Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Speed
Message-ID: <8441@cbmvax.UUCP>
Date: 8 Nov 89 17:06:14 GMT
References: <649@milton.acs.washington.edu>
Organization: Commodore Technology, West Chester, PA
Lines: 26

in article <649@milton.acs.washington.edu>, axjjb@acad3.fai.alaska.edu (BRYANT JOHN J) says:
> 
> I am hopelessly confused about the 25 mhz accelerator board.  What do they mean
> by 32 bit ram?  Is it accessing Ram at 32 bit increments?  I am also wondering
> what is the exact bit access of the cpu..(ie..16 or 32)

On the basic Amiga, all memory is accessed by the 68000 on a 16 bit wide data bus.
This is of course a perfect match to the CPU, because the 68000 only has a 16 bit
data bus.  

Most accelerator cards use either the 68020 or 68030, both of which have a 32 bit
wide external data bus (the 68030's internal data bus is actually 64 bits wide;
there are separate 32 bit paths for Instruction and Data).   While both chips can
dynamically size their logical bus for 8, 16, and 32 bit transfers, the 32 bit
transfer is the most efficient.  In fact, due to the way these chips pre-fetch
instructions, they're actually less efficient on a 16 bit bus than a 68000 (if
you turn off the caching, even fast 68020s or 68030s will run as slow or slower
than the 68000 in most things on a 16 bit bus).  So to really take advantage of
a 68020 or 68030 (for anything other than its floating point unit), you need memory
with a full 32 bit wide bus.

> 					Joe.
-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
                    Too much of everything is just enough