[Info-vax] Chinese Alpha?

[Arne Vajhøj]

On 5/4/2012 9:27 AM, Bob Koehler wrote:
> In article<4fa2eef6$0$288$14726298 at news.sunsite.dk>, =?ISO-8859-1?Q?Arne_Vajh=F8j?=<arne at vajhoej.dk>  writes:
>>
>> I would think that x86-64 is similar to x86 in somewhat
>> the same way as VAX was similar to PDP-11.
>
>     I was under the impression that adding 64 bit to x86 was something
>     like adding 23 bit to PDP-10, or 32 bit to 8086.  Existing code
>     continues to run.

Yes and no.

Different instruction formats, different registers etc. for 64 bit.

But a x86-64 supports both 64 bit and 32 bit mode on a per
process basis.

So a 32 bit app will continue to run by putting the CPU in 32 bit
mode.

But you can not mix things. A 64 bit process runs 64 bit code and
a 32 bit process runs 32 bit code.

This means that you can not run a 32 bit DLL from a 64 bit EXE
or a 64 bit DLL from a 32 bit EXE.

>     VAX to PDP-11 was different.  The VAX instruction set is not the
>     PDP-11 instruction set, although early VAXen had two instruction
>     sets:  VAX and compatability mode (a subset of PDP-11).  CHMU was
>     used to switch to the compatability mode instruction set when
>     running PDP-11 user mode code.

Not that far from x86-64 and x86.

Many instructions in x86-64 has the same binary format as they
had in x86 though. Then some new formats were added. Some old
formats changed semantics (ooops). And some old formats were
removed/deprecated.

I don't know how different or how similar VAX and PDP-11
instruction set were.

Arne