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Re: Little Big Mode

Cross-posted to 3 groups.

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Cecil Bayona wrote:

>>> Let me start by saying that I know zip about Intel Assembler code on
>>> modern CPUs (used to work with the I8085 CPU eons ago) but I'm
>>> interested in the subject.

>>> I will soon need to write a preamble to a compilers output to set
>>> things right for the code to follow which is in Protected mode code.
>>> Eventually I will be looking for a snippet of code to setup the CPU in
>>> Protected Mode, setup a stack and then proceed to run the compiler code.

>>> At this point I know nothing on the subject and I don't even know if
>>> other than setting up the Stack Pointer is needed in a Windows
>>> program, later today I will be looking for books so I can learn about
>>> the subject of Intel Real/Protected mode assembly language.

>>> I read through the post and the responses and it seems that in
>>> accessing 1 MB in Real Mode the behavior is not consistent on every
>>> CPU, so my question will display my less than newbie status. Are there
>>> instructions or registers in the CPU/MMU that can be used to make sure
>>> all machines work by setting the segment length to be 1 MB in Real Mode?

>> After checking I can say that all (386+) Intel/AMD CPUs work identical
>> in TrueRealMode, that mean segments are limited to 64Kb (aka 16bit).
>> There aren't single instructions to override this limits, but we got the
>> well known trick to have Unreal Mode:

>> 1. check if A20 is ON (only 1 MB available if OFF) 2. enter PM32 and set 
>> segment limits, ie: base 0 and full 4GB range.
>> 3. switch to PM16 (just far jump) for a 16-bit code-segment limit *).
>> 4. go back to RM without restoring the data segment limits.

>> in this mode all memory up to 4GB is accessible by 32-bit addressing.
>>
>> *) to add to the confusion there is also BIG real 32-bit code-segment
>> possible, but I wont recommend to use it.

>>> It seems that there should be as there is a discussion on maybe the
>>> Bios leaving the segment length to 1 MB in some cases.

>> No, I wont blame the BIOS, it seems to come from bootloaders, emulators
>> and tools (like in my case).
>>> By the way are there any books recommended for a newbie to learn about
>>> Intel Assembly language and Windows/Linux programming?

>> I haven't seen books which talk about modern CPUs ...
>> You might find ASM-tutorials in FASM and NASM foren. Both Intel and AMD
>> support us with pdf-books about all available instructions for free.

> So what would be the advantage in Unreal Mode over using Protected Mode?

I use Ureal RM for direct write to Text/Graphic sreens while in RM.
The Graphics LFB is usually far above in the 4th GB (ie: 0xD000_0000).

> Does it save code space compared to Protected Mode?

yes. RM Code is shorter and faster (no paraniod protection checks).

The main advantage from RM over PM is that there are no protection
checks and only hardware covered limits which make RM much faster than
any PM. Even Windoze and Loonix have to fallback to RM BIOS calls
when they dont got a hint from to be installed "drivers".

> One advantage I can see is that you can still access the Bios calls 
> without going through an act of congress, while accessing a lot more 
> memory, so it benefits small to medium size programs, I'm inferring this 
> but not 100% sure.

Yeah you got it right, while you are in true RM you can use all BIOS
and VBIOS functions without fear to break or end up in the wild.
And with Unreal mode you can also use BIOS functions (better have
all register highwords zeroed during it) as long functions supported.

But be aware that most BIOS dont like or even use itself Unreal mode
so you bettter call BIOS functions with clean 16-bit (zeroed upper reg)
and some functions may not accept Unreal settingd at all (depends on
the BIOS variant) .

> I have purchased several books on x86 Assembly language and most of these 
> books barely mention Windows Protected Mode programming, one does in one 
> chapter out of 24 chapters.

I hope you didn't step into the HLA/AoA trap :)
"Sandpile.org" is a good place to look for many important info.

> It basically mentions that I don't have to do much of anything, Windows 
> sets things up, and all I have to do is setup the assembler to use the 
> .586 instructions set, use the Flat model, and use the Windows DLLs to do 
> the interaction to the OS, no mention of access to the Bios.

Windoze and Loonix are heavy restricted environments and wont even
know how to access hardware without a hint from so called "drivers".
they use a HAL (Hardware Abstruction Layer) to talk with devices!

But yes, you can write code in ASM for it with many different tools.

> I will keep searching, but some of the other books I have coming deal 
> specifically with Windows and Win32 programs, one is about Windows/Linux 
> Protected Mode assembler usage.

> Is this the right forum? or should I use the c.l.a.x86 instead, I see a 
> lot of subjects there, this one is empty.

CLAX is a moderated group, so be patient to see your own post in time.

AOD isn't a forum, it rather is a place to ask questions about OS 
programming and the like. Our group is multilingual ordered :)

I'm the machine-code hardwarefreak and know nothing about Loonix nor
Windoze, while other long time posters are better in finding (instead
of searching) useful info on the net and others seem to work in the
museum and know everything about older standards.

If you got questions not covered elsewhere you are welcome in
either AOD or ALA. AOA seem to be dead already.

looks like you know what you are searching for, so your questions
will be answered in a logical way.

__
wolfgang  (welcome to the jungle of truth) 

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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

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