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RE: Legacy Terminology RE: DMA across PCI
DMA is Not an ISA only term,
A DMA is a chunk of hardware that
- can become a bus master of the bus in question
- has a register to store source address
- has a register to store destination address
- has a register to store transfer length
- has a control/status register to start,stop, report done
- has possibly interrupt capability.
You load up the registers, hit the start button, and away it goes to
do a block
transfer for you.
Yes, this can and does happen on PCI. PLX 9060,9080, 9054 and I'm sure
others have very fancy DMA logic
inside to allow the PLX pci bridge in question to copy data from/to PCI
space to the local bus (other side)
of the device. Some VGA controllers can do this, allowing the VGA device to
copy large amounts of
data from Pentium memory to Video memory with little effort by the Pentium.
The advantage is the
Pentium (or host processor in a more generic term) can go back to processing
from internal cache
in parallel to the dma transfer running..
Even PCI hard drive controllers have this ability on PCI. It's not marketing
buzz, its useful
and the preferred method of moving data around quickly on a shared bus,
likely predates ISA.
Actually the ISA "DMA" controllers were kind of useless, if I remember - the
8086 ended up stalled
waiting for the bus while the "DMA" controller ran. No real benefit there..
Spectrum Signal Processing Inc.
From: Lame Brooks-G14738 [mailto:email@example.com]
Sent: Thursday, December 14, 2000 2:01 PM
Cc: PCISIG List (E-mail)
Subject: RE: Legacy Terminology RE: DMA across PCI
I think you just said it yourself, Ted: a "DMA engine." That's the old ISA
terminology refering to a state machine that manages data transfers. I'm
only pointing out that the term DMA in reference to PCI does not indicate
any specific implementation nor performance capabilities. And, because the
term is so non-specific, it has little meaning to engineers in PCI. With
marketing liturature on the other hand, they throw in all the buzz words
they can. :P -- BrooksL
> -----Original Message-----
> From: Ted Firlit [mailto:firstname.lastname@example.org]
> Sent: Thursday, 14 December, 2000 12:46
> To: Lame Brooks-G14738
> Cc: 'Daniel DeConinck'; PCISIG List (E-mail)
> Subject: Re: Legacy Terminology RE: DMA across PCI
> I'm using a Galileo PCI controller chip (GT64120A) which specifically
> implements a 4-channel DMA engine for the host CPU to set up DMA
> transfers. I didn't think this was so unusual for a PCI
> controller chip
> to do. If and when we change out this chip, I'll expect the
> to implement DMA as well, unless you know something about DMA and PCI
> which I don't.
> Lame Brooks-G14738 wrote:
> > Yeah, years ago when I was a newbie to PCI, I got smacked
> on the head for
> > using DMA in reference to PCI. DMA is an ISA bus related
> acronym that
> > stands for Direct Memory Access. PCI is by definition all
> about direct
> > memory access - that's one of the features that made it
> superior to ISA!
> > Therefore, the old term DMA itself has little meaning in
> reference to PCI.
> > I think most of the people who try to use it now days are
> meaning a concept
> > analgous to the legacy ISA DMA Transfer Engine which is
> simply a state
> > machine that can manage a block data transfer given the
> start and end
> > addresses. I'd stay away from legacy terminology when talking about
> > specific bus behaviour like burst transfers. -- BrooksL
> > -----Original Message-----
> > From: Daniel DeConinck [mailto:email@example.com]
> > Sent: Saturday, 14 October, 2000 09:48
> > To: firstname.lastname@example.org
> > Subject: DMA across PCI
> > Hello,
> > I looked in the index of the PCI spec 2.1 and the index of
> "PCI System
> > Architecture", by Mindshare, Inc. Niether had a reference to DMA.
> > I am guessing that there is no such thing as DMA in the PCI
> world. Is that
> > the case ?
> > I further guess that the closest thing to DMA is a Master
> read or a Master
> > write cylce.
> > Please enlighten me.
> > Sincerely
> > Daniel DeConinck
> > High Res Technologies, Inc.
> Ted Firlit (719) 594-8138
> Senior Design Engineer
> UTMC Microelectronics Systems
> 4350 Centennial Blvd., MS 1004
> Colorado Springs, CO 80907