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

[Daniel Jacobowitz <drow@mvista.com>]

On Thu, Aug 29, 2002 at 02:43:04PM -0700, Kevin Buettner wrote:
> On Aug 29,  5:36pm, Daniel Jacobowitz wrote:
> 
> > 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.
> 
> I think that's right.  My (hazy) recollection from my days of programming
> x86 before there was an 80386 is that an address was formed by shifting
> the segment registers left by four bits and adding some other sixteen bit
> register to it.

That's positively perverse.  Thanks.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer