.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 1988, 2006 On-Line Applications Research Corporation (OAR)

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.

TBD

- See other CPUs for documentation reference formatting examples.

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.

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.

Calling Conventions
===================

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.

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).

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.

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.

Interrupt Processing
====================

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

Vectoring of an Interrupt Handler
---------------------------------

TBD

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.

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.

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.

Symmetric Multiprocessing
=========================

SMP is not supported.

Thread-Local Storage
====================

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

Board Support Packages
======================

System Reset
------------

TBD
