Due to restrictions of the ARM EABI, uC/OS-II task stacks must be 8-byte aligned for proper printing of floating point types, in addition to using the large or full printf formatter. The printf formatter can be found in the IAR project menu under Options... > General Options > Library Options.
This can be accomplished using the data_alignment pragma, e.g.
//app variables #pragma data_aligment = 8 OS_STK myTaskStack[STACK_SIZE]; ... // prototypes static void myTask(void* unused); ... //create the task osErr = OSTaskCreateExt( myTask, (void *) 0, (OS_STK*) (&myTaskStack[STACK_SIZE-1]), PRIORITY, PRIORITY, (OS_STK*) &myTaskStack, STACK_SIZE, (void *) 0, OS_TASK_OPT_STK_CHK | OS_TASK_OPT_STK_CLR ); ...
See also http://supp.iar.com/Support/?Note=85413.