3.19.6 AVR Options

These options are defined for AVR implementations:


Specify the AVR instruction set architecture (ISA) or device type. The default for this option is avr2.

The following AVR devices and ISAs are supported. Note: A complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs file specs-mcu. Only the latter is provided by the compiler according the supported mcus below. The rest is supported by AVR-LibC, or by means of atpack files from the hardware manufacturer.


“Classic” devices with up to 8 KiB of program memory.
mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.


“Classic” devices with up to 8 KiB of program memory and with the MOVW instruction.
mcu = attiny13, attiny13a, attiny24, attiny24a, attiny25, attiny261, attiny261a, attiny2313, attiny2313a, attiny43u, attiny44, attiny44a, attiny45, attiny48, attiny441, attiny461, attiny461a, attiny4313, attiny84, attiny84a, attiny85, attiny87, attiny88, attiny828, attiny841, attiny861, attiny861a, ata5272, ata6616c, at86rf401.


“Classic” devices with 16 KiB up to 64 KiB of program memory.
mcu = at76c711, at43usb355.


“Classic” devices with 128 KiB of program memory.
mcu = atmega103, at43usb320.


“Classic” devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction.
mcu = attiny167, attiny1634, atmega8u2, atmega16u2, atmega32u2, ata5505, ata6617c, ata664251, at90usb82, at90usb162.


“Enhanced” devices with up to 8 KiB of program memory.
mcu = atmega48, atmega48a, atmega48p, atmega48pa, atmega48pb, atmega8, atmega8a, atmega8hva, atmega88, atmega88a, atmega88p, atmega88pa, atmega88pb, atmega8515, atmega8535, ata5795, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.


“Enhanced” devices with 16 KiB up to 64 KiB of program memory.
mcu = atmega16, atmega16a, atmega16hva, atmega16hva2, atmega16hvb, atmega16hvbrevb, atmega16m1, atmega16u4, atmega161, atmega162, atmega163, atmega164a, atmega164p, atmega164pa, atmega165, atmega165a, atmega165p, atmega165pa, atmega168, atmega168a, atmega168p, atmega168pa, atmega168pb, atmega169, atmega169a, atmega169p, atmega169pa, atmega32, atmega32a, atmega32c1, atmega32hvb, atmega32hvbrevb, atmega32m1, atmega32u4, atmega32u6, atmega323, atmega324a, atmega324p, atmega324pa, atmega324pb, atmega325, atmega325a, atmega325p, atmega325pa, atmega328, atmega328p, atmega328pb, atmega329, atmega329a, atmega329p, atmega329pa, atmega3250, atmega3250a, atmega3250p, atmega3250pa, atmega3290, atmega3290a, atmega3290p, atmega3290pa, atmega406, atmega64, atmega64a, atmega64c1, atmega64hve, atmega64hve2, atmega64m1, atmega64rfr2, atmega640, atmega644, atmega644a, atmega644p, atmega644pa, atmega644rfr2, atmega645, atmega645a, atmega645p, atmega649, atmega649a, atmega649p, atmega6450, atmega6450a, atmega6450p, atmega6490, atmega6490a, atmega6490p, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5787, ata5835, ata5700m322, ata5702m322, at90pwm161, at90pwm216, at90pwm316, at90can32, at90can64, at90scr100, at90usb646, at90usb647, at94k, m3000.


“Enhanced” devices with 128 KiB of program memory.
mcu = atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.


“Enhanced” devices with 3-byte PC, i.e. with more than 128 KiB of program memory.
mcu = atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.


“XMEGA” devices with more than 8 KiB and up to 64 KiB of program memory.
mcu = atxmega8e5, atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5, avr64da28, avr64da32, avr64da48, avr64da64, avr64db28, avr64db32, avr64db48, avr64db64, avr64dd14, avr64dd20, avr64dd28, avr64dd32, avr64du28, avr64du32, avr64ea28, avr64ea32, avr64ea48.


