1.. _module-pw_boot: 2 3------- 4pw_boot 5------- 6Pigweed's boot module should provide a linker script and some early 7initialization of static memory regions and C++ constructors. This is enough to 8get many targets booted and ready to run C++ code. 9 10This module is split into two components: 11 121. The facade (this module) which declares the symbols exported by the backend 132. The backend, provided elsewhere, that provides the implementation 14 15.. warning:: 16 This module is currently NOT stable! Depending on this module may cause 17 breakages as this module is updated. 18 19Sequence 20======== 21The high level pw_boot boot sequence looks like the following pseudo-code 22invocation of the user-implemented functions: 23 24.. code:: cpp 25 26 void pw_boot_Entry() { // Boot entry point provided by backend. 27 pw_boot_PreStaticMemoryInit(); // User-implemented function. 28 // Static memory initialization. 29 pw_boot_PreStaticConstructorInit(); // User-implemented function. 30 // C++ static constructors are invoked. 31 pw_boot_PreMainInit(); // User-implemented function. 32 main(); // User-implemented function. 33 pw_boot_PostMain(); // User-implemented function. 34 PW_UNREACHABLE; 35 } 36 37Setup 38===== 39 40User-Implemented Functions 41-------------------------- 42This module expects all of these extern "C" functions to be defined outside this 43module: 44 45 - ``int main()``: This is where applications reside. 46 - ``void pw_boot_PreStaticMemoryInit()``: This function executes just before 47 static memory has been zeroed and static data is intialized. This function 48 should set up any early initialization that should be done before static 49 memory is initialized, such as: 50 51 - Enabling the FPU or other coprocessors. 52 - Opting into extra restrictions such as disabling unaligned access to ensure 53 the restrictions are active during static RAM initialization. 54 - Initial CPU clock, flash, and memory configurations including potentially 55 enabling extra memory regions with .bss and .data sections, such as SDRAM 56 or backup powered SRAM. 57 - Fault handler initialization if required before static memory 58 initialization. 59 60 .. warning:: 61 Code running in this hook is violating the C spec as static values are not 62 yet initialized, meaning they have not been loaded (.data) nor 63 zero-initialized (.bss). 64 65 - ``void pw_boot_PreStaticConstructorInit()``: This function executes just 66 before C++ static constructors are called. At this point, other static memory 67 has been zero or data initialized. This function should set up any early 68 initialization that should be done before C++ static constructors are run, 69 such as: 70 71 - Run time dependencies such as Malloc, and ergo sometimes the RTOS. 72 - Persistent memory that survives warm reboots. 73 - Enabling the MPU to catch nullptr dereferences during construction. 74 - Main stack watermarking. 75 - Further fault handling configuration necessary for your platform which 76 were not safe before pw_boot_PreStaticRamInit(). 77 - Boot count and/or boot session UUID management. 78 79 - ``void pw_boot_PreMainInit()``: This function executes just before main, and 80 can be used for any device initialization that isn't application specific. 81 Depending on your platform, this might be turning on a UART, setting up 82 default clocks, etc. 83 84 - ``PW_NO_RETURN void pw_boot_PostMain()``: This function executes after main 85 has returned. This could be used for device specific teardown such as an 86 infinite loop, soft reset, or QEMU shutdown. In addition, if relevant for 87 your application, this would be the place to invoke the global static 88 destructors. This function must not return! 89 90 91If any of these functions are unimplemented, executables will encounter a link 92error. 93 94Backend-Implemented Functions 95----------------------------- 96Backends for this module must implement the following extern "C" function: 97 98 - ``void pw_boot_Entry()``: This function executes as the entry point for 99 the application, and must perform call the user defined methods in the 100 appropriate sequence for the target (see Sequence above). 101 102Dependencies 103============ 104 * ``pw_preprocessor`` module 105