Atmel Studio executes instructions outside my source files - debugging

Question

I'm trying to write and debug a C program for an ATMega8 device using the Atmel Studio Simulator.
For example, let's say I'm trying to debug this piece of code:
int main(void)
{
while(1)
{
SPI_transaction(0xFF);
}
}
uint8_t SPI_transaction(unsigned char byte_to_send){
value = byte_to_send;
SPDR = byte_to_send; //Sends command
while (checkBitStatus(SPSR,SPIF) == 0){}; //Do nothing until transfer is completed
value = SPDR; //Receives command
CLEARBIT(SPSR, SPIF);
return value;
}
unsigned char checkBitStatus(unsigned char byte, unsigned char bit){
unsigned char bit_status;
if ((byte & (1 << bit)) == 0){
bit_status = 0;
}
else{
bit_status = 1;
}
return bit_status;
}
This code buids without any problem but when I try to debug this happens:
At some points the program counter goes outside my source files. I've been trying to find out why this happens but I have ot been able yet to find an answer. I leave here the disassembly code for you to have a look. I find it really strange.
Init of main function:
00000024 PUSH R28 Push register on stack
00000025 PUSH R29 Push register on stack
00000026 IN R28,0x3D In from I/O location
00000027 IN R29,0x3E In from I/O location
SPI_transaction(0xFF);
00000028 SER R24 Set Register
00000029 RCALL PC+0x0002 Relative call subroutine
uint8_t SPI_transaction(unsigned char byte_to_send){
0000002B PUSH R28 Push register on stack
0000002C PUSH R29 Push register on stack
0000002D PUSH R1 Push register on stack
0000002E IN R28,0x3D In from I/O location
0000002F IN R29,0x3E In from I/O location
00000030 STD Y+1,R24 Store indirect with displacement
while (checkBitStatus(SPSR,SPIF) == 0){}; //Do nothing until transfer is completed
00000039 NOP No operation
--- No source file -------------------------------------------------------------
0000003A LDI R24,0x2E Load immediate
0000003B LDI R25,0x00 Load immediate
0000003C MOVW R30,R24 Copy register pair
--- No source file -------------------------------------------------------------
0000003D LDD R24,Z+0 Load indirect with displacement
0000003E LDI R22,0x07 Load immediate
0000003F RCALL PC+0x0018 Relative call subroutine
From that it jumps here:
unsigned char checkBitStatus(unsigned char byte, unsigned char bit){
00000057 PUSH R28 Push register on stack
00000058 PUSH R29 Push register on stack
00000059 RCALL PC+0x0001 Relative call subroutine
0000005A PUSH R1 Push register on stack
0000005B IN R28,0x3D In from I/O location
0000005C IN R29,0x3E In from I/O location
0000005D STD Y+2,R24 Store indirect with displacement
0000005E STD Y+3,R22 Store indirect with displacement
if ((byte & (1 << bit)) == 0){
0000005F LDD R24,Y+2 Load indirect with displacement
00000060 MOV R24,R24 Copy register
00000061 LDI R25,0x00 Load immediate
00000062 LDD R18,Y+3 Load indirect with displacement
00000063 MOV R18,R18 Copy register
00000064 LDI R19,0x00 Load immediate
00000065 MOV R0,R18 Copy register
00000066 RJMP PC+0x0003 Relative jump
00000067 ASR R25 Arithmetic shift right
00000068 ROR R24 Rotate right through carry
00000069 DEC R0 Decrement
0000006A BRPL PC-0x03 Branch if plus
0000006B ANDI R24,0x01 Logical AND with immediate
0000006C CLR R25 Clear Register
0000006D SBIW R24,0x00 Subtract immediate from word
0000006E BRNE PC+0x03 Branch if not equal
bit_status = 0;
0000006F STD Y+1,R1 Store indirect with displacement
00000070 RJMP PC+0x0003 Relative jump
bit_status = 1;
00000071 LDI R24,0x01 Load immediate
00000072 STD Y+1,R24 Store indirect with displacement
return bit_status;
00000073 LDD R24,Y+1 Load indirect with displacement
}
00000074 POP R0 Pop register from stack
00000075 POP R0 Pop register from stack
00000076 POP R0 Pop register from stack
00000077 POP R29 Pop register from stack
00000078 POP R28 Pop register from stack
00000079 RET Subroutine return
The function returns to somewhere out my source files:
--- No source file -------------------------------------------------------------
00000040 TST R24 Test for Zero or Minus
00000041 BREQ PC-0x07 Branch if equal
0000003A LDI R24,0x2E Load immediate
0000003B LDI R25,0x00 Load immediate
0000003C MOVW R30,R24 Copy register pair
0000003D LDD R24,Z+0 Load indirect with displacement
0000003E LDI R22,0x07 Load immediate
0000003F RCALL PC+0x0018 Relative call subroutine
And the goes again to CheckBitStatus() function.
Any help will be really appreciated.
Alex

Answer 1

The code executed outside your source is most likely an interrupt service routine for a hardware interrupt handled by an atmel or c standard library. These routinely occur for stuff like USB events that must be handled asynchronously.

Answer 2

Another simple reason for the debugger to display the assembly view and the message "no source file found" , if you attempt to use the debugger while the configuration manager (Under Build tap) has the configuration option set for release, the program compilation will go to the release directory and not to the debug directory.