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What does “faster than 400MHz” mean in this context? Does that mean you can’t ask for a unit of memory from it more than 400M times a second? If so, what’s the
by kstrauser 2y ago
What does “faster than 400MHz” mean in this context? Does that mean you can’t ask for a unit of memory from it more than 400M times a second? If so, what’s the basic unit there, a bit? A word?
I built a little CPU in undergrad but never got around to building RAM and admit it’s still kind of a black box to me.
Bonus question: When I had an Amiga, we’d buy 50 or 60ns RAM. Any idea what that number meant, or what today’s equivalent would be?
- hajile 2y agoThe capacitors take time to charge and discharge. You can't do that more than around 400MHz with current materials. You are correct that it means you can't access the same bit of memory more than 400M/sec. This is the same whether you are accessing 1bit or 1M bits because the individual capacitors that make up those bits can't be charged/discharged any faster. When we moved from SDR to DDR1, latencies dropped from 20-25ns to about 15ns too, but if you run the math, we've been at 13-17ns of latency ever since.
- rasz 2y agoYep, we pretty much hit a wall during DDR2 rein https://en.wikipedia.org/wiki/CAS_latency#Memory_timing_examples https://en.wikipedia.org/wiki/CAS_latency#Memory_timing_exam...
- anticensor 2y agoNo, 7.5 to 9.75ns during the DDR4 rein (DDR4-4266), according to that page.
- rasz 2y agoDDR2-1066 CL4 is also 7.5ns to first data.
- anticensor 2y agoBut DDR2-1066 is worse in second and later accesses.
- rasz 2y agoConsequent accesses are measuring bus speed, not actual DRAM cell latency. At that point data is already loaded into a row of sense amplifiers.
- deleted 2y ago[deleted]