1.. _module-pw_boot_armv7m:
2
3--------------
4pw_boot_armv7m
5--------------
6
7The ARMv7-M boot module provides a linker script and some early initialization
8of static memory regions and C++ constructors. This is enough to get many
9ARMv7-M cores booted and ready to run C++ code.
10
11This module is currently designed to support a very minimal device memory layout
12configuration:
13
14 - One contiguous region for RAM.
15 - One contiguous region for flash.
16 - Static, in-flash vector table at the default location expected by the SoC.
17
18Note that this module is not yet particularly suited for projects that utilize
19a bootloader, as it's relatively opinionated regarding where code is stored.
20
21.. warning::
22  This module is currently NOT stable! Depending on this module may cause
23  breakages as this module is updated.
24
25Sequence
26========
27
28The high level pw_boot_armv7m boot sequence looks like the following psuedo-code
29invocation of the user-implemented functions:
30
31.. code:: cpp
32
33  void pw_boot_Entry() {  // Boot entry point.
34    pw_boot_PreStaticMemoryInit();  // User-implemented function.
35    // Static memory initialization.
36    pw_boot_PreStaticConstructorInit();  // User-implemented function.
37    // C++ static constructors are invoked.
38    pw_boot_PreMainInit();  // User-implemented function.
39    main();  // User-implemented function.
40    pw_boot_PostMain();  // User-implemented function.
41    PW_UNREACHABLE;
42  }
43
44Setup
45=====
46
47User-Implemented Functions
48--------------------------
49This module expects all of these extern "C" functions to be defined outside this
50module:
51
52 - ``int main()``: This is where applications reside.
53 - ``void pw_boot_PreStaticMemoryInit()``: This function executes just before
54   static memory has been zerod and static data is intialized. This function
55   should set up any early initialization that should be done before static
56   memory is initialized, such as:
57
58   - Enabling the FPU or other coprocessors.
59   - Opting into extra restrictions such as disabling unaligned access to ensure
60     the restrictions are active during static RAM initialization.
61   - Initial CPU clock, flash, and memory configurations including potentially
62     enabling extra memory regions with .bss and .data sections, such as SDRAM
63     or backup powered SRAM.
64   - Fault handler initialization if required before static memory
65     initialization.
66
67   .. warning::
68     Code running in this hook is violating the C spec as static values are not
69     yet initialized, meaning they have not been loaded (.data) nor
70     zero-initialized (.bss).
71
72 - ``void pw_boot_PreStaticConstructorInit()``: This function executes just
73   before C++ static constructors are called. At this point, other static memory
74   has been zero or data initialized. This function should set up any early
75   initialization that should be done before C++ static constructors are run,
76   such as:
77
78   - Run time dependencies such as Malloc, and ergo sometimes the RTOS.
79   - Persistent memory that survives warm reboots.
80   - Enabling the MPU to catch nullptr dereferences during construction.
81   - Main stack watermarking.
82   - Further fault handling configuration necessary for your platform which
83     were not safe before pw_boot_PreStaticRamInit().
84   - Boot count and/or boot session UUID management.
85
86 - ``void pw_boot_PreMainInit()``: This function executes just before main, and
87   can be used for any device initialization that isn't application specific.
88   Depending on your platform, this might be turning on a UART, setting up
89   default clocks, etc.
90
91 - ``PW_NO_RETURN void pw_boot_PostMain()``: This function executes after main
92   has returned. This could be used for device specific teardown such as an
93   infinite loop, soft reset, or QEMU shutdown. In addition, if relevant for
94   your application, this would be the place to invoke the global static
95   destructors. This function must not return!
96
97
98If any of these functions are unimplemented, executables will encounter a link
99error.
100
101Required Configs
102----------------
103This module has a number of required configuration options that mold the linker
104script to fit to a wide variety of ARMv7-M SoCs. The ``pw_boot_armv7m_config``
105GN variable has a ``defines`` member that can be used to modify these linker
106script options. See the documentation section on configuration for information
107regarding which configuration options are required.
