[LinuxBIOS] General question abouts jumps in machine code

[todthgie]

Hi,

X86 is a little endian machine so e9 bd ff  -> JMP relative 0xFFBD or JMP relative -0x43 (i might be of by one ...)  = -67 decimal (64 for the nops and 3 for the e9 bd ff ) its is relative to the next instruction if i remember right ...

greetings,
todthgie

  ----- Original Message ----- 
  From: Shadravan Fontanov 
  To: linuxbios at linuxbios.org 
  Sent: Friday, June 29, 2007 19:03
  Subject: [LinuxBIOS] General question abouts jumps in machine code


  Hello,

  I wanted to verify that the relative jump in my program is properly transalated into machine code.
  Consider following example:
  <code begins>
  extern char handler[], endhandler[];   /* C-code glue for the asm insert */
  asm(
      ".data\n"
      ".code16\n"
      ".globl handler, endhandler\n"
      "\n"
      "handler:\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
      "jmp handler\n"
      "endhandler:\n"
      "\n"
      ".text\n"
      ".code32\n"
  );

  int main(void) 
  {
      return 0;
  }
  <code ends>

  There are 64 NOPs, on the basis of these I want to locate my jmp instruction after compiling. I am interested about "jmp handler\n"-instruction.

  I compile the example above, then start
   hexdump -vC compiled_example | less
  and look for 64 times of 0x90 (opcode for NOP). I find them:
  <snippet of hexdump starts>
  00005740  00 00 00 00 00 00 00 00  84 e5 04 08 90 90 90 90  |................|
  00005750  90 90 90 90 90 90 90 90  90 90 90 90 90 90 90 90  |................|
  00005760  90 90 90 90 90 90 90 90  90 90 90 90 90 90 90 90  |................|
  00005770  90 90 90 90 90 90 90 90  90 90 90 90 90 90 90 90  |................|
  00005780  90 90 90 90 90 90 90 90  90 90 90 90 e9 bd ff 00  |................|
  00005790  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
  <snippet of hexdump end>
  Exactly 64 times 0x90, fine... The next code after the last 0x90 is 0xe9. Look at "Intel Architecture Software Developers Manual Vol2" tells: its jump instruction, the next two bytes (bd ff) specify the relative address to jump. bdff must be  in second complement and  represent  minus 4201. But offset 4201 does not jumps to the start of the NOP sequence, it jumps to a very smaller address...

  Do you have any hints?

  Regards

  Shadravan




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