Created Boot Process (markdown)

Boot-Process.md

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+The boot process is a bit of madness, but justifiable madness.
+
+### Overview
+
+The LPC4320 bootloader initializes the Cortex-M4F core to boot from the start of external SPI flash. The M0 core stays in reset. The [bootstrap code](https://github.com/sharebrained/portapack-hackrf/blob/master/firmware/bootstrap/bootstrap.c) runs from SPI flash, on the Cortex-M4F. The bootstrap initializes the Cortex-M0 to execute the application code from SPI flash, then sleeps. The application code copies the baseband code into RAM, configures the Cortex-M4F to run from RAM, then resets the Cortex-M4F to begin baseband execution.
+
+(TODO: A diagram would be helpful, showing the M4F and M0 activities vs. time.)
+
+### Bootstrap
+
+In the PortaPack image, the Cortex-M4F "bootstrap" image is located at the start of SPI flash. It's executed by the LPC4320 built-in bootloader. The M4 clock is already set to 96MHz. The bootstrap code configures SPIFI to run at (approximately) maximum speed. Then, it initializes the Cortex-M0's memory map to point at the "application" image in SPI flash, and releases the Cortex-M0 from reset. The bootstrap then sleeps the Cortex-M4 until the M0 application needs to run baseband firmware on it.
+
+### Application
+
+On the Cortex-M0 core, boot time looks like this:
+
+    ResetHandler:
+        Initialize process stack pointer
+        Initialize stack RAM regions (fill with pattern)
+        __early_init()
+            Enable extra processor exceptions for debugging
+        Init data segment (copy SPI flash -> data region in RAM)
+        Initialize BSS (fill RAM region with 0)
+        __late_init()
+            reset()
+                Reset most peripherals -- not SCU, SPIFI, or M0APP
+            halInit()
+                hal_lld_init()
+                    Init timer 3 as cycle counter (no DWT on M0)
+                    Init RIT as SysTick (no SysTick on M0)
+                palInit()
+                gptInit()
+                i2cInit()
+                sdcInit()
+                spiInit()
+                rtcInit()
+                boardInit()
+            chSysInit()
+                port_init()
+                _scheduler_init()
+                _vt_init()
+                _core_init()
+                _heap_init()
+                chSysEnable()
+                chThdCreateStatic(_idle_thread_wa, ...)
+        Constructors
+        main()
+        Destructors
+        _default_exit()
+            while(1);
+
+### Baseband
+
+On the Cortex-M4F core, these are the stages a baseband image moves through:
+
+    ResetHandler:
+        Initialize process stack pointer
+        Initialize FPU
+        Initialize stack RAM regions (fill with pattern)
+        __early_init()
+            Enable extra processor exceptions for debugging
+        Init data segment (copy SPI flash -> data region in RAM)
+        Initialize BSS (fill RAM region with 0)
+        __late_init()
+            halInit()
+                hal_lld_init()
+                    Init SysTick
+                    Init DWT as cycle counter
+                # Baseband controls no hardware, so no hardware init here.
+                boardInit()
+            chSysInit()
+                port_init()
+                _scheduler_init()
+                _vt_init()
+                _core_init()
+                _heap_init()
+                chSysEnable()
+                chThdCreateStatic(_idle_thread_wa, ...)
+        Constructors
+        main()
+        Destructors
+        _default_exit()
+            while(1);
+
+##
+Original Wiki by sharebrained at [Boot Process](https://github.com/sharebrained/portapack-hackrf/wiki/Boot-Process)