tinySA/os/ports/GCC/ARM7/port.dox

/*
    ChibiOS/RT - Copyright (C) 2006-2007 Giovanni Di Sirio.

    This file is part of ChibiOS/RT.

    ChibiOS/RT is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    ChibiOS/RT is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

/**
 * @defgroup ARM7 ARM7TDMI
 * @details The ARM7 architecture is quite complex for a microcontroller and
 * some explanations are required about the port choices.
 *
 * @section ARM7_NOTES The ARM7 modes
 * The ARM7 port supports three modes:
 * - Pure ARM mode, this is the preferred mode for code speed. The code size
 *   is larger however. This mode is enabled when all the modules are compiled
 *   in ARM mode, see the Makefiles.
 * - Pure THUMB mode, this is the preferred mode for code size. In this mode
 *   the execution speed is slower than the ARM mode. This mode is enabled
 *   when all the modules are compiled in THUMB mode, see the Makefiles.
 * - Interworking mode, when in the system there are ARM modules mixed with
 *   THUMB modules then the interworking compiler option is enabled. This is
 *   usually the slowest mode and the code size is not as good as in pure
 *   THUMB mode.
 * .
 * @section ARM7_STATES Mapping of the System States in the ARM7 port
 * The ChibiOS/RT logical @ref system_states are mapped as follow in the ARM7
 * port:
 * - <b>Init</b>. This state is represented by the startup code and the
 *   initialization code before @p chSysInit() is executed. It has not a
 *   special hardware state associated, usually the CPU goes through several
 *   hardware states during the startup phase.
 * - <b>Normal</b>. This is the state the system has after executing
 *   @p chSysInit(). In this state the ARM7TDMI has both the interrupt sources
 *   (IRQ and FIQ) enabled and is running in ARM System Mode.
 * - <b>Suspended</b>. In this state the IRQ sources are disabled but the FIQ
 *   sources are served, the core is running in ARM System Mode.
 * - <b>Disabled</b>. Both the IRQ and FIQ sources are disabled, the core is
 *   running in ARM System Mode.
 * - <b>Sleep</b>. The ARM7 code does not have any built-in low power mode but
 *   there are clock stop modes implemented in custom ways by the various
 *   silicon vendors. This state is implemented in each microcontroller support
 *   code in a different way, the core is running (or freezed...) in ARM
 *   System Mode.
 * - <b>S-Locked</b>. IRQ sources disabled, core running in ARM System Mode.
 * - <b>I-Locked</b>. IRQ sources disabled, core running in ARM IRQ Mode. Note
 *   that this state is not different from the SRI state in this port, the
 *   @p chSysLockI() and @p chSysUnlockI() APIs do nothing (still use them in
 *   order to formally change state because this may change).
 * - <b>Serving Regular Interrupt</b>. IRQ sources disabled, core running in
 *   ARM IRQ Mode. See also the I-Locked state.
 * - <b>Serving Fast Interrupt</b>. IRQ and FIQ sources disabled, core running
 *   in ARM FIQ Mode.
 * - <b>Serving Non-Maskable Interrupt</b>. There are no asynchronous NMI
 *   sources in ARM7 architecture but synchronous SVC, ABT and UND exception
 *   handlers can be seen as belonging to this category.
 * - <b>Halted</b>. Implemented as an infinite loop after disabling both IRQ
 *   and FIQ sources. The ARM state is whatever the processor was running when
 *   @p chSysHalt() was invoked.
 * .
 * @section ARM7_NOTES The ARM7 port notes
 * The ARM7 port makes some assumptions on the application code organization:
 * - The @p main() function is invoked in system mode.
 * - Each thread has a private user/system stack, the system has a single
 *   interrupt stack where all the interrupts are processed.
 * - The threads are started in system mode.
 * - The threads code can run in system mode or user mode, however the
 *   code running in user mode cannot invoke the ChibiOS/RT APIs directly
 *   because privileged instructions are used inside.<br>
 *   The kernel APIs can be eventually invoked by using a SWI entry point
 *   that handles the switch in system mode and the return in user mode.
 * - Other modes are not preempt-able because the system code assumes the
 *   threads running in system mode. When running in supervisor or other
 *   modes make sure that the interrupts are globally disabled.
 * - Interrupts nesting is not supported in the ARM7 code because their
 *   implementation, even if possible, is not really efficient in this
 *   architecture.
 * - FIQ sources can preempt the kernel (by design) so it is not possible to
 *   invoke the kernel APIs from inside a FIQ handler. FIQ handlers are not
 *   affected by the kernel activity so there is not added jitter.
 * .
 * @section ARM7_IH ARM7 Interrupt Handlers
 * ARM7 Interrupt handlers do not save function-saved registers so you need to
 * make sure your code saves them or does not use them (this happens
 * because in the ARM7 port all the OS interrupt handler functions are declared
 * naked).<br>
 * Function-trashed registers (R0-R3, R12, LR, SR) are saved/restored by the
 * system macros @p CH_IRQ_PROLOGUE() and @p CH_IRQ_EPILOGUE().<br>
 * The easiest way to ensure this is to just invoke a normal function from
 * within the interrupt handler, the function code will save all the required
 * registers.<br>
 * Example:
 * @code
 * CH_IRQ_HANDLER(irq_handler) {
 *   CH_IRQ_PROLOGUE();
 *
 *   serve_interrupt();
 *
 *   VICVectAddr = 0; // This is LPC214x-specific.
 *   CH_IRQ_EPILOGUE();
 * }
 * @endcode
 * This is not a bug but an implementation choice, this solution allows to
 * have interrupt handlers compiled in thumb mode without have to use an
 * interworking mode (the mode switch is hidden in the macros), this
 * greatly improves code efficiency and size. You can look at the serial
 * driver for real examples of interrupt handlers.
 *
 * @ingroup Ports
 */

/**
 * @defgroup ARM7_CONF Configuration Options
 * @brief ARM7 specific configuration options.
 * @details The ARM7 port allows some architecture-specific configurations
 * settings that can be specified externally, as example on the compiler
 * command line:
 * - @p INT_REQUIRED_STACK, this value represent the amount of stack space used
 *   by an interrupt handler between the @p extctx and @p intctx
 *   structures.<br>
 *   In practice this value is the stack space used by the chSchDoReschedule()
 *   stack frame.<br>
 *   This value can be affected by a variety of external things like compiler
 *   version, compiler options, kernel settings (speed/size) and so on.<br>
 *   The default for this value is @p 0x10 which should be a safe value, you
 *   can trim this down by defining the macro externally. This would save
 *   some valuable RAM space for each thread present in the system.<br>
 *   The default value is set into <b>./ports/ARM7/chcore.h</b>.
 * .
 * @ingroup ARM7
 */

/**
 * @defgroup ARM7_CORE Core Port Implementation
 * @brief ARM7 specific port code, structures and macros.
 *
 * @ingroup ARM7
 * @file ARM7/chtypes.h Port types.
 * @file ARM7/chcore.h Port related structures and macros.
 * @file ARM7/chcore.c Port related code.
 */

/**
 * @defgroup ARM7_STARTUP Startup Support
 * @brief ARM7 startup code support.
 * @details ChibiOS/RT provides its own generic startup file for the ARM7 port.
 * Of course it is not mandatory to use it but care should be taken about the
 * startup phase details.
 *
 * <h2>Startup Process</h2>
 * The startup process, as implemented, is the following:
 * -# The stacks are initialized by assigning them the sizes defined in the
 *    linker script (usually named @p ch.ld). Stack areas are allocated from
 *    the highest RAM location downward.
 * -# The ARM state is switched to System with both IRQ and FIQ sources
 *    disabled.
 * -# An early initialization routine @p hwinit0 is invoked, if the symbol is
 *    not defined then an empty default routine is executed (weak symbol).
 * -# DATA and BSS segments are initialized.
 * -# A late initialization routine @p hwinit1 is invoked, if the symbol not
 *    defined then an empty default routine is executed (weak symbol).<br>
 *    This late initialization function is also the proper place for a
 *    @a bootloader, if your application requires one.
 * -# The @p main() function is invoked with the parameters @p argc and @p argv
 *    set to zero.
 * -# Should the @p main() function return a branch is performed to the weak
 *    symbol MainExitHandler. The default code is an endless empty loop.
 * .
 * <h2>Expected linker symbols</h2>
 * The startup code starts at the symbol @p ResetHandler and expects the
 * following symbols to be defined in the linker script:
 * - @p __ram_end__ RAM end location +1.
 * - @p __und_stack_size__ Undefined Instruction stack size.
 * - @p __abt_stack_size__ Memory Abort stack size.
 * - @p __fiq_stack_size__ FIQ service stack size.
 * - @p __irq_stack_size__ IRQ service stack size.
 * - @p __svc_stack_size__ SVC service stack size.
 * - @p __sys_stack_size__ System/User stack size. This is the stack area used
 *      by the @p main() function.
 * - @p _textdata address of the data segment source read only data.
 * - @p _data data segment start location.
 * - @p _edata data segment end location +1.
 * - @p _bss_start BSS start location.
 * - @p _bss_end BSS end location +1.
 * .
 * @ingroup ARM7
 * @file ARM7/crt0.s Startup code.
 */