Add a function to transfer control to another app

Add the function mbed_start_application() which allows a bootloader to
transfer control to an application.

mbed.h

@@ -49,6 +49,7 @@
 
 #include "platform/toolchain.h"
 #include "platform/platform.h"
+#include "platform/mbed_application.h"
 
 // Useful C libraries
 #include <math.h>

platform/mbed_application.c

@@ -0,0 +1,127 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2017-2017 ARM Limited
+ *
+ * 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
+ *
+ *     http://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.
+ */
+
+#include <stdlib.h>
+#include <stdarg.h>
+#include "device.h"
+#include "platform/mbed_application.h"
+
+#if MBED_APPLICATION_SUPPORT
+
+static void powerdown_nvic(void);
+static void powerdown_scb(uint32_t vtor);
+static void start_new_application(void *sp, void *pc);
+
+void mbed_start_application(uintptr_t address)
+{
+    void *sp;
+    void *pc;
+
+    // Interrupts are re-enabled in start_new_application
+    __disable_irq();
+
+    SysTick->CTRL = 0x00000000;
+    powerdown_nvic();
+    powerdown_scb(address);
+
+    sp = *((void**)address + 0);
+    pc = *((void**)address + 1);
+    start_new_application(sp, pc);
+}
+
+static void powerdown_nvic()
+{
+    int isr_count;
+    int i;
+    int j;
+
+    isr_count = (SCnSCB->ICTR & SCnSCB_ICTR_INTLINESNUM_Msk) >> SCnSCB_ICTR_INTLINESNUM_Pos;
+    for (i = 0; i < isr_count; i++) {
+        NVIC->ICER[i] = 0xFFFFFFFF;
+        NVIC->ICPR[i] = 0xFFFFFFFF;
+        for (j = 0; j < 8; j++) {
+            NVIC->IP[i * 8 + j] = 0x00000000;
+        }
+    }
+}
+
+static void powerdown_scb(uint32_t vtor)
+{
+    int i;
+
+    // SCB->CPUID   - Read only CPU ID register
+    SCB->ICSR = SCB_ICSR_PENDSVCLR_Msk | SCB_ICSR_PENDSTCLR_Msk;
+    SCB->VTOR = vtor;
+    SCB->AIRCR = 0x05FA | 0x0000;
+    SCB->SCR = 0x00000000;
+    // SCB->CCR     - Implementation defined value
+    for (i = 0; i < 12; i++) {
+#if defined(__CORTEX_M7)
+        SCB->SHPR[i] = 0x00;
+#else
+        SCB->SHP[i] = 0x00;
+#endif
+    }
+    SCB->SHCSR = 0x00000000;
+    SCB->CFSR = 0xFFFFFFFF;
+    SCB->HFSR = SCB_HFSR_DEBUGEVT_Msk | SCB_HFSR_FORCED_Msk | SCB_HFSR_VECTTBL_Msk;
+    SCB->DFSR = SCB_DFSR_EXTERNAL_Msk | SCB_DFSR_VCATCH_Msk |
+                SCB_DFSR_DWTTRAP_Msk | SCB_DFSR_BKPT_Msk | SCB_DFSR_HALTED_Msk;
+    // SCB->MMFAR   - Implementation defined value
+    // SCB->BFAR    - Implementation defined value
+    // SCB->AFSR    - Implementation defined value
+    // SCB->PFR     - Read only processor feature register
+    // SCB->DFR     - Read only debug feature registers
+    // SCB->ADR     - Read only auxiliary feature registers
+    // SCB->MMFR    - Read only memory model feature registers
+    // SCB->ISAR    - Read only instruction set attribute registers
+    // SCB->CPACR   - Implementation defined value
+}
+
+#if defined (__CC_ARM)
+
+__asm static void start_new_application(void *sp, void *pc)
+{
+    MOV R2, #0
+    MSR CONTROL, R2         // Switch to main stack
+    MOV SP, R0
+    MSR PRIMASK, R2         // Enable interrupts
+    BX R1
+}
+
+#elif defined (__GNUC__) || defined (__ICCARM__)
+
+void start_new_application(void *sp, void *pc)
+{
+    __asm volatile (
+        "mov    r2, #0      \n"
+        "msr    control, r2 \n" // Switch to main stack
+        "mov    sp, %0      \n"
+        "msr    primask, r2 \n" // Enable interrupts
+        "bx     %1          \n"
+        :
+        : "l" (sp), "l" (pc)
+        : "r2", "cc", "memory"
+    );
+}
+
+#else
+
+#error "Unsupported toolchain"
+
+#endif
+
+#endif /* MBED_APPLICATION_SUPPORT */

platform/mbed_application.h

@@ -0,0 +1,50 @@
+
+/** \addtogroup platform */
+/** @{*/
+/* mbed Microcontroller Library
+ * Copyright (c) 2017-2017 ARM Limited
+ *
+ * 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
+ *
+ *     http://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.
+ */
+#ifndef MBED_APPLICATION_H
+#define MBED_APPLICATION_H
+
+#include<stdint.h>
+
+#define MBED_APPLICATION_SUPPORT (defined(__CORTEX_M3) || defined(__CORTEX_M4) || defined(__CORTEX_M7))
+#if MBED_APPLICATION_SUPPORT
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ *  Start the application at the given address. This function does
+ *  not return. It is the applications responsibility for flushing to
+ *  or powering down external components such as filesystems or
+ *  socket connections before calling this function. For Cortex-M
+ *  devices this function powers down generic system components such as
+ *  the NVIC and set the vector table to that of the new image followed
+ *  by jumping to the reset handler of the new image.
+ *
+ *  @param address    Starting address of next application to run
+ */
+void mbed_start_application(uintptr_t address);
+
+#ifdef __cplusplus
+}
+#endif
+#endif
+
+#endif
+
+/** @}*/