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Re: 64-bit embedded computing is here and now

From Dimiter_Popoff <dp@tgi-sci.com>
Newsgroups comp.arch.embedded
Subject Re: 64-bit embedded computing is here and now
Date 2021-06-09 21:00 +0300
Organization TGI
Message-ID <s9qvji$1nu$2@dont-email.me> (permalink)
References (2 earlier) <s9n10p$t4i$1@dont-email.me> <Iyz*Vc+ly@news.chiark.greenend.org.uk> <s9ojag$4ce$1@dont-email.me> <s9okhu$e58$1@dont-email.me> <s9pvth$b1c$1@dont-email.me>

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On 6/9/2021 11:59, David Brown wrote:
> On 08/06/2021 22:39, Dimiter_Popoff wrote:
>> On 6/8/2021 23:18, David Brown wrote:
>>> On 08/06/2021 16:46, Theo wrote:
>>>> ......
>>>
>>>> Memory bus/cache width
>>>
>>> No, that is not a common way to measure cpu "width", for many reasons.
>>> A chip is likely to have many buses outside the cpu core itself (and the
>>> cache(s) may or may not be considered part of the core).  It's common to
>>> have 64-bit wide buses on 32-bit processors, it's also common to have
>>> 16-bit external databuses on a microcontroller.  And the cache might be
>>> 128 bits wide.
>>
>> I agree with your points and those of Theo, but the cache is basically
>> as wide as the registers? Logically, that is; a cacheline is several
>> times that, probably you refer to that.
>> Not that it makes much of a difference to the fact that 64 bit data
>> buses/registers in an MCU (apart from FPU registers, 32 bit FPUs are
>> useless to me) are unlikely to attract much interest, nothing of
>> significance to be gained as you said.
>> To me 64 bit CPUs are of interest of course and thankfully there are
>> some available, but this goes somewhat past what we call  "embedded".
>> Not long ago in a chat with a guy who knew some of ARM 64 bit I gathered
>> there is some real mess with their out of order execution, one needs to
>> do... hmmmm.. "sync", whatever they call it, all the time and there is
>> a huge performance cost because of that. Anybody heard anything about
>> it? (I only know what I was told).
>>
> 
> sync instructions of various types can be needed to handle
> thread/process synchronisation, atomic accesses, and coordination
> between software and hardware registers.  Software normally runs with
> the idea that it is the only thing running, and the cpu can re-order and
> re-arrange the instructions and execution as long as it maintains the
> illusion that the assembly instructions in the current thread are
> executed one after the other.  These re-arrangements and parallel
> execution can give very large performance benefits.
> 
> But it also means that when you need to coordinate with other things,
> you need syncs, perhaps cache flushes, etc.  Full syncs can take
> hundreds of cycles to execute on large processors.  So you need to
> distinguish between reads and writes, acquires and releases, syncs on
> single addresses or general memory syncs.  Big processors are optimised
> for throughput, not latency or quick reaction to hardware events.
> 
> There are good reasons why big cpus are often paired with a Cortex-M
> core in SOCs.
> 
> 

Of course I know all that David, I have been using power processors
which do things out of order for over 20 years now.
What I was told was something about a real mess, like system memory
accesses getting wrong because of out of order execution hence
plenty of syncs needed to keep the thing working. I have not
even tried to verify that, only someone with experience with 64 bit
ARM can do that - so far none here seems to have that.

Dimiter

======================================================
Dimiter Popoff, TGI             http://www.tgi-sci.com
======================================================
http://www.flickr.com/photos/didi_tgi/

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Thread

64-bit embedded computing is here and now James Brakefield <jim.brakefield@ieee.org> - 2021-06-07 07:47 -0700
  Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-07 14:13 -0700
  Re: 64-bit embedded computing is here and now Paul Rubin <no.email@nospam.invalid> - 2021-06-07 22:31 -0700
    Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-08 07:59 +0200
      Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-08 00:39 -0700
        Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-08 13:04 +0200
          Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-08 17:30 -0700
            Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-09 09:17 +0200
              Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-09 03:12 -0700
                Re: 64-bit embedded computing is here and now George Neuner <gneuner2@comcast.net> - 2021-06-12 12:58 -0400
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-12 12:40 -0700
              Re: 64-bit embedded computing is here and now Paul Rubin <no.email@nospam.invalid> - 2021-06-09 09:41 -0700
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-09 10:12 -0700
                Re: 64-bit embedded computing is here and now DJ Delorie <dj@delorie.com> - 2021-06-09 13:16 -0400
                Re: 64-bit embedded computing is here and now Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2021-06-09 13:44 -0400
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-09 22:03 +0300
                Re: 64-bit embedded computing is here and now Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2021-06-09 15:22 -0400
                Re: 64-bit embedded computing is here and now Paul Rubin <no.email@nospam.invalid> - 2021-06-09 12:58 -0700
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-09 16:26 -0700
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-09 23:12 +0300
        Re: 64-bit embedded computing is here and now James Brakefield <jim.brakefield@ieee.org> - 2021-06-08 12:38 -0700
          Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-08 22:11 +0200
            Re: 64-bit embedded computing is here and now James Brakefield <jim.brakefield@ieee.org> - 2021-06-08 14:25 -0700
            Re: 64-bit embedded computing is here and now George Neuner <gneuner2@comcast.net> - 2021-06-09 00:16 -0400
              Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-09 10:40 +0200
                Re: 64-bit embedded computing is here and now Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2021-06-09 22:52 +0200
                Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-10 09:37 +0200
          Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-09 01:01 +0300
            Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-08 18:29 -0700
              Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-09 20:56 +0300
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-09 17:12 -0700
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-10 13:45 +0300
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-10 06:55 -0700
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-10 18:32 +0300
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-10 14:09 -0700
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-11 01:13 +0300
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-10 21:55 -0700
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-11 14:14 +0300
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-11 05:10 -0700
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-11 16:35 +0300
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-11 16:24 -0700
              Re: 64-bit embedded computing is here and now George Neuner <gneuner2@comcast.net> - 2021-06-14 05:41 -0400
          Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-08 16:51 -0700
          Re: 64-bit embedded computing is here and now Paul Rubin <no.email@nospam.invalid> - 2021-06-08 18:20 -0700
            Re: 64-bit embedded computing is here and now Theo <theom+news@chiark.greenend.org.uk> - 2021-06-09 18:07 +0100
              Re: 64-bit embedded computing is here and now Paul Rubin <no.email@nospam.invalid> - 2021-06-09 10:34 -0700
                Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-09 16:37 -0700
              Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-09 16:43 -0700
      Re: 64-bit embedded computing is here and now Theo <theom+news@chiark.greenend.org.uk> - 2021-06-08 15:46 +0100
        Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-08 22:18 +0200
          Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-08 23:39 +0300
            Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-08 18:33 -0700
            Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-09 10:59 +0200
              Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-09 21:00 +0300
                Re: 64-bit embedded computing is here and now David Brown <david.brown@hesbynett.no> - 2021-06-09 20:55 +0200
                Re: 64-bit embedded computing is here and now Dimiter_Popoff <dp@tgi-sci.com> - 2021-06-09 22:06 +0300
        Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-08 16:00 -0700
          Re: 64-bit embedded computing is here and now Theo <theom+news@chiark.greenend.org.uk> - 2021-06-09 13:10 +0100
            Re: 64-bit embedded computing is here and now Don Y <blockedofcourse@foo.invalid> - 2021-06-09 06:19 -0700

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