assembler - .TH "assembler" 3 "Thu Aug 12 2010" "Version 1.6.8" "avr-
assembler - .SH "Introduction"
There might be several reasons to write code for AVR microcontrollers
using plain assembler source code. Among them are:
o Code for devices that do not have RAM and are thus not supported by
the C compiler.
o Code for very time-critical applications.
o Special tweaks that cannot be done in C.
Usually, all but the first could probably be done easily using the
inline assembler facility of the compiler.
Although avr-libc is primarily targeted to support programming AVR
microcontrollers using the C (and C++) language, there's limited
support for direct assembler usage as well. The benefits of it are:
o Use of the C preprocessor and thus the ability to use the same
symbolic constants that are available to C programs, as well as a
flexible macro concept that can use any valid C identifier as a macro
(whereas the assembler's macro concept is basically targeted to use a
macro in place of an assembler instruction).
o Use of the runtime framework like automatically assigning interrupt
vectors. For devices that have RAM, initializing the RAM variables
can also be utilized.
Invoking the compiler
For the purpose described in this document, the assembler and linker
are usually not invoked manually, but rather using the C compiler
frontend (avr-gcc) that in turn will call the assembler and linker as
This approach has the following advantages:
o There is basically only one program to be called directly, avr-gcc,
regardless of the actual source language used.
o The invokation of the C preprocessor will be automatic, and will
include the appropriate options to locate required include files in
o The invokation of the linker will be automatic, and will include the
appropriate options to locate additional libraries as well as the
application start-up code (crtXXX.o) and linker script.
Note that the invokation of the C preprocessor will be automatic when
the filename provided for the assembler file ends in .S (the capital
letter 's'). This would even apply to operating systems that use case-
insensitive filesystems since the actual decision is made based on the
case of the filename suffix given on the command-line, not based on the
actual filename from the file system.
Alternatively, the language can explicitly be specified using the -x
The following annotated example features a simple 100 kHz square wave
generator using an AT90S1200 clocked with a 10.7 MHz crystal. Pin PD6
will be used for the square wave output.
#include <avr/io.h> ; Note 
work = 16 ; Note 
tmp = 17
inttmp = 19
intsav = 0
SQUARE = PD6 ; Note 
; Note :
tmconst= 10700000 / 200000 ; 100 kHz => 200000 edges/s
fuzz= 8 ; # clocks in ISR until TCNT0 is set
.global main ; Note 
rjmp 1b ; Note 
.global TIMER0_OVF_vect ; Note 
ldi inttmp, 256 - tmconst + fuzz
out _SFR_IO_ADDR(TCNT0), inttmp ; Note 
in intsav, _SFR_IO_ADDR(SREG) ; Note 
sbic _SFR_IO_ADDR(PORTD), SQUARE
sbi _SFR_IO_ADDR(PORTD), SQUARE
1: cbi _SFR_IO_ADDR(PORTD), SQUARE
out _SFR_IO_ADDR(SREG), intsav
sbi _SFR_IO_ADDR(DDRD), SQUARE
ldi work, _BV(TOIE0)
out _SFR_IO_ADDR(TIMSK), work
ldi work, _BV(CS00) ; tmr0: CK/1
out _SFR_IO_ADDR(TCCR0), work
ldi work, 256 - tmconst
out _SFR_IO_ADDR(TCNT0), work
.global __vector_default ; Note 
As in C programs, this includes the central processor-specific file
containing the IO port definitions for the device. Note that not all
include files can be included into assembler sources.
Assignment of registers to symbolic names used locally. Another option
would be to use a C preprocessor macro instead:
#define work 16
Our bit number for the square wave output. Note that the right-hand
side consists of a CPP macro which will be substituted by its value (6
in this case) before actually being passed to the assembler.
The assembler uses integer operations in the host-defined integer size
(32 bits or longer) when evaluating expressions. This is in contrast to
the C compiler that uses the C type int by default in order to
calculate constant integer expressions.
