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 [1]

work    =       16              ; Note [2]
tmp     =       17

inttmp  =       19

intsav  =       0

SQUARE  =       PD6             ; Note [3]

                                ; Note [4]:
tmconst= 10700000 / 200000      ; 100 kHz => 200000 edges/s
fuzz=   8                       ; # clocks in ISR until TCNT0 is set

        .section .text

        .global main                            ; Note [5]
        rcall   ioinit
        rjmp    1b                              ; Note [6]

        .global TIMER0_OVF_vect                 ; Note [7]
        ldi     inttmp, 256 - tmconst + fuzz
        out     _SFR_IO_ADDR(TCNT0), inttmp     ; Note [8]

        in      intsav, _SFR_IO_ADDR(SREG)      ; Note [9]

        sbic    _SFR_IO_ADDR(PORTD), SQUARE
        rjmp    1f
        sbi     _SFR_IO_ADDR(PORTD), SQUARE
        rjmp    2f
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 [10]


Note [1]

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.

Note [2]

Assignment of registers to symbolic names used locally. Another option would be to use a C preprocessor macro instead:

#define work 16 

Note [3]

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.

Note [4]

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.

Note [5]

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.

Note [6]

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.

Note [7]

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.)

Note [8]

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.

Note [9]

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.

Note [10]

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 instead.)