Joe Smith said:
> 
> > From: cs_yus@cs4.lamar.edu (YU SONG)
> > To: amiga@NetBSD.ORG
> > Subject: cache
> > 
> > However, as I was 
> > told before that the memory access in stock Amiga ( A4000 in my case )
> > is quite slow.
> 
> You have been misinformed.  Memory access in the Amiga between the CPU and
> fast RAM is quite fast.  So fast, that adding an L2 cache will not make it
> any faster.

	Well, actually it's that the Amiga's CPU's (well, all 680x0 CPU's) are
clocked so slow (relative to other CPU's), and that the motherboard data paths 
are well designed - resulting in 80ns RAM being all that's needed without 
a significant bottleneck.

> Memory access between the Amiga CPU and the "chip" RAM (also known as the
> "graphics" RAM) is slow because the CPU has to go through the Amiga's custom
> graphics chips to get to the graphics RAM.  Adding an L2 cache there is not
> possible.

	Trua, but mostly irrelevant to NetBSD.

> You seem fixated on the idea that adding a Level-2 cache to the Amiga will
> fix all of your problems.  It doesn't work that way.  Please accept that
> fact and move on.

        However:
In the A3000 and A4000, a CPU in the "CPU Slot" will have pathetically slow -
in terms of MB/s, about 1.6 - access to motherboard memory..  While on L2 cache
on the CPU board would alleviate the effects of this bottleneck somewhat, the 
expense of cache RAM makes it more economical to put the memory itself on the 
board (It's probably better, too).  This is done on all of the "CPU slot" 
boards the I know of except the 3640 - the stock board in the A3000T/040 and 
A4000/040.
	The problem is not that the motherboard datapaths are bad, but because 
the CPU and motherboard are on different clocks.  

	When Amiga's (finally!) get serious CPU's - with high external clock 
rates  and double, triple, and quadruple clocked internally - then we'll start
wanting L2 caches (beats the hell outta buying 16Megs of 40ns RAM!) to 
alleviate the bottlenecks created by fast CPU's and slower, cheaper memory.
The '060 comes close, being double clocked even with a decent external 
clock rate, but I think it had a pretty big onboard cache.  Don't worry about 
an 040. 


-- 
Stephen Champion                 Workstation and Network Consultant
steve@onshore.com                Onshore, Inc.