> On Thu, 10 Feb 2000, Christophe Kumsta wrote:
>
> >
> > I don't think that's really right, cause the usage of memory directly
> > from PCI is available too.
I was referring to the quote in BYTE magazine in a previous email, where
a graphics vendor said that his AGP card doesn't use the CPU much. My
point was that it is a good thing for the AGP card not to bother the
CPU.
I was not trying to say that PCI is unable to access system memory.
> > It's called bus-master transfert ( from or to the PCI-card ), and this
> > operation is done withoutmain CPU.
> >
> > The first advantage of AGP bus is that this one can be considered as 'a
> > second' PCI bus
An AGP card can be designed for 'AGP only operation' or 'AGP and PCI
operation' according to the AGP 2.0 Specification. If it is designed
for 'AGP only operation' the PCI IDSEL pin can be omitted. In AGP mode
unlike PCI mode a card can be designed to send one data, two data, or
four data transfers per 66 MHz clock cycle. Also, in AGP the address
transfer travels on a separate set of bus lines than the data transfer.
There are many other differences between AGP and PCI.
> > with only one peripheral ( this explain the full
> > bandwith operation for one slot ) and the frequency of the bus which is
> > 66MHz against 33MHz for the PCI.
Chapter 7 of the PCI 2.1 Specification is titled, "66 MHz PCI
Specification". PCI can do 33 and 66 MHz if the PCI chipset allows it.
If a PCI card can do 66 MHz and it is in a 33 MHz system it will do 33
MHz. If it is in a 66 MHz system it will do 66 MHz.
> >
> > This point of view explain why there is only one AGP bus on the system.
> > ( if you look at linux /proc/pci file info, you will see that the AGP
> > card is considered as a PCI card on bus=1 while others cards are all on
> > bus=0 )
That it is on bus=1, just means that it is a different bus. PCI allows
different buses and different bus types. PCI does not place a
limitation on the number of devices that can exist on a AGP bus. It is
the AGP 2.0 and AGP 2.1 specifications that prohibit only one active
card on an AGP bus at one time.
Download the specifications and check it out.
- Cheers, Kal.
> >
> > Chris.
> >
> > --
> > Kumsta Christophe
> > <[EMAIL PROTECTED]>
> > Real-Time System developper
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