These options are defined for AVR implementations:
-mmcu=mcuThe default for this option is ‘avr2’.
GCC supports the following AVR devices and ISAs:
avr2attiny22, attiny26, at90c8534, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90s8535.
avr25MOVW instruction.
ata5272, ata6616c, attiny13, attiny13a, attiny2313, attiny2313a, attiny24, attiny24a, attiny25, attiny261, attiny261a, attiny43u, attiny4313, attiny44, attiny44a, attiny441, attiny45, attiny461, attiny461a, attiny48, attiny828, attiny84, attiny84a, attiny841, attiny85, attiny861, attiny861a, attiny87, attiny88, at86rf401.
avr3at43usb355, at76c711.
avr31atmega103, at43usb320.
avr35MOVW instruction.
ata5505, ata6617c, ata664251, atmega16u2, atmega32u2, atmega8u2, attiny1634, attiny167, at90usb162, at90usb82.
avr4ata6285, ata6286, ata6289, ata6612c, atmega48, atmega48a, atmega48p, atmega48pa, atmega48pb, atmega8, atmega8a, atmega8hva, atmega8515, atmega8535, atmega88, atmega88a, atmega88p, atmega88pa, atmega88pb, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.
avr5ata5702m322, ata5782, ata5790, ata5790n, ata5791, ata5795, ata5831, ata6613c, ata6614q, ata8210, ata8510, 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, atmega325, atmega325a, atmega325p, atmega325pa, atmega3250, atmega3250a, atmega3250p, atmega3250pa, atmega328, atmega328p, atmega328pb, atmega329, atmega329a, atmega329p, atmega329pa, atmega3290, atmega3290a, atmega3290p, atmega3290pa, atmega406, atmega64, atmega64a, atmega64c1, atmega64hve, atmega64hve2, atmega64m1, atmega64rfr2, atmega640, atmega644, atmega644a, atmega644p, atmega644pa, atmega644rfr2, atmega645, atmega645a, atmega645p, atmega6450, atmega6450a, atmega6450p, atmega649, atmega649a, atmega649p, atmega6490, atmega6490a, atmega6490p, at90can32, at90can64, at90pwm161, at90pwm216, at90pwm316, at90scr100, at90usb646, at90usb647, at94k, m3000.
avr51atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.
avr6atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.
avrxmega2atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5, atxmega8e5.
avrxmega3attiny1614, attiny1616, attiny1617, attiny212, attiny214, attiny3216, attiny3217, attiny412, attiny414, attiny416, attiny417, attiny814, attiny816, attiny817.
avrxmega4atxmega64a3, atxmega64a3u, atxmega64a4u, atxmega64b1, atxmega64b3, atxmega64c3, atxmega64d3, atxmega64d4.
avrxmega5atxmega64a1, atxmega64a1u.
avrxmega6atxmega128a3, atxmega128a3u, atxmega128b1, atxmega128b3, atxmega128c3, atxmega128d3, atxmega128d4, atxmega192a3, atxmega192a3u, atxmega192c3, atxmega192d3, atxmega256a3, atxmega256a3b, atxmega256a3bu, atxmega256a3u, atxmega256c3, atxmega256d3, atxmega384c3, atxmega384d3.
avrxmega7atxmega128a1, atxmega128a1u, atxmega128a4u.
avrtinyattiny10, attiny20, attiny4, attiny40, attiny5, attiny9.
avr1attiny11, attiny12, attiny15, attiny28, at90s1200.
-mabsdata-maccumulate-argsPopping 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 to 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-mcall-prologues-mint8int 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.
-mn-flash=num-mno-interrupts-mrelaxCALL 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.
-mrmwXCH, LAC, LAS and LAT.
-mshort-callsRJMP 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.
-msp8This 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-XX 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-mfract-convert-truncate-nodeviceliblib<mcu>.a.
-Waddr-space-convert-Wmisspelled-isrEIND and Devices with More Than 128 Ki Bytes of FlashPointers 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 implicitely 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 RegistersSome 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 contentent 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__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____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__atmega8.
If device is not a device but only a core architecture like
‘avr51’, this macro is not defined.
__AVR_XMEGA____AVR_HAVE_ELPM__ELPM instruction.
__AVR_HAVE_ELPMX__ELPM Rn,Z and ELPM
Rn,Z+ instructions.
__AVR_HAVE_MOVW__MOVW instruction to perform 16-bit
register-register moves.
__AVR_HAVE_LPMX__LPM Rn,Z and
LPM Rn,Z+ instructions.
__AVR_HAVE_MUL____AVR_HAVE_JMP_CALL__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__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____AVR_HAVE_8BIT_SP____AVR_HAVE_16BIT_SP____AVR_HAVE_SPH____AVR_SP8____AVR_HAVE_RAMPD____AVR_HAVE_RAMPX____AVR_HAVE_RAMPY____AVR_HAVE_RAMPZ__RAMPD, RAMPX, RAMPY,
RAMPZ special function register, respectively.
__NO_INTERRUPTS____AVR_ERRATA_SKIP____AVR_ERRATA_SKIP_JMP_CALL__SBRS, SBRC, SBIS, SBIC and CPSE.
The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also
set.
__AVR_ISA_RMW____AVR_SFR_OFFSET__=offsetIN, 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____AVR_PM_BASE_ADDRESS__=addrLD*
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__