From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nicolas Bock <nicolasbock@gmail.com>
Subject: writing a jump table
Date: Tue, 15 Mar 2011 18:10:05 -0600
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Hello list,

I am trying to write a jump table, but unfortunately with limited
success. When I compile the code and disassemble it, the offset of
"table" is 0, which I guess means that something didn't work out. Any
help would be gratefully appreciated.

The assembly code, jump_table.S:

  .text
  .global jump_table
  .type jump_table, @function

jump_table:
  push %rax

  mov $0x02, %rax # Move index into rax; 2 is supposed to end up at
label_02.
  jmp *table(, %rax, 4) # Jump into the table.

  .align 8
table:
  .long label_00
  .long label_01
  .long label_02

label_00:
  nop

label_01:
  nop

label_02:
  nop

done:
  pop %rax
  ret

  .size jump_table, .-jump_table


compiled with: gcc -c -g -o jump_table.o jump_table.S

disassembled code:

(gdb) disassemble jump_table
Dump of assembler code for function jump_table:
   0x0000000000000000 <+0>:     push   %rax
   0x0000000000000001 <+1>:     mov    $0x2,%rax
   0x0000000000000008 <+8>:     jmpq   *0x0(,%rax,4)
   0x000000000000000f <+15>:    nop
   0x0000000000000010 <+16>:    add    %al,(%rax)
   0x0000000000000012 <+18>:    add    %al,(%rax)
   0x0000000000000014 <+20>:    add    %al,(%rax)
   0x0000000000000016 <+22>:    add    %al,(%rax)
   0x0000000000000018 <+24>:    add    %al,(%rax)
   0x000000000000001a <+26>:    add    %al,(%rax)
   0x000000000000001c <+0>:     nop
   0x000000000000001d <+0>:     nop
   0x000000000000001e <+0>:     nop
   0x000000000000001f <+0>:     pop    %rax
   0x0000000000000020 <+1>:     retq
End of assembler dump.



Thanks already, nick


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