> In general, most people want ISA. There are one or two people
> that would rather have a simple, 'riscy' specific, cheap 8-bit bus.
> Then there is the issue of how many ISA sockets to provide?
>
>
> No external bus at all:
>
> Advantages:
> * Very cheap
> Disadvantages:
> * Make expansion rather difficult
I would say: Makes expansion impossible.
> R3000 specific high bandwidth bus (local bus?):
>
> Advantages:
> * Peripherals that we add later can go fast.
> Disadvantages:
> * Designing a high speed expander bus is problematical
> (lots of wave calculations)
> * Connectors for high speed busses are expensive.
> * No cards available, unless we make them.
Do we really want to invest the wheel once again ?
There are *so* many bus systems yet... I don't think
that we have the power and market to introduce a new
bus no matter how good and fast it is.
> R3000 specific slow speed 8-bit bus:
>
> Advantages:
> * Cheap, small connectors
> * Easy to provide for on the motherboard
> * Simple to make cards for
> Disadvantages:
> * No cards available, unless we make them
As above, plus the disadvantage that it will be slow :-)
> ISA bus:
>
> Advantages:
> * Supports many, many peripherals
> * Allows other options for video on the motherboard
> * Allows other options if SCSI is not used
> Disadvantages:
> * Large connectors are not the cheapest (though given the quantities
> those things are made in, they probably are cheap)
> * Will could not hope to support everything ISA (such as bus-
> mastering mode, and maybe the bus sizing stuff).
> * Uses lots of board space
In fact, the connectors are old-fashioned and use a
lot of boardspace. But we don't need that many slots
as a PC has, because we have most of the necessary
I/O stuff on-board. One might need two, perhaps 3 slots.
That will be sufficient for 99% of all applications.
> A full AT ISA bus implementation provides:
> * 8-bit or 16-bit data path
> * Flexible throttling mechanism (automatic wait state generator)
> * Slave and Master DMA modes
> * Automatic 16 to 8-bit translation.
> * 8.33 MHz clock rate
The discussion about DMA modes is already in progress.
Perhaps there's no need to support DMA on the ISA bus.
Comments ?
Andy
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