[Simh] VAX Emulation Speed Ramblings...

[Kevin Handy]

Robert Lawrence wrote:

> According to 
> http://www.comwaretech.com/products/legacy_solutions/microvax.shtml,
> "The REVIVER-VAX performance scales linearly with the PC's CPU speed. 
> For integer operations, REVIVER-VAX provides a performance of about 
> one VAX Unit of Processing (VUP) per 100 MHz of CPU clock. Disk I/O is 
> 5 to 20 times faster than on the original VAX."
>
> How are they doing this?  Are they using some sort of JIT?  I get 
> about 5 VUPs from a 1.2ghz using simh.  According to their 
> calculations, 12 VUPs should be possible.
>
> What methods can be used to increase performance of simh/vax on any 
> given pc?
>
You are only looking at a ~2x difference in speed. Some possibilities:

1. Hand tuned machine language code.

Compilers can do a good job, but handcrafted machine code can
make a large improvement, and a 2x difference should be possible.

However, you would then tie the emulator to run on only one
specific CPU.

2. Improve the optimizers.

Make the compilers produce better/faster code.

3. Hand optimization.

Look for hot-spots in the simh code, and speed them up.

4. Less accurate emulation.

If it doesn't have to produce the correct answer, you can make it
as fast as you want.

> Thanks,