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Can someone more knowledgeable than me elaborate on this statement? > when compared to application processors, MMUless microcontrollers are horribly expensive,

by gspr 6y ago

Can someone more knowledgeable than me elaborate on this statement? > when compared to application processors, MMUless microcontrollers are horribly expensive, power-hungry, and slow. What is it about the lack of an MMU that causes a hunger for power?

magicalhippo 6y ago
I might be wrong, but the key point between a microcontroller and a application processor is deterministic execution. When controlling a motor say, it might be vital that your interrupt handler finishes in less than 100 clock cycles. A microcontroller usually[1] doesn't have fancy out-of-order execution, fancy caches etc as that would make the execution less deterministic in time. A MMU would as well. Lacking these features also make microcontrollers a lot slower, and I'm guessing he's thinking about cost/watts per MIPS or something like that. Yes the application processors draw more power overall, but (I assume) they are so much faster it more than makes up for it in dollars per MIPS or watts per MIPS. So it appears more of a symptom than a cause. But again, I might be wrong. [1]: https://en.wikipedia.org/wiki/ARM_Cortex-M#Silicon_customization https://en.wikipedia.org/wiki/ARM_Cortex-M#Silicon_customiza...
- jaydcarlson 6y ago
  None of these application processors I reviewed have out-of-order execution. Cortex-M7 microcontrollers have icache/dcache. You can run bare-metal code on any of these application processors and it will behave more or less like a microcontroller. The lines are really pretty blurry, but the MMU is the big dividing line in my opinion (but it's obviously open for discussion).
  - magicalhippo 6y ago
    > None of these application processors I reviewed have out-of-order execution. You're right, I was thinking of the A8, A9 and similar. > the MMU is the big dividing line in my opinion I'm no expert, but seems like a reasonable line in the sand to me.
jaydcarlson 6y ago
"MMUless" was intended as a nonrestrictive modifier --- I'll remove it to avoid ambiguity. Microcontrollers tend to be built on larger processes than microprocessors are, so they eat through more active-mode power. The point I'm trying to get across is while you can run Linux on a microcontroller (which doesn't have an MMU), there's not a lot of good reasons to do so. Thanks for the feedback!

by gspr 6y ago

magicalhippo 6y ago

I might be wrong, but the key point between a microcontroller and a application processor is deterministic execution. When controlling a motor say, it might be vital that your interrupt handler finishes in less than 100 clock cycles. A microcontroller usually[1] doesn't have fancy out-of-order execution, fancy caches etc as that would make the execution less deterministic in time. A MMU would as well. Lacking these features also make microcontrollers a lot slower, and I'm guessing he's thinking about cost/watts per MIPS or something like that. Yes the application processors draw more power overall, but (I assume) they are so much faster it more than makes up for it in dollars per MIPS or watts per MIPS. So it appears more of a symptom than a cause. But again, I might be wrong. [1]: https://en.wikipedia.org/wiki/ARM_Cortex-M#Silicon_customization https://en.wikipedia.org/wiki/ARM_Cortex-M#Silicon_customiza...

jaydcarlson 6y ago
None of these application processors I reviewed have out-of-order execution. Cortex-M7 microcontrollers have icache/dcache. You can run bare-metal code on any of these application processors and it will behave more or less like a microcontroller. The lines are really pretty blurry, but the MMU is the big dividing line in my opinion (but it's obviously open for discussion).
- magicalhippo 6y ago
  > None of these application processors I reviewed have out-of-order execution. You're right, I was thinking of the A8, A9 and similar. > the MMU is the big dividing line in my opinion I'm no expert, but seems like a reasonable line in the sand to me.

jaydcarlson 6y ago

"MMUless" was intended as a nonrestrictive modifier --- I'll remove it to avoid ambiguity. Microcontrollers tend to be built on larger processes than microprocessors are, so they eat through more active-mode power. The point I'm trying to get across is while you can run Linux on a microcontroller (which doesn't have an MMU), there's not a lot of good reasons to do so. Thanks for the feedback!