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Old 10-11-2016, 07:13 PM   #1
david_8274
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Why concern about memory size during micro processor selection?


Hi all,

Generally in the early stage of an embedded system project, we need to select a micro processor first. People seem to concern about how much memory a candidate processor has. My question is, can't we always add more external memory to accommodate our need if the internal RAM that comes with the processor is not enough(as long as the data bus has enough bits for addressing the additional memory)? Is cost the only concern here?

Regards,
Wei Xu
 
Old 10-11-2016, 07:48 PM   #2
I'am
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Don't get me wrong, Wei Xu, but buy more memory or buy better CPU. Alright?
 
Old 10-11-2016, 07:50 PM   #3
david_8274
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Quote:
Originally Posted by I'am View Post
Don't get me wrong, Wei Xu, but buy more memory or buy better CPU. Alright?
unfortunately when it comes to a project decisions are not made casual like that...
 
Old 10-11-2016, 10:13 PM   #4
jpollard
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Quote:
Originally Posted by david_8274 View Post
Hi all,

Generally in the early stage of an embedded system project, we need to select a micro processor first. People seem to concern about how much memory a candidate processor has. My question is, can't we always add more external memory to accommodate our need if the internal RAM that comes with the processor is not enough(as long as the data bus has enough bits for addressing the additional memory)? Is cost the only concern here?

Regards,
Wei Xu
Depends on the processor/chip vendor/...

some chips do not pass the memory buss out of the chip - only one or more peripheral buss

Always check the design against requirements.
 
Old 10-11-2016, 10:50 PM   #5
Timothy Miller
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Quote:
Originally Posted by david_8274 View Post
Hi all,

Generally in the early stage of an embedded system project, we need to select a micro processor first. People seem to concern about how much memory a candidate processor has. My question is, can't we always add more external memory to accommodate our need if the internal RAM that comes with the processor is not enough(as long as the data bus has enough bits for addressing the additional memory)? Is cost the only concern here?

Regards,
Wei Xu
Accessing ram is MUCH, MUCH, MUCH slower than accessing internal caches. Even each stage of cache is (generally, but not always) slower than the previous. We like to think of RAM as fast, but compared with the onboard cache, it's pretty slow due to the limited bus that it's on. One of the best demonstrations is the v1 Celeron. They removed the L2 cache from the Pentium II and released it as a Celeron. Even configured with the same amount of RAM as a P-II 266 (the celeron was also 266 MHz), the Celeron was DECIMATED in performance for gaming, photo/video editing, basically anything that was at all processor intensive. Lacking the high speed l2 cache brought the processor to it's knees in performance, thus bringing out the second generation that had 1/2 the l2 cache (although running at full processor speed instead of 1/2 speed), and was a rousing success, AND a great overclocker (loved my Celeron-A slot 1 300 MHz that ran at 450 MHz without issue).

So onboard memory is all about getting the sizes right. After a certain point, it doesn't help the processor anymore, but under a certain point, and it hurts the processor's performance quite heavily.

Last edited by Timothy Miller; 10-11-2016 at 10:51 PM.
 
  


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