arm9 memory throughput

Hi folks.

I'm working on an ARM9 system (the DaVinci). Yesterday a workmate and I made some tests to measure the memory speed of the system and we wonder a bit about the numbers we get.

In short the system during our tests looked like this:

* RAM is DDR2 at roughly 160Mhz
* The CPU is an ARM9 and runs at roughly 300Mhz
* We disabled all components that may access the RAM (e.g. video out, DSP ect).

In our tests we cleared a 3Mb chunk of memory using 32 bit writes and measured a troughput of 200 Mb/s. We tried everything possible to improve the speed, e.g. unroll the loop, use store-multiple instructions ect. We always get the 200Mb/s. We can even put up to four nops between the writes and the numbers don't change.


Doing the same using DMA I get numbers around 1.3Gb/s on the same system.

I know that I never get the full theoretic memory throughput but 200mb/s is a lot less than we have expected. Now I want to understand why this happends. Unfortunately I know s**t about memory interfaces, memory latencies and all the other stuff.

Could somone please explain me what the memory and CPU does between the writes?

Thanks
Nils

.

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