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Re: Program in 32-bit, run on 64-bit OK?

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On 11/15/2012 5:08 PM, Stuart wrote:
[snip]
>> The early 286 processor had a register width of
>> 16 bits but an address bus with 24 bits (4MB), so they had to invent
>> this segmentation model in order to compute a 20bit address from two 16
>> bit registers.

On 11/16/12 5:46 BGB wrote:
> corrections:
> 24 bits is 16MB (because 2^24 = 16777216), this bus size was used by
> both the 286 and 386SX (the 386DX and 486 used full 32-bits);
> the segments:offset -> 20 bit address thing was there since the 8086,
> which could only address 1MB of RAM (in contrast to 64kB on the 8080);
> on current 64-bit chips, the segmentation is also non-functional in 64
> bit mode with the partial exception of the FS and GS segments (used
> mostly for implementing TLS and similar).

Gosh, I got it all mixed up. Of course 24 bits is 16MB. I can't say 
where my brain got the number 4MB from, probably because most 286 boards 
could only handle 4MBs of memory at that time. Or was that only for EMS 
modules?

> (so, while the segment registers are still present, they don't really do
> all that much anymore).

I don't know. As far as I can see it, it should still be possible under 
a 64 bit processor to set up segment based memory protection using call 
gates and such, so all the privilege level checking stuff should still 
be present in 64 bit CPUs (albeit noone uses it anymore).

> in effect, the chip is using a 64-bit flat address model, but may remap
> the addresses mostly through the use of page-based address translation
> (which maps from the program's virtual address space to the physical/RAM
> address space).
>
> yes, the CPU has backwards compatibility, but it actually involves a
> fair amount of mode-changing (as control is passed from one program to
> another, the CPU mode changes), but there are limits to this.
>
> however, real-mode software (IOW: most of the software from 1985) can't
> actually run on the CPU if it is running in "long mode", so it actually
> requires use of either hardware emulation or virtualization to make this
> work (for example, DOSbox does emulation, and VMware does virtualization).
>
> as-is, about the oldest software that can be run directly in modern
> Windows is roughly mid-to-late 90s era software (IOW: from when the
> world moved solidly over to 32 bits), although the hardware can
> technically still support running 16-bit protected-mode software in
> long-mode (theoretically, MS could have made Win3.x apps work without
> emulation, had they wanted to do so...).

Um, does that mean that I can no longer install DOS on a modern 64 bit 
PC? Not so long ago I bought a HP laptop without an OS (planned to run 
Linux on it). It shipped with FreeDOS. Awesome. Reminded me of the good 
old times when we messed with the LAN cards of four PCs for hours just 
to be able to play Doom2 on Christmas Eve.

Regards,
Stuart

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Program in 32-bit, run on 64-bit OK? "sl@exabyte" <sb5309@hotmail.com> - 2012-11-14 21:58 +0800
  Re: Program in 32-bit, run on 64-bit OK? "sl@exabyte" <sb5309@hotmail.com> - 2012-11-14 22:01 +0800
  Re: Program in 32-bit, run on 64-bit OK? Arne Vajhøj <arne@vajhoej.dk> - 2012-11-14 09:12 -0500
    Re: Program in 32-bit, run on 64-bit OK? Cholo Lennon <chololennon@hotmail.com> - 2012-11-14 14:11 -0300
      Re: Program in 32-bit, run on 64-bit OK? Arne Vajhøj <arne@vajhoej.dk> - 2012-11-14 12:14 -0500
        Re: Program in 32-bit, run on 64-bit OK? Joerg Meier <joergmmeier@arcor.de> - 2012-11-14 18:36 +0100
          Re: Program in 32-bit, run on 64-bit OK? "sl@exabyte" <sb5309@hotmail.com> - 2012-11-15 10:48 +0800
            Re: Program in 32-bit, run on 64-bit OK? Stuart <DerTopper@web.de> - 2012-11-16 00:08 +0100
              Re: Program in 32-bit, run on 64-bit OK? "SL@maxis" <ecp_gen@my-rialto.com> - 2012-11-16 09:23 +0800
                Re: Program in 32-bit, run on 64-bit OK? BGB <cr88192@hotmail.com> - 2012-11-15 22:52 -0600
                Re: Program in 32-bit, run on 64-bit OK? Fritz Wuehler <fritz@spamexpire-201211.rodent.frell.theremailer.net> - 2012-11-16 16:44 +0100
                Re: Program in 32-bit, run on 64-bit OK? Joerg Meier <joergmmeier@arcor.de> - 2012-11-17 13:18 +0100
                Re: Program in 32-bit, run on 64-bit OK? BGB <cr88192@hotmail.com> - 2012-11-17 12:42 -0600
              Re: Program in 32-bit, run on 64-bit OK? BGB <cr88192@hotmail.com> - 2012-11-15 22:46 -0600
                Re: Program in 32-bit, run on 64-bit OK? Stuart <DerTopper@web.de> - 2012-11-16 12:08 +0100
                Re: Program in 32-bit, run on 64-bit OK? BGB <cr88192@hotmail.com> - 2012-11-16 18:08 -0600
                Re: Program in 32-bit, run on 64-bit OK? Stuart <DerTopper@web.de> - 2012-11-17 23:59 +0100
              Re: Program in 32-bit, run on 64-bit OK? Anonymous <nobody@remailer.paranoici.org> - 2012-11-16 12:42 +0000
            Re: Program in 32-bit, run on 64-bit OK? Roedy Green <see_website@mindprod.com.invalid> - 2012-11-16 05:17 -0800
    Re: Program in 32-bit, run on 64-bit OK? Jan Burse <janburse@fastmail.fm> - 2012-11-14 20:08 +0100

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