0 0 WDR
    1 0 0 1 0 1 0 1 1 0 1 1 1 0 0 0 (reserved)
    1 0 0 1 0 1 0 1 1 1 0 q 1 0 0 0 LPM/ELPM
    1 0 0 1 0 1 0 1 1 1 1 0 1 0 0 0 SPM
    1 0 0 1 0 1 0 1 1 1 1 1 1 0 0 0 SPM Z+
    1 0 0 1 0 1 0 c 0 0 0 e 1 0 0 1 Indirect jump/call to Z or EIND:Z
    1 0 0 1 0 1 0 d d d d d 1 0 1 0 DEC Rd
    1 0 0 1 0 1 0 0 k k k k 1 0 1 1 DES round k
    1 0 0 1 0 1 0 k k k k k 1 1 c k align=left rowspan=2JMP/CALL abs22
    k k k k k k k k k k k k k k k k
    1 0 0 1 0 1 1 0 k k p p k k k k ADIW Rp,uimm6
    1 0 0 1 0 1 1 1 k k p p k k k k SBIW Rp,uimm6
    1 0 0 1 1 0 B 0 a a a a a b b b CBI/SBI a,b (clear/set I/O bit)
    1 0 0 1 1 0 B 1 a a a a a b b b SBIC/SBIS a,b (I/O bit test)
    1 0 0 1 1 1 r d d d d d r r r r MUL, unsigned: R1:R0 = Rr × Rd
    1 0 1 1 s a a d d d d d a a a a IN/OUT to I/O space
    1 1 0 c 12 bit signed offset RJMP/RCALL to PC + simm12
    1 1 1 0 K K K K d d d d K K K K LDI Rd,K
    1 1 1 1 0 7-bit signed offset b b b Conditional branch on status register bit
    1 1 1 1 1 0 s d d d d d 0 b b b BLD/BST register bit to STATUS.T
    1 1 1 1 1 1 B d d d d d 0 b b b SBRC/SBRS skip if register bit equals B
    1 1 1 1 1 x x d d d d d 1 b b b (reserved)

    External links

    Notes and References

    1. Web site: AVR Instruction Set Manual . November 2016 . Atmel . Atmel-0856L.
    2. Web site: Using the GNU tools. AVR Libc Manual. 6 May 2018.