[Simh] vax vs. vax780

[Jeremy Begg]

Hi,

>>Just out of curiosity, what's the difference between the vax and
>>vax780 simulators? I realize that vax emulates a MicroVAX and vax780
>>emulates a VAX-11/780, but other than that what is the difference as
>>far as OpenVMS (or any other operating system) is concerned?
>
>As far as I/O is concerned, there is quite a difference between a
>VAX-11/780 and a MicroVAX.  Although I believe the MicroVAX emulated
>is a Q-Bus system, so it's not as big as it could be.
>
>Also the VAX-11/780 and the MicroVAX should have some pretty serious
>differences at the CPU level I should think with regards to PDP-11
>emulation.

That, and other VAX instructions which were emulated on the MicroVAX II.
For example, the Decimal String and Edit instructions (chapters 11 and 12 in
my trusty "VAX Architecture Handbook").  On the 780 all instructions were in
microcode.  On the MicroVAXes, emulated instructions caused a trap to an RTL
routine which performed the instruction using other VAX instructions.

I suppose that if your code does a lot of packed decimal arithmetic (e.g.
COBOL) then the simh-780 emulator might be faster than the simh-microvax
emulator. 

>Hopefully someone more clueful on this subject than myself chips in.

I don't know about *more* clue, but maybe I have *different* clue :-)

Regards,

        Jeremy Begg

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