On Tue, Aug 03, 2004 at 03:16:21PM +1000, Ross Vumbaca wrote:
> Hi,
>
> Geert Uytterhoeven wrote:
>
> >Hmm, last time I used a floppy on my LongTrail, it did work (to my
> >surprise,
> >since a few years earlier it was broken ;-)
>
> >Yes, CHRP had PC-style floppy controllers. And decent South Bridges used
> >on PPC
> >(e.g. W53C883) usually support 32-bit ISA DMA.
>
> The South Bridge on the "AmigaOne" is a VIA686B. It's a common PC
> component, and it implements an i8259 ISA DMA controller, which can't do
> DMA above 16MB.
Hmm, the DMA controller is called the 8237. The 8259 is the PIC, aka
Painful Interrupt Controller.
Once upon a while, Intel introduced a couple of PCI/ISA bridge with
enhanced DMA capabilities:
- the 82378ZB with scatter/gather DMA with 32 bit addressing
capability
- the 82379AB with an I/O APIC (meaning Awfully Painful...) and
another kind of enhanced DMA without scatter/gather but with
guess what ... 27 bit (yes!) addressing capability.
If you want it, I still have the pdf from March 1996 on my disk.
Note that 27 bit was not bad since very few machines at the time
could even have such a tremendous (128MB) amount of RAM.
The PreP specification said that the ISA bridge should
be the Intel 82378ZB, so Intel stopped producing it and
no other Intel bridges since then have to my knowledge
implemented the same ISA DMA engine. Only Winbond (again to
my knowledge) copied it in the 83C553 and 83C554 bridges.
> I think it _might_ have an option for 32 Bit ISA DMA, but due to the
> docs being so poor, and no other information being available, it's
> probably easier to do what I believe x86 Linux does on similar hardware
> - use ISA DMA below 16MB only.
I have Winbond 83C553 or 83C554 on my boards, but I have
no devices that use ISA DMA (well I could connect a floppy
but what for?).
Regards,
Gabriel
