[Info-vax] VAX vs. MV/8000 [was Re: Hard links on VMS ODS5 disks]

[John Dallman]

In article <uci441$lgj$2 at news.misty.com>, bqt at softjar.se (Johnny
Billquist) wrote:

> I've tried to stay out, and don't want to get too deep in.
> But in a sense, my question/issue would be: Can you take a binary 
> for x86 and run it without any "mode bit", or anything else, on an 
> x86-64 and it works?

Depends on how much setup you're willing to do before kicking off the
binary. As Dan says, if you set up the segments right, many of the
instructions will execute, but you'll hit problems with others, and
certainly when you start trying to call OS functions that are at all
complicated. 

> Individual instructions is not all. Will the stack offsets remain 
> the same when I push stuff, will address references be fine when I 
> just use 32 bits, and do arithmetic and address offsets and so on.

Stack offsets and arithmetic operations will likely be OK; there may be
some corner cases. The problem comes with memory references being passed
between 32-bit and 64-bit code. Those are offsets within segments, and
there is no intrinsic or canonical mapping between 32-bit segments and
64-bit segments on x86. 

To run executables intended for a 32-bit OS on a 64-bit OS, the usual
route on x86 is "thunks", small chunks of code that receive OS calls made
by the 32-bit code map the data into 64-bit addressing, and then call the
OS' native 64-bit API. That's how 64-bit Windows and Linux run 32-bit
executable. 64-bit macOS did that too, until Apple removed its ability to
run 32-bit executables. The thunks are collected into special versions of
the various run-time libraries that 32-bit code calls. Sometimes, for
performance reasons, chunks of the "real" 32-bit run0time libraries are
included. 

> If that is the case, then yes, it has been done the same way DG did 
> it. Which truly meant you could take your old binaries and run them 
> on the newer hardware without any added trickery.

No, it isn't the full DG job. But it isn't a VAX-style separate mode
either. It's something in between. 

An example of a modern separate mode comes with ARM. The ARM32 and ARM64
instruction sets are /very/ different. Most ARM32 instructions are
conditionally executed using predicate flags in the instructions, taking
up 5 bits in almost all instructions. ARM64 threw that out, and some old
ARM hands reckon ARM64 is more like MIPS than ARM32. 

John