Date   : Mon, 25 Jul 1994 09:28:24 WET DST
From   : Bonfield James <jkb@...>
Subject: Re: Beeb Emulators

Stephen Quan writes:

>        opcode = 0xa9;
>        switch (opcode)
>        {
>          case 0x00 : PC += 0x00; break; /* fake execution of opcode 0x00. */
>          case 0x01 : PC += 0x01; break; /* fake execution of opcode 0x01. */
>                   ....
>          case 0xff : PC += 0xff; break; /* fake execution of opcode 0xff. */
>        }
>      }
>    }
>
>This code executed on my PC in 50 seconds when compiled exactly the
>same way as my emulator.  Meaning that the execution cycle of our
>emulator only account for 9 seconds in 59.  If optimisations are to

I tried using gcc with:

-O2 -fforce-mem -fforce-addr -ffast-math -fomit-frame-pointer -fcaller-saves

Much of those were just experimental tests, but the -fomit-frame-pointer in
particular is important. With this, I found that the switch system, and the
'opcode[x]()' systems were implemented in exactly the same manner - by a large
lookup table and a jump. The lookups didn't seem to be particularly complex
either. I'm suprised at the 50/59 seconds result! Maybe this will help aim our
directions to the real slow down.

       James B