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Why program switches from thumb to ARM

Note: This was originally posted on 24th November 2011 at http://forums.arm.com

Hi Everyone

I've got beginner question (hope this is right place to do it, if not please direct me). I 've prepared whole environment for my STM32F103VCT6 Cortex-M3 processor using Sourcery G++ Lite, Eclipse and Segger GBD Server. I have problem with debugging, for simplest possible program it is available and working, but for more "complicated" like new function it stops. My compilation flags are:

C_FLAGS := -c -ggdb -mcpu=cortex-m3 -mthumb

my script linker:

MEMORY 
{ 
   FLASH : ORIGIN = 0x08000000, LENGTH = 256K 
   RAM  : ORIGIN = 0x20000000, LENGTH = 48K 
} 

SECTIONS 
{ 
   .text : 
   { 
      KEEP(*(.interrupt_vector)) 
      *(.text) 
   } > FLASH 
    
   .data : 
   { 
      _data_begin_ = .; 
      *(.data) 
      _data_end_ = .; 
   } > FLASH 
    
   .bss : 
   { 
      _bss_begin_ = .; 
      *(.bss) 
      _bss_end_ = .; 
   } > RAM 
    
   _stack_size_ = 1024; 
   _stack_end_ = ORIGIN(RAM) + LENGTH(RAM); 
   _stack_begin_ = _stack_end_ - _stack_size_; 
    
   ._stack : 
   { 
      . = . + _stack_size_; 
   } > RAM 
} 

ENTRY(handler_reset)

my startup code:

extern unsigned long _stack_end_;
 
typedef void (*interrupt_routine)( void );
void handler_reset( void );
void handler_default( void );
 
// vector interrupt table
__attribute__ ( ( section( ".interrupt_vector" ) ) )
interrupt_routine routines[] =
{
 (void*) &_stack_end_,
 handler_reset,
 handler_default,
 // ... itd...
};
 
 
void handler_reset( void )
{
 main();
}
 
void handler_default( void )
{
 while ( 1 )
 {
 }
}

and my program (working):

int main( void )
{
 int  a = 3;
 int  b = 2;
 int  c = 3 + 2;
 
 a = c;
 
 while( 1 )
 {
  // infinite loop
 }
 
 return 0;
}

and my program (no working, added aa() function):

void aa()
{
}
 
int main( void )
{
 int  a = 3;
 int  b = 2;
 int  c = 3 + 2;
 
 a = c;
 aa();
 
 while( 1 )
 {
  // infinite loop
 }
 
 return 0;
}

I've got message from Segger debugger:

Setting breakpoint @ address 0x08000034, Size = 2, BPHandle = 0x0023
Starting target CPU... WARNING: T-bit of XPSR is 0 but should be 1. Changed to 1. ...Target halted (PC = 0xF04FAF00)
Reading all registers
Removing breakpoint @ address 0x08000034, Size = 2

Does it mean that my program switches from thumb to arm? If so, why it is done?