Re: arm9 memory throughput

Nils wrote:

.... snip ...

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?

Let's assume a simple testing mechanism. The actual assembly code
will be something like:

call recordtime
mov r1, #I; number of tests to apply
mov r2, A1; starting address to use
mov r3, #0; initialize counter
; start of loop
lp: mov f4, r2+r3; where to write
mov (r4), #0; what we are measuring!!!
inc r3
cmp r3, r1
jnz lp; do it again
; end of loop
call recordtime
call computeanddisplay

Now look at the work done within the loop compared to the writes.
Each instruction requires a memory read just to access it. There
are 5 of these. At best the COU requires no time to execute
things, in which case there is already a 6 : 1 reduction in writing
speed from memory access speed.

Smart use of caches etc. can improve this ratio. It will never
become 1. And any such improvement costs money.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.


.

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