Re: Large RAM sizes in embedded systems

On 25 Dec 2006 18:14:48 -0800, "larwe" <zwsdotcom@xxxxxxxxx> wrote:

I'd be curious to hear from anyone who has worked on embedded systems
with relatively large amounts of directly-addressable RAM.

Hence I would say my RAM requirement would be speced at 1TB of
error-correcting RAM.

If the largest COTS memory modules are 4 GiB, you would still need 256
of these, taking some considerable board area or total volume, so the
wire connection lengths would be considerable. Unfortunately the speed
of light is unfortunately not very fast and in PCB tracks the
propagation velocity is 200000 km/s or even less depending on the
dielectric.

In a truly random access system, the performance would suffer very
badly due to the propagation delay (i.e. this would basically be a
half-duplex environment), but in some block transfer system, such as
cache line loading, block loading or "DMA" systems, the propagation
delay/line turnaround delay would only be suffered once for each block
transferred.

It might be preferable to store any index structures (etc.) into a
smaller RAM with shorter propagation delays and use block transfers
for getting the actual data from memory physically located at a larger
distance from the processing power.

Paul

.