5. Atmel AVR Specific Information

This chapter discusses the AVR architecture dependencies in this port of RTEMS.

Architecture Documents

For information on the AVR architecture, refer to the following documents available from Atmel.


  • See other CPUs for documentation reference formatting examples.

5.1. CPU Model Dependent Features

CPUs of the AVR 53X only differ in the peripherals and thus in the device drivers. This port does not yet support the 56X dual core variants.

5.1.1. Count Leading Zeroes Instruction

The AVR CPU has the XXX instruction which could be used to speed up the find first bit operation. The use of this instruction should significantly speed up the scheduling associated with a thread blocking.

5.2. Calling Conventions

5.2.1. Processor Background

The AVR architecture supports a simple call and return mechanism. A subroutine is invoked via the call (call) instruction. This instruction saves the return address in the RETS register and transfers the execution to the given address.

It is the called funcions responsability to use the link instruction to reserve space on the stack for the local variables. Returning from a subroutine is done by using the RTS (RTS) instruction which loads the PC with the adress stored in RETS.

It is is important to note that the call instruction does not automatically save or restore any registers. It is the responsibility of the high-level language compiler to define the register preservation and usage convention.

5.2.2. Register Usage

A called function may clobber all registers, except RETS, R4-R7, P3-P5, FP and SP. It may also modify the first 12 bytes in the caller’s stack frame which is used as an argument area for the first three arguments (which are passed in R0…R3 but may be placed on the stack by the called function).

5.2.3. Parameter Passing

RTEMS assumes that the AVR GCC calling convention is followed. The first three parameters are stored in registers R0, R1, and R2. All other parameters are put pushed on the stack. The result is returned through register R0.

5.3. Memory Model

The AVR family architecutre support a single unified 4 GB byte address space using 32-bit addresses. It maps all resources like internal and external memory and IO registers into separate sections of this common address space.

The AVR architcture supports some form of memory protection via its Memory Management Unit. Since the AVR port runs in supervisior mode this memory protection mechanisms are not used.

5.4. Interrupt Processing

Discussed in this chapter are the AVR’s interrupt response and control mechanisms as they pertain to RTEMS.

5.4.1. Vectoring of an Interrupt Handler


5.4.2. Disabling of Interrupts by RTEMS

During interrupt disable critical sections, RTEMS disables interrupts to level N (N) before the execution of this section and restores them to the previous level upon completion of the section. RTEMS uses the instructions CLI and STI to enable and disable Interrupts. Emulation, Reset, NMI and Exception Interrupts are never disabled.

5.4.3. Interrupt Stack

The AVR Architecture works with two different kind of stacks, User and Supervisor Stack. Since RTEMS and its Application run in supervisor mode, all interrupts will use the interrupted tasks stack for execution.

5.5. Default Fatal Error Processing

The default fatal error handler for the AVR performs the following actions:

  • disables processor interrupts,

  • places the error code in r0, and

  • executes an infinite loop (while(0); to simulate a halt processor instruction.

5.6. Symmetric Multiprocessing

SMP is not supported.

5.7. Thread-Local Storage

Thread-local storage is not supported due to a broken tool chain.

5.8. Board Support Packages

5.8.1. System Reset