void op_dec_addr(void)
{
  // tmp used: rax, rdi, rdx
  //
  // statically (must be preserved)
  // vm_data: rsi
  // vm_head: rbx
  // vm_tail: rcx

  // if (--(vm->vm_data) < vm->vm_head)
  asm ( "dec %rsi"           );
  asm ( "cmp %rbx, %rsi"     );
  asm ( "jl _exit2"          );

  // pos = vm->vm_data - vm->vm_head;
  asm ( "mov %rsi, %rdx"     );
  asm ( "sub %rbx, %rdx"     );
  // data_size = (vm->vm_tail - vm->vm_head) + 0x20;
  asm ( "mov %rcx, %rdi"     );
  asm ( "sub %rbx, %rdi"     );
  asm ( "add $0x20, %rdi"    );

  // save our structure
  asm ( "push %rsi"          );
  asm ( "push %rbx"          );
  asm ( "push %rcx"          );
  asm ( "push %rdi"          ); // save data_size

  // if (!(vm->vm_head = realloc(vm->vm_head, data_size)))
  asm ( "mov %rdi, %rsi"     );
  asm ( "mov %rbx, %rdi"     );
  asm ( "call realloc"       ); // realloc(rdi, rsi)

  // restore our structure
  asm ( "pop %rdi"           ); // restore data_size
  asm ( "pop %rcx"           );
  asm ( "pop %rbx"           );
  asm ( "pop %rsi"           );

  asm ( "cmp $0, %rax"       );
  asm ( "jne _continue_alloc2");
  //  _abort(ABORT_ALLOC);
  asm ( "push $1"            );
  asm ( "call _abort"        );

  asm ( "_continue_alloc2:"  );
  asm ( "mov %rax, %rbx"     );
  // vm->vm_data = vm->vm_head + 0x20 + pos;
  asm ( "mov %rbx, %rsi"     );
  asm ( "add $0x20, %rsi"    );
  asm ( "add %rdx, %rsi"     );
  // vm->vm_tail = vm->vm_head + data_size;
  asm ( "mov %rbx, %rcx"     );
  asm ( "add %rdi, %rcx"     );

  // memset(vm->vm_head, 0, 0x20);
  asm ( "_zero_memset2:"     );
  asm ( "movb $0, (%rax)"    );
  asm ( "inc %rax"           );
  asm ( "cmp %rsi, %rax"     );
  asm ( "jne _zero_memset2"  );

  asm ( "_exit2:"            );
}