Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!oliveb!amiga!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: DMA controllers and 32bit RAM
Message-ID: <7122@cbmvax.UUCP>
Date: 23 Jun 89 07:16:57 GMT
References: <14391@ut-emx.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 56

in article <14391@ut-emx.UUCP>, sjk@ut-emx.UUCP (bob) says:

> Just a question I haven't been able to get a straight answer about:
> What happens if you use a DMA hard disk controller, but have 32bit
> expansion RAM that does not have it's own DMA controller?  

Lots of interesting things can happen, depending on what you've got.
32 bit RAM never has it's own DMA controller, but it needs to have 
DMA kept in mind.  This basically means one of two things:

	[a]	You locate the memory somewhere that's easy to keep
		DMA away from.  I consider this to be some place 
		outside of the normal 68000 address space.  With 
		memory starting, say, at $1000000, it's easy to keep
		DMA devices away from it by specifying a Mask value
		in their mountlist entry, providing the device will
		support Mask.

	[b]	You build a 32 bit memory system that can support
		16 bit DMA.  It does add to the complexity of the
		memory system.  There are two basic approaches.  The
		simplest is to make the memory respond like a 68000
		memory board during DMA, and like a 68020/30 memory board
		during CPU operation.  The second approach is to build 
		a bus converter that turns 68000 signals into slow
		68020/30 signals.  The A2620 and A2630 use the first
		approach; the only thing they all DMA to access is the
		on-board DRAM (pretty much the only thing on-board that
		would be interesting to a DMA device anyhow).

> My own system is an Amiga 1000 with LUCAS and (soon to be) FRANCES
> 32 bit memory board and a Toolbox expansion chassis where my hard
> drive controller will go, but I suppose people with A2000's and 
> a third party 68020 board (ex. Ronin) will face similar problems, as
> the 2620 is the only 020 and 32 bit RAM board with its own DMA support
> routines, that I know of.

Ronin does indeed have a problem with DMA.  They ship their memory board
non-autoconfiguring but allow it to sit in expansion space.  That's a bad
idea.  They also recommend a Mask=0x0ffffe, which restricts DMA to CHIP
memory, an even worse idea.  Fortunately, with the right jumper settings the
Ronin memory can be moved above the 24 bit mark and the right 0xfffffe mask
can be used (restrict DMA to where the hardware allows it to go in the 
first place).

I talked with Brad on this some before, and briefly at DevCon, so I'd expect
his board to make the right kind of decisions for the type of board it'll
be.

> Scot Kleinman
> sjk@astro.as.utexas.edu
> Astronomy Dept. U. Texas Austin TX 78712
-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
           Be careful what you wish for -- you just might get it