“XMEGA” devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space.
mcu = attiny202, attiny204, attiny212, attiny214, attiny402, attiny404, attiny406, attiny412, attiny414, attiny416, attiny416auto, attiny417, attiny424, attiny426, attiny427, attiny804, attiny806, attiny807, attiny814, attiny816, attiny817, attiny824, attiny826, attiny827, attiny1604, attiny1606, attiny1607, attiny1614, attiny1616, attiny1617, attiny1624, attiny1626, attiny1627, attiny3214, attiny3216, attiny3217, attiny3224, attiny3226, attiny3227, atmega808, atmega809, atmega1608, atmega1609, atmega3208, atmega3209, atmega4808, atmega4809, avr16dd14, avr16dd20, avr16dd28, avr16dd32, avr16du14, avr16du20, avr16du28, avr16du32, avr16ea28, avr16ea32, avr16ea48, avr16eb14, avr16eb20, avr16eb28, avr16eb32, avr32da28, avr32da32, avr32da48, avr32db28, avr32db32, avr32db48, avr32dd14, avr32dd20, avr32dd28, avr32dd32, avr32du14, avr32du20, avr32du28, avr32du32, avr32ea28, avr32ea32, avr32ea48.


“XMEGA” devices with more than 64 KiB and up to 128 KiB of program memory.
mcu = atxmega64a3, atxmega64a3u, atxmega64a4u, atxmega64b1, atxmega64b3, atxmega64c3, atxmega64d3, atxmega64d4, avr128da28, avr128da32, avr128da48, avr128da64, avr128db28, avr128db32, avr128db48, avr128db64.


“XMEGA” devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM.
mcu = atxmega64a1, atxmega64a1u.


“XMEGA” devices with more than 128 KiB of program memory.
mcu = atxmega128a3, atxmega128a3u, atxmega128b1, atxmega128b3, atxmega128c3, atxmega128d3, atxmega128d4, atxmega192a3, atxmega192a3u, atxmega192c3, atxmega192d3, atxmega256a3, atxmega256a3b, atxmega256a3bu, atxmega256a3u, atxmega256c3, atxmega256d3, atxmega384c3, atxmega384d3.


“XMEGA” devices with more than 128 KiB of program memory and more than 64 KiB of RAM.
mcu = atxmega128a1, atxmega128a1u, atxmega128a4u.


“Reduced Tiny” Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory.
mcu = attiny4, attiny5, attiny9, attiny10, attiny102, attiny104, attiny20, attiny40.


This ISA is implemented by the minimal AVR core and supported for assembler only.
mcu = attiny11, attiny12, attiny15, attiny28, at90s1200.


Assume that all data in static storage can be accessed by LDS / STS instructions. This option has only an effect on reduced Tiny devices like ATtiny40. See also the absdata variable attribute.


Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards.

Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call.

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack like calls to printf-like functions.


Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0.


Functions prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller.


Optimize indirect memory accesses on reduced Tiny devices. The default uses level=1 for optimizations -Og and -O1, and level=2 for higher optimizations. Valid values for level are 0, 1 and 2.


Set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether or not a specific value for bits is allowed depends on the --with-double= and --with-long-double= configure options, and the same applies for the default values of the options.


Interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the no_gccisr function attribute. This feature is activated per default if optimization is on (but not with -Og, see Options That Control Optimization), and if GNU Binutils support PR21683.


Assume int to be 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size.


Do not save registers in main. The effect is the same like attaching attribute OS_task to main. It is activated per default if optimization is on.


Generated code is not compatible with hardware interrupts. Code size is smaller.


Try to replace CALL resp. JMP instruction by the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler’s command line and the --relax option to the linker’s command line.

Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code.

Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below.


Locate the .rodata sections for read-only data in RAM resp. in program memory. For most devices, there is no choice and this option acts rather like an assertion.

Since v14 and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default, provided the required GNU Binutils support (PR31124) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM.


Use address register X in a way proposed by the hardware. This means that X is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the X register may be used in the same way as Y or Z which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as

adiw r26, const   ; X += const
ld   Rn, X        ; Rn = *X
sbiw r26, const   ; X -= const

Only change the lower 8 bits of the stack pointer.


Allow to use truncation instead of rounding towards zero for fractional fixed-point types.


Don’t link against AVR-LibC’s device specific library lib<mcu>.a.


Don’t add -specs=device-specs/specs-mcu to the compiler driver’s command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. This means that the user has to supply her private device specs file by means of -specs=path-to-specs-file. There is no more need for option -mmcu=mcu.

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named device-specs which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu.


Warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.


Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default. EIND and Devices with More Than 128 Ki Bytes of Flash

Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used.

Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations: Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers

Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a RAMP register is used as high part of the address: The X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing. AVR Built-in Macros

GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.


Build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu option. Possible values are:

2, 25, 3, 31, 35, 4, 5, 51, 6

for mcu=avr2, avr25, avr3, avr31, avr35, avr4, avr5, avr51, avr6,

respectively and

100, 102, 103, 104, 105, 106, 107

for mcu=avrtiny, avrxmega2, avrxmega3, avrxmega4, avrxmega5, avrxmega6, avrxmega7, respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.


Setting -mmcu=device defines this built-in macro which reflects the device’s name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.

The built-in macros’ names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.

If device is not a device but only a core architecture like ‘avr51’, this macro is not defined.


Setting -mmcu=device defines this built-in macro to the device’s name. For example, with -mmcu=atmega8 the macro is defined to atmega8.

If device is not a device but only a core architecture like ‘avr51’, this macro is not defined.


The device / architecture belongs to the XMEGA family of devices.


The device has the ADIW and SBIW instructions.


The device has the ELPM instruction.


The device has the ELPM Rn,Z and ELPM Rn,Z+ instructions.


The device has the LPM Rn,Z and LPM Rn,Z+ instructions.


The device has the MOVW instruction to perform 16-bit register-register moves.


The device has a hardware multiplier.


The device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory.


The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.


The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.


The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.


The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.


The device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively.


This macro reflects the -mno-interrupts command-line option.


Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.


The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).


Instructions that can address I/O special function registers directly like IN, OUT, SBI, etc. may use a different address as if addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.


The -mshort-calls command line option is set.


Some devices support reading from flash memory by means of LD* instructions. The flash memory is seen in the data address space at an offset of __AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put .rodata into RAM. This is handled by the default linker description file, and is currently available for avrtiny and avrxmega3. Even more convenient, there is no need to use address spaces like __flash or features like attribute progmem and pgm_read_*.


This macro is defined provided the following conditions are met:

  • The device has the NVMCTRL_CTRLB.FLMAP bitfield. This applies to the AVR64* and AVR128* devices.
  • It’s not known at assembler-time which emulation will be used.

This implies the compiler was configured with GNU Binutils that implement PR31124.


This macro is undefined when the code is compiled for a core architecture.

When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data is located in RAM; and defined to 0, otherwise.


The compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option.


Defined if -mdouble= acts as a multilib option.


Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=.


The size in bits of double if -mdouble= is not set. To test the layout of double in a program, use the built-in macro __SIZEOF_DOUBLE__.


Same as above, but for long double instead of double.


Reflects the --with-double-comparison={tristate|bool|libf7} configure option and is defined to 2 or 3.


Reflects the --with-libf7={libgcc|math|math-symbols} configure option. AVR Internal Options

The following options are used internally by the compiler and to communicate between device specs files and the compiler proper. You don’t need to set these options by hand, in particular they are not optimization options. Using these options in the wrong way may lead to sub-optimal or wrong code. They are documented for completeness, and in order to get a better understanding of device specs files.


Assume that the flash memory has a size of num times 64 KiB. This determines which __flashN address spaces are available.


The device has the FLMAP bit field located in special function register NVMCTRL_CTRLB.


Assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT.


Assume that RJMP and RCALL can target the whole program memory. This option is used for multilib generation and selection for the devices from architecture avrxmega3.


Generate code without skips (CPSE, SBRS, SBRC, SBIS, SBIC) over 32-bit instructions.


Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. This option is used by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH.