pw_boot_armv7m/public/pw_boot_armv7m/boot.h

// Copyright 2020 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#pragma once

// This module is similar to a traditional assembly startup file paired with a
// linker script. It turns out that everything typically done in ARMv7-M
// assembly startup can be done straight from C code. This makes startup code
// easier to maintain, modify, and read.
//
// Core initialization is comprised of two primary parts:
//
// 1. Load boot information from ARMv7-M Vector Table: The ARMv7-M vector table
//    (See ARMv7-M Architecture Reference Manual DDI 0403E.b section B1.5)
//    dictates the starting Program Counter (PC) and Stack Pointer (SP) when the
//    SoC powers on. The vector table also contains a number of other vectors to
//    handle different exceptions. This module does not provide a vector table,
//    but it does account for it in the linker script.
//
// 2. Initialize static memory: When execution begins due to SoC power-on (or
//    the device is reset), static memory regions must be initialized to ensure
//    they contains the expected values when code begins to run. The SoC doesn't
//    inherently have a notion of how to do this, so before ANYTHING else the
//    memory must be initialized. This is done at the beginning of
//    pw_BootEntry().
//
//
// The simple flow is as follows:
//   Power on -> PC and SP set (from vector_table by SoC) -> pw_BootEntry()
//
// In pw_BootEntry():
//   Initialize memory -> pw_PreMainInit() -> main()

#include "pw_preprocessor/compiler.h"
#include "pw_preprocessor/util.h"

PW_EXTERN_C_START

// The following extern symbols are provided by the linker script, and their
// values are accessible via the reference of the symbol.
//
// Example:
//   if (stack_pointer < &pw_stack_low_addr) {
//     PW_LOG_ERROR("Main stack overflowed!")
//   }

// pw_stack_[low/high]_addr indicate the range of the main stack. Note that this
// might not be the only stack in the system.
//
// The main stack pointer (sp_main) should be initialized to pw_stack_high_addr.
// This can be done by inserting the address into index 0 of the ARMv7-M vector
// table. (See ARMv7-M Architecture Reference Manual DDI 0403E.b section B1.5.3)
extern uint8_t pw_stack_low_addr;
extern uint8_t pw_stack_high_addr;

// pw_heap_[low/high]_addr indicate the address range reserved for the heap.
extern uint8_t pw_heap_low_addr;
extern uint8_t pw_heap_high_addr;

// The address that denotes the beginning of the .vector_table section. This
// can be used to set VTOR (vector table offset register) by the bootloader.
extern uint8_t pw_vector_table_addr;

// Forward declaration of main. Pigweed applications are expected to implement
// this function. An implementation of main() is NOT provided by this module.
int main();

// For this module to work as expected, index 1 of the ARMv7-M vector table
// (which usually points to Reset_Handler) must be set to point to this
// function. This function is implemented by pw_boot_armv7m, and does early
// memory initialization.
PW_NO_PROLOGUE void pw_BootEntry();

// This function is called by pw_BootEntry() after memory initialization but
// before main. This allows targets to have pre-main initialization of the
// device and seamlessly swap out the main() implementation. This function is
// NOT implemented by pw_boot_armv7m.
void pw_PreMainInit();

PW_EXTERN_C_END