From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gdb-patches-return-18253-listarch-gdb-patches=sourceware.cygnus.com@sources.redhat.com>
Received: (qmail 18912 invoked by alias); 29 Aug 2002 21:46:04 -0000
Mailing-List: contact gdb-patches-help@sources.redhat.com; run by ezmlm
Precedence: bulk
List-Subscribe: <mailto:gdb-patches-subscribe@sources.redhat.com>
List-Archive: <http://sources.redhat.com/ml/gdb-patches/>
List-Post: <mailto:gdb-patches@sources.redhat.com>
List-Help: <mailto:gdb-patches-help@sources.redhat.com>, <http://sources.redhat.com/ml/#faqs>
Sender: gdb-patches-owner@sources.redhat.com
Received: (qmail 18890 invoked from network); 29 Aug 2002 21:46:03 -0000
Received: from unknown (HELO localhost.redhat.com) (216.138.202.10)
  by sources.redhat.com with SMTP; 29 Aug 2002 21:46:03 -0000
Received: from ges.redhat.com (localhost [127.0.0.1])
	by localhost.redhat.com (Postfix) with ESMTP
	id F09B33F38; Thu, 29 Aug 2002 17:46:00 -0400 (EDT)
Message-ID: <3D6E9618.2070609@ges.redhat.com>
Date: Thu, 29 Aug 2002 14:46:00 -0000
From: Andrew Cagney <ac131313@ges.redhat.com>
User-Agent: Mozilla/5.0 (X11; U; NetBSD macppc; en-US; rv:1.0.0) Gecko/20020824
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: Daniel Jacobowitz <drow@mvista.com>
Cc: gdb-patches@sources.redhat.com
Subject: Re: [rfc] 16 bit real-mode for the i386
References: <3D6E8453.5080306@ges.redhat.com> <20020829213653.GA24048@nevyn.them.org>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
X-SW-Source: 2002-08/txt/msg01003.txt.bz2


> 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.

It is.  From an ia32 manual (www.intel.com):

16.1.1.    Address Translation in Real-Address Mode
In real-address mode, the processor does not interpret segment selectors 
as indexes into a descriptor table; instead, it uses them directly to 
form linear addresses as the 8086 processor does. It shifts the segment 
selector left by 4 bits to form a 20-bit base address (see Figure 16-1).
The offset into a segment is added to the base address to create a 
linear address that maps directly
to the physical address space.

Andrew