AVR Options#

These options are defined for AVR implementations:

-mmcu=mcu#

Specify Atmel AVR instruction set architectures (ISA) or MCU type.

The default for this option is avr2.

GCC supports the following AVR devices and ISAs:

avr2

“Classic” devices with up to 8 KiB of program memory.

mcu= attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.

avr25

“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.

avr3

“Classic” devices with 16 KiB up to 64 KiB of program memory.

mcu= at76c711, at43usb355.

avr31

“Classic” devices with 128 KiB of program memory.

mcu= atmega103, at43usb320.

avr35

“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.

avr4

“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, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.

avr5

“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, ata5795, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5702m322, at90pwm161, at90pwm216, at90pwm316, at90can32, at90can64, at90scr100, at90usb646, at90usb647, at94k, m3000.

avr51

“Enhanced” devices with 128 KiB of program memory.

mcu= atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.

avr6

“Enhanced” devices with 3-byte PC, i.e.: with more than 128 KiB of program memory.

mcu= atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.

avrxmega2

“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.

avrxmega3

“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, attiny417, attiny804, attiny806, attiny807, attiny814, attiny816, attiny817, attiny1604, attiny1606, attiny1607, attiny1614, attiny1616, attiny1617, attiny3214, attiny3216, attiny3217, atmega808, atmega809, atmega1608, atmega1609, atmega3208, atmega3209, atmega4808, atmega4809, avr32da28, avr32da32, avr32da48, avr32db28, avr32db32, avr32db48.

avrxmega4

“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.

avrxmega5

“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.

avrxmega6

“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.

avrxmega7

“XMEGA” devices with more than 128 KiB of program memory and more than 64 KiB of RAM.

mcu= atxmega128a1, atxmega128a1u, atxmega128a4u.

avrtiny

“TINY” Tiny core devices with 512 B up to 4 KiB of program memory.

mcu= attiny4, attiny5, attiny9, attiny10, attiny20, attiny40.

avr1

This ISA is implemented by the minimal AVR core and supported for assembler only.

mcu= attiny11, attiny12, attiny15, attiny28, at90s1200.

-mabsdata#

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.

-maccumulate-args#

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.

-mbranch-cost=cost#

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.

-mcall-prologues#

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

-mdouble=bits#

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.

-mgas-isr-prologues#

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 AVR Function Attributes 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.

-mint8#

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.

-mmain-is-OS_task#

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

-mn-flash=num#

Assume that the flash memory has a size of num times 64 KiB.

-mno-interrupts#

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

-mrelax#

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.

-mrmw#

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

-mshort-calls#

Assume that RJMP and RCALL can target the whole program memory.

This option is used internally for multilib selection. It is not an optimization option, and you don’t need to set it by hand.

-msp8#

Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don’t need to set this option by hand.

This option is used internally by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH. For any setting other than -mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this option from the compiler proper’s command line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no SPH register or not.

-mstrict-X#

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
-mtiny-stack#

Only change the lower 8 bits of the stack pointer.

-mfract-convert-truncate#

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

-nodevicelib#

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

-nodevicespecs#

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.

-Waddr-space-convert#

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

-Wno-addr-space-convert#

Default setting; overrides -Waddr-space-convert.

-Wmisspelled-isr#

Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.

-Wno-misspelled-isr#

Default setting; overrides -Wmisspelled-isr.

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:

  • The compiler never sets EIND.

  • The compiler uses EIND implicitly in EICALL / EIJMP instructions or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.

  • The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue.

  • For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.

  • Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message.

  • The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.

  • The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual.

  • It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located.

    #include <avr/io.h>

static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
  __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                  "out %i0,r24" :: "n" (&EIND) : "r24","memory");
}

    The __trampolines_start symbol is defined in the linker script.

  • Stubs are generated automatically by the linker if the following two conditions are met:

    • The address of a label is taken by means of the gs modifier (short for generate stubs) like so:

      LDI r24, lo8(gs(func))
LDI r25, hi8(gs(func))

    • The final location of that label is in a code segment outside the segment where the stubs are located.

  • The compiler emits such gs modifiers for code labels in the following situations:

    • Taking address of a function or code label.

    • Computed goto.

    • If prologue-save function is used, see -mcall-prologues command-line option.

    • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.

    • C and C++ constructors/destructors called during startup/shutdown.

    • If the tools hit a gs() modifier explained above.

  • Jumping to non-symbolic addresses like so is not supported:

    int main (void)
{
    /* Call function at word address 0x2 */
    return ((int(*)(void)) 0x2)();
}

    Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):

    int main (void)
{
    extern int func_4 (void);

    /* Call function at byte address 0x4 */
    return func_4();
}

    and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script.

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.

  • The startup code initializes the RAMP special function registers with zero.

  • If a AVR Named Address Spaces other than generic or __flash is used, then RAMPZ is set as needed before the operation.

  • If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation.

  • If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.

  • RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access.

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.

__AVR_ARCH__

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.

__AVR_Device__

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.

__AVR_DEVICE_NAME__

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.

__AVR_XMEGA__

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

__AVR_HAVE_ELPM__

The device has the ELPM instruction.

__AVR_HAVE_ELPMX__

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

__AVR_HAVE_MOVW__

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

__AVR_HAVE_LPMX__

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

__AVR_HAVE_MUL__

The device has a hardware multiplier.

__AVR_HAVE_JMP_CALL__

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

__AVR_HAVE_EIJMP_EICALL__ __AVR_3_BYTE_PC__

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.

__AVR_2_BYTE_PC__

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

__AVR_HAVE_8BIT_SP__ __AVR_HAVE_16BIT_SP__

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.

__AVR_HAVE_SPH__ __AVR_SP8__

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.

__AVR_HAVE_RAMPD__ __AVR_HAVE_RAMPX__ __AVR_HAVE_RAMPY__ __AVR_HAVE_RAMPZ__

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

__NO_INTERRUPTS__

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

__AVR_ERRATA_SKIP__ __AVR_ERRATA_SKIP_JMP_CALL__

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.

__AVR_ISA_RMW__

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

__AVR_SFR_OFFSET__=offset

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.

__AVR_SHORT_CALLS__

The -mshort-calls command line option is set.

__AVR_PM_BASE_ADDRESS__=addr

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_*.

__WITH_AVRLIBC__

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

__HAVE_DOUBLE_MULTILIB__

Defined if -mdouble= acts as a multilib option.

__HAVE_DOUBLE32__ __HAVE_DOUBLE64__

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

__DEFAULT_DOUBLE__

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__.

__HAVE_LONG_DOUBLE32__ __HAVE_LONG_DOUBLE64__ __HAVE_LONG_DOUBLE_MULTILIB__ __DEFAULT_LONG_DOUBLE__

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

__WITH_DOUBLE_COMPARISON__

Reflects the --with-double-comparison and is defined to 2 or 3.

__WITH_LIBF7_LIBGCC__ __WITH_LIBF7_MATH__ __WITH_LIBF7_MATH_SYMBOLS__

Reflects the --with-libf7.