[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: [f-cpu] F-CPU architecture...
Michael Riepe wrote:
I'm even thinking about putting a simple RISC like a LEON on die as
well and let it handle the I/O and selftest. If done it switches to
I/O pass through mode :)
Well, the I/O thing is still more or less undesigned anyway. We had
plans for the "G" (as in "glue") chip which is a kind of crossbar
switch supporting three or four F-CPUs and maybe also including some
I/O devices. It could be used to distribute boot code to the CPUs, too.
With the advent of a new organisation (with FC0 having one specific I/O
channel),
the "G chip" can be a simple FPGA. This makes it much more
cost-effective and practical :
some FPGAs can be had with a thousand pins, expensive but much less
than if we designed our own G-chip with as many pins. And we can develop
the routing algorithms, etc, without having to going to fundry.
The basic idea is that the F-CPU contains a DMA engine that transfers
data from I/O to memory and vice versa. When the chip is reset, the
engine would be preset to receive a fixed amount of data from the I/O
bus and transfer it to a fixed physical address (most likely zero)
where the F-CPU can execute it.
That, or a simpler way :
init PC=0,
which triggers cache fault, (because RESET has flushed everything)
so reads from outside memory (private memory is not mapped to address 0)
---> no need to design the DMA specificically for booting.
And don't tell me: "OMG. So much die space wasted!" If F-CPU is to be
high end then the size of a wasted LEON is almost 0 in comparison!
I'd rather use a downscaled version of the F-CPU itself. Maybe even
the mythical 32-bit version that some people already have asked for,
with all the unnecessary stuff stripped off (e.g. FPU, multiplication
and division).
If the "scaled-down core" is never going to run applications or user code,
then use the VSP :-P
YG
*************************************************************
To unsubscribe, send an e-mail to majordomo@xxxxxxxx with
unsubscribe f-cpu in the body. http://f-cpu.seul.org/