[NUC472/M453] Refine comment for two-region model

Author: ccli8

platform/retarget.cpp

@@ -574,7 +574,10 @@ register unsigned char * stack_ptr __asm ("sp");
 
 // Dynamic memory allocation related syscall.
 #if defined(TARGET_NUMAKER_PFM_NUC472) || defined(TARGET_NUMAKER_PFM_M453)
-// Overwrite _sbrk() to support two region model.
+// Overwrite _sbrk() to support two region model (heap and stack are two distinct regions).
+// __wrap__sbrk() is implemented in:
+// TARGET_NUMAKER_PFM_NUC472    hal/targets/cmsis/TARGET_NUVOTON/TARGET_NUC472/TARGET_NUMAKER_PFM_NUC472/TOOLCHAIN_GCC_ARM/retarget.c
+// TARGET_NUMAKER_PFM_M453      hal/targets/cmsis/TARGET_NUVOTON/TARGET_M451/TARGET_NUMAKER_PFM_M453/TOOLCHAIN_GCC_ARM/retarget.c
 extern "C" void *__wrap__sbrk(int incr);
 extern "C" caddr_t _sbrk(int incr) {
     return (caddr_t) __wrap__sbrk(incr);

targets/TARGET_NUVOTON/TARGET_NUC472/device/TOOLCHAIN_GCC_ARM/retarget.c

@@ -15,9 +15,11 @@
 extern uint32_t __mbed_sbrk_start;
 extern uint32_t __mbed_krbs_start;
 
-// NOTE: The implementation of _sbrk (in common/retarget.cpp) for GCC_ARM requires one-region model for heap and stack, which doesn't
-//       meet the layout of e.g. Nu-mbed-NUC472 board where heap is located on external SRAM. 
-//       Because the symbol is not weak and cannot be overwritten, wrap calls to it by hooking command line linker with '-Wl,--wrap,_sbrk'.
+/**
+ * The default implementation of _sbrk() (in common/retarget.cpp) for GCC_ARM requires one-region model (heap and stack share one region), which doesn't
+ * fit two-region model (heap and stack are two distinct regions), for example, NUMAKER-PFM-NUC472 locates heap on external SRAM. Define __wrap__sbrk() to
+ * override the default _sbrk(). It is expected to get called through gcc hooking mechanism ('-Wl,--wrap,_sbrk') or in _sbrk().
+ */
 void *__wrap__sbrk(int incr)
 {
     static uint32_t heap_ind = (uint32_t) &__mbed_sbrk_start;

targets/cmsis/TARGET_NUVOTON/TARGET_M451/TARGET_NUMAKER_PFM_M453/TOOLCHAIN_GCC_ARM/retarget.c

@@ -15,9 +15,11 @@
 extern uint32_t __mbed_sbrk_start;
 extern uint32_t __mbed_krbs_start;
 
-// NOTE: The implementation of _sbrk (in common/retarget.cpp) for GCC_ARM requires one-region model for heap and stack, which doesn't
-//       meet the layout of e.g. Nu-mbed-NUC472 board where heap is located on external SRAM. 
-//       Because the symbol is not weak and cannot be overwritten, wrap calls to it by hooking command line linker with '-Wl,--wrap,_sbrk'.
+/**
+ * The default implementation of _sbrk() (in common/retarget.cpp) for GCC_ARM requires one-region model (heap and stack share one region), which doesn't
+ * fit two-region model (heap and stack are two distinct regions), for example, NUMAKER-PFM-NUC472 locates heap on external SRAM. Define __wrap__sbrk() to
+ * override the default _sbrk(). It is expected to get called through gcc hooking mechanism ('-Wl,--wrap,_sbrk') or in _sbrk().
+ */
 void *__wrap__sbrk(int incr)
 {
     static uint32_t heap_ind = (uint32_t) &__mbed_sbrk_start;