Re: Little Big Mode

From	CN <qmbmnp3799@pacbell.net>
Newsgroups	alt.lang.asm, alt.os.assembly, alt.os.development
Subject	Re: Little Big Mode
Date	2016-06-15 03:08 -0700
Organization	A noiseless patient Spider
Message-ID	<njr9e3$5js$1@dont-email.me> (permalink)
References	<nj3hgq$1nvk$1@gioia.aioe.org>

Cross-posted to 3 groups.

Show all headers | View raw

On 6/6/2016 2:58 AM, Mike Gonta wrote:
> Apparently when using 32 bit instructions in real mode on a
> modern PC the 24 bit address calculation (segment register *
> 16 + offset) utilizes 24 bits of the offset register.
> This is an interesting _discovery_.
> By using 32 bit instructions and setting the segment
> registers to zero the entire 1MB *flat* address space is
> accessible in real mode.

I didn't follow the links, but this sounds wrong. In fact, the range of 
memory you can access in real mode with 32-bit instructions depends on 
the segment limit. By default, segment limits are 64KB in real mode, so 
when you attempt to use an offset that is greater than 64KB, you get a 
GPF. The trick is, therefore, to set the segment limits to 4GB by 
entering protected mode, loading the segment registers from a GDT with 
4GB limits, then going back to real mode. Voila, you can access the 
entire 32-bit address space from real mode using instructions with the 
"address size" prefix. This is called "big real mode" or "unreal mode".

Back to alt.lang.asm | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

Little Big Mode "Mike Gonta" <mikegonta@gmail.com> - 2016-06-06 05:58 -0400
  Re: Little Big Mode Cecil Bayona <cbayona@cbayona.com> - 2016-06-06 10:00 -0500
    Re: Little Big Mode "wolfgang kern" <nowhere@never.at> - 2016-06-14 09:45 +0200
      Re: Little Big Mode Cecil Bayona <cbayona@cbayona.com> - 2016-06-14 10:08 -0500
        Re: Little Big Mode "wolfgang kern" <nowhere@never.at> - 2016-06-14 21:18 +0200
      Re: Little Big Mode Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-06-14 17:14 -0400
        Re: Little Big Mode "wolfgang kern" <nowhere@never.at> - 2016-06-15 10:00 +0200
  Re: Little Big Mode JJ <jj4public@vfemail.net> - 2016-06-07 13:21 +0700
  Re: Little Big Mode CN <qmbmnp3799@pacbell.net> - 2016-06-15 03:08 -0700

csiph-web