108
109Vector Table
110------------
111Targets using ``pw_boot_armv7m`` will need to provide an ARMv7-M interrupt
112vector table (ARMv7-M Architecture Reference Manual DDI 0403E.b section B1.5.2
113and B1.5.3). This is done by storing an array into the ``.vector_table``
114section, and properly configuring ``PW_BOOT_VECTOR_TABLE_*`` preprocessor
115defines to cover the address region your SoC expects the vector table to be
116located at (often the beginning of the flash region). If using a bootloader,
117ensure VTOR (Vector Table Offset Register) is configured to point to the vector
118table. Otherwise, refer to the hardware vendor's documentation to determine
119where the vector table should be located such that it resides where VTOR is
120initialized to by default.
121
122Example vector table:
123
124.. code-block:: cpp
125
126  typedef void (*InterruptHandler)();
127
128  PW_KEEP_IN_SECTION(".vector_table")
129  const InterruptHandler vector_table[] = {
130      // The starting location of the stack pointer.
131      // This address is NOT an interrupt handler/function pointer, it is simply
132      // the address that the main stack pointer should be initialized to. The
133      // value is reinterpret casted because it needs to be in the vector table.
134      [0] = reinterpret_cast<InterruptHandler>(&pw_boot_stack_high_addr),
135
136      // Reset handler, dictates how to handle reset interrupt. This is the
137      // address that the Program Counter (PC) is initialized to at boot.
138      [1] = pw_boot_Entry,
139
140      // NMI handler.
141      [2] = DefaultFaultHandler,
142      // HardFault handler.
143      [3] = DefaultFaultHandler,
144      ...
145  };
146
147Usage
148=====
149
150Publicly exported symbols
151-------------------------
152The linker script provided by this module exports a number of symbols that
153may be used to retrieve the locations of specific memory regions at runtime.
154These symbols are declared as ``uint8_t`` variables. The variables themselves
155do not contain the addresses, they only reside at the memory location they
156reference. To retrieve the memory locations, simply take the reference of the
157symbol (e.g. ``&pw_boot_vector_table_addr``).
158
159``pw_boot_heap_[low/high]_addr``: Beginning and end of the memory range of the heap.
160These addresses may be identical, indicating a heap with a size of zero bytes.
161
162``pw_boot_stack_[low/high]_addr``: Beginning and end of the memory range of the main
163stack. This might not be the only stack in the system.
164
165``pw_boot_vector_table_addr``: Beginning of the ARMv7-M interrupt vector table.
166
167Configuration
168=============
169These configuration options can be controlled by appending list items to
170``pw_boot_armv7m_LINK_CONFIG_DEFINES`` as part of a Pigweed target
171configuration.
172
173``PW_BOOT_HEAP_SIZE`` (required):
174How much memory (in bytes) to reserve for the heap. This can be zero.
175
176``PW_BOOT_MIN_STACK_SIZE`` (required):
177The minimum size reserved for the main stack. If statically allocated memory
178begins to cut into the minimum, a link error will be emitted.
179
180``PW_BOOT_FLASH_BEGIN`` (required):
181The start address of the MCU's flash region. This region must NOT include the
182vector table. (i.e. if the VECTOR_TABLE is in flash, the flash region
183should begin *after* the vtable)
184
185``PW_BOOT_FLASH_SIZE`` (required):
186Size of the flash region in bytes.
187
188``PW_BOOT_RAM_BEGIN`` (required):
189The start address of the MCU's RAM region.
190
191``PW_BOOT_RAM_SIZE`` (required):
192Size of the RAM region in bytes.
193
194``PW_BOOT_VECTOR_TABLE_BEGIN`` (required):
195Address the target MCU expects the link-time vector table to be located at. This
196is typically the beginning of the flash region. While the vector table may be
197changed later in the boot process, a minimal vector table MUST be present for
198the MCU to operate as expected.
199
200``PW_BOOT_VECTOR_TABLE_SIZE`` (required):
201Number of bytes to reserve for the ARMv7-M vector table.
202
203Dependencies
204============
205  * ``pw_preprocessor`` module
206