[TUHS] machine code translation,as mental architecture models

[Marc Donner]

I never dug that deep into her work.  If I were to guess, I would say that
this was aimed at user-space sorts of code.  It was aimed at making it
possible to run large old 370 binaries in Power PC architecture machines.
If I recall correctly, there was some discussion of migrating off of the
S370 (later S390 and then z-system) ISA to the Power PC architecture.  My
sense was that this was a research study to understand whether a naive
translation like this could work.

I can not imagine trying to translate system code in this way.
=====
nygeek.net
mindthegapdialogs.com/home <https://www.mindthegapdialogs.com/home>


On Wed, Jul 10, 2024 at 1:07 PM segaloco via TUHS <tuhs at tuhs.org> wrote:

> On Wednesday, July 10th, 2024 at 7:12 AM, Marc Donner <
> marc.donner at gmail.com> wrote:
>
> > The basic technique was to load pages of 360 machine code unmodified
> into Power PC memory, marking the pages as dirty. If the machine branched
> into one of these pages the VM trap handler would take the page and
> translate all of its code from 370 instructions to Power PC instructions.
> Once that was done the branch, suitably recalculated, was reinstated.
> >
> > ...
> >
> > Marc
> > =====
> > nygeek.net
> > mindthegapdialogs.com/home
>
> Was this under the assumption everything else going on (bus architecture,
> memory map, exception/trap handling, etc.) was exactly the same or was the
> MMU for instance in the picture translating from 370 addresses to their
> equivalents on whatever PowerPC machine was in play?
>
> That or was this only expected to work on PIC, user-level code and
> anything touching for instance privilege escalation or with explicit
> address pointers, I/O port access, etc. just understood to not be
> applicable to such a direct translation?  To tie it back to UNIX, for
> instance, was this thing also able to detect that a UNIX system call was
> being made and translate the 370 syscall mechanism into the PowerPC syscall
> mechanism?
>
> My main experience with emulation is console video games so in that realm,
> you're dealing not only with a different CPU architecture but system bus,
> memory map, peripheral controllers, etc.  Essentially you're not just
> speaking another language, you're on another planet entirely.  This has
> lead to a entrenched belief in my mind that this sort of "direct
> translation" of CPU operations is a non-starter in emulation hence my
> curiosity.  If there's some historic approach to emulation efficiently
> handling system architecture diversity beyond just CPU instructions, I'm
> certainly interested!
>
> - Matt G.
>
> P.S. Feeling trepid about continuing this thread on TUHS rather than COFF,
> but resisting the urge to bump it (again).  To keep it on topic, I'd mostly
> be interested in how this sort of thing would handle the UNIX system call
> mechanism, because that would certainly illuminate the general idea of
> translating something more complicated than an algorithm.
>
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