Re: [rfc] 16 bit real-mode for the i386

[Daniel Jacobowitz <drow at mvista dot com>]

On Thu, Aug 29, 2002 at 04:30:11PM -0400, Andrew Cagney wrote:
> Hello,
> 
> The attached modifies i386-tdep.c so that there is a ``set i386 
> real-mode'' command (doco to go).
> 
> When real-mode is enabled, GDB computes the ``20 bit'' ``stop address'' 
> (aka PC but not to be confused with $pc ... :-) from both the $cs and 
> $pc registers.  That way, core GDB sees a cannonical PC address that 
> (regardless of $cs) will match a ``20 bit'' breakpoint address.
> 
> Thoughts?
> 
> I'm desperatly trying to come up with a test-case mind.  This is a 
> rewrite of an old old patch (that hacked breakpoint.c) and the original 
> testcase has been lost :-(
> 
> Andrew

Now, my i386 knowledge is a bit rusty.  But:

> +static CORE_ADDR
> +i386_read_pc (ptid_t ptid)
> +{
> +  CORE_ADDR pc = read_register_pid (PC_REGNUM, ptid);
> +  if (i386_real_mode_p)
> +    {
> +      CORE_ADDR cs = read_register_pid (CS_REGNUM, ptid);
> +      pc = pc + (cs << 4);
> +    }
> +  return pc;
> +}
> +
> +static void
> +i386_write_pc (CORE_ADDR pc, ptid_t ptid)
> +{
> +  if (i386_real_mode_p)
> +    {
> +      CORE_ADDR cs = read_register_pid (CS_REGNUM, ptid);
> +      pc = pc - (cs << 4);
> +    }
> +  write_register_pid (PC_REGNUM, pc, ptid);
> +}
> +

Left shift of _four_?  Surely the PC is more than four bits.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer