SCB->SHPR3 = 0
            | (3 << 22)     // PendSV priority = 3, lowest
            | (2 << 30)     // SysTick priority = 2, little higher
           ;

#define CORE_PRIORITY_BITS  2

#if (!defined CORE_PRIORITY_BITS)
#    define SYS_TIMER_PRIO (0xFF)
#else
#    define SYS_TIMER_PRIO (0xFE << (8-(CORE_PRIORITY_BITS)))
#endif

#define CORE_PRIORITY_BITS  2

" LDR R1, =os_context_switch_hook \n" // call os_context_switch_hook();
" BLX R1                          \n"

" BL =os_context_switch_hook \n" // call os_context_switch_hook();

#pragma weak PendSVC_ISR = PendSV_Handler

Invoking: Cross ARM GNU C++ Linker
arm-none-eabi-g++ -mcpu=cortex-m3 -mthumb -Os -fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -ffreestanding
-flto -Wunused -Wuninitialized -Wall -Wextra -Wmissing-declarations -Wconversion -Wpointer-arith -Wpadded -Wshadow -Wlogical-op
-Waggregate-return -Wfloat-equal  -g -T mem.ld -T libs.ld -T sections.ld -nostartfiles -Xlinker --gc-sections -L"../ldscripts"
-Wl,-Map,"scmRTOS.map" --specs=nano.specs -o "scmRTOS.elf"  
./src/src/_cxx.o ./src/src/_exit.o ./src/src/_sbrk.o ./src/src/_syscalls.o ./src/src/assert.o  ./src/_write.o ./src/main.o ./src/os_kernel.o
./src/os_services.o ./src/os_target.o ./src/recursive_mutex.o ./src/startup.o ./src/sysinit.o ./src/usrlib.o  

C:\Users\ESAULE~1\AppData\Local\Temp\ccEIkvE6.ltrans0.ltrans.o: In function `PendSV_Handler':
<artificial>:(.text.PendSV_Handler+0x1c): undefined reference to `os_context_switch_hook'
C:\Users\ESAULE~1\AppData\Local\Temp\ccEIkvE6.ltrans0.ltrans.o: In function `Reset_Handler':
<artificial>:(.text.Reset_Handler+0x190): undefined reference to `__init_system_timer'

extern "C" {
    extern stack_item_t* os_context_switch_hook(stack_item_t* sp)    __attribute__((used));
    extern void __init_system_timer()                                 __attribute__((used));
};