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.
avr1This 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
variable attribute.
-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 ¶-mlong-double=bitsSet 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 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.
-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 OS_task
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.
-Wmisspelled-isr ¶Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.
EIND and Devices with More Than 128 Ki Bytes of FlashRAMPD, RAMPX, RAMPY and RAMPZ Special Function RegistersEIND 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:
EIND.
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.
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.
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.
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.
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.
gs modifier
(short for generate stubs) like so:
LDI r24, lo8(gs(func)) LDI r25, hi8(gs(func))
gs modifiers for code labels in the
following situations:
gs() modifier explained above.
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.
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.
RAMP special function
registers with zero.
__flash is used, then RAMPZ is set
as needed before the operation.
RAMPZ to accomplish an operation, RAMPZ
is reset to zero after the operation.
RAMP register, the ISR
prologue/epilogue saves/restores that SFR and initializes it with
zero in case the ISR code might (implicitly) use it.
RAMP registers,
you must reset it to zero after the access.
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__=offsetInstructions 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__=addrSome 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={tristate|bool|libf7}
configure option
and is defined to 2 or 3.
__WITH_LIBF7_LIBGCC____WITH_LIBF7_MATH____WITH_LIBF7_MATH_SYMBOLS__Reflects the --with-libf7={libgcc|math|math-symbols}
configure option.