In order to get a 100 kHz output, we need to toggle the PD6 line
200000 times per second. Since we use timer 0 without any prescaling
options in order to get the desired frequency and accuracy, we already
run into serious timing considerations: while accepting and processing
the timer overflow interrupt, the timer already continues to count.
When pre-loading the TCCNT0 register, we therefore have to account for
the number of clock cycles required for interrupt acknowledge and for
the instructions to reload TCCNT0 (4 clock cycles for interrupt
acknowledge, 2 cycles for the jump from the interrupt vector, 2 cycles
for the 2 instructions that reload TCCNT0). This is what the constant
fuzz is for.
External functions need to be declared to be .global. main is the
application entry point that will be jumped to from the ininitalization
routine in crts1200.o.
The main loop is just a single jump back to itself. Square wave
generation itself is completely handled by the timer 0 overflow
interrupt service. A sleep instruction (using idle mode) could be used
as well, but probably would not conserve much energy anyway since the
interrupt service is executed quite frequently.
Interrupt functions can get the usual names that are also available to
C programs. The linker will then put them into the appropriate
interrupt vector slots. Note that they must be declared .global in
order to be acceptable for this purpose. This will only work if
<avr/io.h> has been included. Note that the assembler or linker have no
chance to check the correct spelling of an interrupt function, so it
should be double-checked. (When analyzing the resulting object file
using avr-objdump or avr-nm, a name like __vector_N should appear, with
N being a small integer number.)
As explained in the section about special function registers, the
actual IO port address should be obtained using the macro _SFR_IO_ADDR.
(The AT90S1200 does not have RAM thus the memory-mapped approach to
access the IO registers is not available. It would be slower than using
in / out instructions anyway.)
Since the operation to reload TCCNT0 is time-critical, it is even
performed before saving SREG. Obviously, this requires that the
instructions involved would not change any of the flag bits in SREG.
Interrupt routines must not clobber the global CPU state. Thus, it is
usually necessary to save at least the state of the flag bits in SREG.
(Note that this serves as an example here only since actually, all the
following instructions would not modify SREG either, but that's not
commonly the case.)
Also, it must be made sure that registers used inside the interrupt
routine do not conflict with those used outside. In the case of a RAM-
less device like the AT90S1200, this can only be done by agreeing on a
set of registers to be used exclusively inside the interrupt routine;
there would not be any other chance to 'save' a register anywhere.
If the interrupt routine is to be linked together with C modules, care
must be taken to follow the register usage guidelines imposed by the C
compiler. Also, any register modified inside the interrupt sevice needs
to be saved, usually on the stack.
As explained in Interrupts, a global 'catch-all' interrupt handler that
gets all unassigned interrupt vectors can be installed using the name
__vector_default. This must be .global, and obviously, should end in a
reti instruction. (By default, a jump to location 0 would be implied
Pseudo-ops and operators
The available pseudo-ops in the assembler are described in the GNU
assembler (gas) manual. The manual can be found online as part of the
current binutils release under http://sources.redhat.com/binutils/.
As gas comes from a Unix origin, its pseudo-op and overall assembler
syntax is slightly different than the one being used by other
assemblers. Numeric constants follow the C notation (prefix 0x for
hexadecimal constants), expressions use a C-like syntax.
Some common pseudo-ops include:
Note that .org is available in gas as well, but is a fairly pointless
pseudo-op in an assembler environment that uses relocatable object
files, as it is the linker that determines the final position of some
object in ROM or RAM.
Along with the architecture-independent standard operators, there are
some AVR-specific operators available which are unfortunately not yet
described in the official documentation. The most notable operators
o lo8 Takes the least significant 8 bits of a 16-bit integer
o hi8 Takes the most significant 8 bits of a 16-bit integer
o pm Takes a program-memory (ROM) address, and converts it into a RAM
address. This implies a division by 2 as the AVR handles ROM
addresses as 16-bit words (e.g. in an IJMP or ICALL instruction), and
can also handle relocatable symbols on the right-hand side.
ldi r24, lo8(pm(somefunc))
ldi r25, hi8(pm(somefunc))
This passes the address of function somefunc as the first parameter to