Re: Custom CPU Designs

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.arch.embedded
Subject	Re: Custom CPU Designs
Date	2020-04-21 17:58 +0200
Organization	A noiseless patient Spider
Message-ID	<r7n57f$mlr$1@dont-email.me> (permalink)
References	(8 earlier) <bdee508d-74fd-44e4-a821-12834f31564f@googlegroups.com> <871rolzzts.fsf@nightsong.com> <387c667c-ae86-413e-b973-65a3fb3db7fc@googlegroups.com> <r7kd1p$kum$1@dont-email.me> <869be946-a22c-4bd9-b3ef-d0180b960343@googlegroups.com>

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On 21/04/2020 03:27, Rick C wrote:
> On Monday, April 20, 2020 at 10:53:49 AM UTC-4, David Brown wrote:
>>
>> Beyond that, you have mostly the same issues.  Deadlock, livelock,
>> synchronisation - they are all something you have to consider whether
>> you are making an FPGA design, multi-tasking on one cpu, or running
>> independent tasks on independent processors.
> 
> I've never heard of livelock until this discussion.  Reading about it I have no idea what the parallel would be in hardware design.

Livelock is when bits of a system are running fine, but the overall 
system is not making progress.  Often it is temporary and resolves 
itself (unlike deadlock).

A hardware example might be if you have a crossbar for multiple bus 
masters to access a single slave device.  You need some way of deciding 
which master gets access when both want it simultaneously.  Perhaps you 
decide that master A is more important, and always gets priority.  Then 
if master A hogs the bus, master B never gets a chance - livelock.

Often these kinds of things are straightforward to avoid as long as you 
think about what can happen.  That applies to software and hardware.

> 
> I don't even know of an example of deadlock in hardware design.

The simplest way to get a deadlock is to have two shared resources, and 
two processes (hardware modules, software tasks, whatever) that need 
both the resources, but acquire them in different orders.  But you don't 
usually get such simple cases, as they are so obvious.

> 
> So clearly these are not such important issues in hardware design.

If your designs don't involve much in the way of shared resources, 
you're not going see them.  (The same applies in software.)  It is also 
perfectly possible that the way you design your systems, they naturally 
don't occur - or that you think of them as bugs, hangs, blocks or stops 
rather than as "deadlocks".  (The same applies in software.)  It is also 
possible, though of course /highly/ unlikely, that your systems /do/ 
have the risk of deadlocks and they just haven't happened yet.  (The 
same applies in software.)

> 
> I expect the difference is that while hardware design uses signals (in fact that is the term for a "variable" in VHDL, not to be confused with a variable in VHDL which has limited scope and other limitations) it does not have resources to be allocated or seized or whatever the correct term is.   If it does for synthesis, I don't know what that would be.
> 

You can have shared resources in hardware too.

It is perhaps fair to say that the way you design hardware makes shared 
resources stand out a bit more - you have explicit sharing with 
cross-switches, multiplexors, etc.  And that might mean that 
deadlock-free solutions are mostly so obvious that you don't see them as 
a potential problem.  I discussed previously about thinking the 
"hardware way" for shared data in software development - that also makes 
it very easy to avoid deadlock.

> 
>> Task prioritising is an important issue.  But it is not just for
>> multitasking on a single cpu.  If you have a high priority task A that
>> sometimes has to wait for the results from a low priority task B, you
>> have an issue to deal with.  That applies whether they are on the same
>> cpu or different ones.  On a single cpu, you have the solution of
>> bumping up the priority for task B for a bit (priority inheritance) - on
>> different cpus, you just have to wait.
> 
> How is that an issue?  Isn't task A stalled when it is waiting which allows task B to run?

Yes.  But it means task A - the high priority task - can't be completed 
as fast as you had wanted.

> 
> I recall reading about priority issues such as "priority inversion" some time back.  But not having written any multitasking software in a long time it has all vanished.  I'm pretty glad of it too.  It was just a lot of ugly stuff I thought.  My biggest problem writing VHDL is trying to manage the verbosity.  But then there are issues in the language I have simply internalized and don't think about.
> 
> I have thought many times about the contradiction of my liking VHDL, a rather restrictive (in theory) and verbose language as well as a much simpler, easy and concise language as Forth.  Coding for Forth is like working in my basement with hand tools.  Coding in VHDL is like working on a sculpture for a museum.  Never the twain shall meet.
> 

I program mostly in C and Python.  It's hard to pick two software 
languages that are further apart - so I understand what you mean here.

Thread

Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-15 20:51 -0700
  Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-16 11:59 +0100
    Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-16 13:28 +0200
    Re: Custom CPU Designs Grant Edwards <invalid@invalid.invalid> - 2020-04-16 13:10 +0000
      Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-16 22:16 +0100
        Re: Custom CPU Designs Grant Edwards <invalid@invalid.invalid> - 2020-04-16 22:01 +0000
          Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-16 17:13 -0700
            Re: Custom CPU Designs jim.brakefield@ieee.org - 2020-04-16 18:10 -0700
            Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-16 18:37 -0700
              Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-16 22:08 -0700
              Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-17 10:02 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-17 10:49 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-17 15:44 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-17 15:23 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-17 18:15 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-17 17:49 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-18 15:06 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-18 16:08 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 15:13 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 14:24 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 16:00 +0200
                Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-20 14:32 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 23:55 +0100
                Re: Custom CPU Designs Reinhardt Behm <rbehm@hushmail.com> - 2020-04-21 15:14 +0800
                Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-18 16:51 +0100
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-18 18:06 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-18 14:53 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-19 10:11 +0100
                Re: Custom CPU Designs upsidedown@downunder.com - 2020-04-19 13:19 +0300
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-19 11:33 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 15:26 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 14:59 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-19 13:37 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-19 22:02 +0100
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-19 22:15 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-19 15:02 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 02:03 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-18 14:44 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 15:20 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 14:57 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-18 12:38 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 15:58 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 16:27 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-20 17:36 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-21 14:02 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-21 06:15 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-21 17:25 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-21 16:56 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-21 12:42 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-21 23:16 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-22 11:10 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-22 08:09 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-22 20:07 +0200
                Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-18 13:09 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-18 21:28 +0100
                Re: Custom CPU Designs upsidedown@downunder.com - 2020-04-21 20:29 +0300
                Re: Custom CPU Designs Grant Edwards <invalid@invalid.invalid> - 2020-04-21 18:10 +0000
                Re: Custom CPU Designs upsidedown@downunder.com - 2020-04-22 09:54 +0300
                Re: Custom CPU Designs Grant Edwards <invalid@invalid.invalid> - 2020-04-22 13:19 +0000
                Re: Custom CPU Designs upsidedown@downunder.com - 2020-04-22 18:09 +0300
                Re: Custom CPU Designs Grant Edwards <invalid@invalid.invalid> - 2020-04-22 17:03 +0000
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-22 11:16 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-17 10:48 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-18 16:57 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-18 14:21 -0700
                Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-18 14:50 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 16:32 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-20 18:12 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-21 10:09 +0100
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 16:31 +0100
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 16:42 +0100
                Re: Custom CPU Designs upsidedown@downunder.com - 2020-04-18 00:43 +0300
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-18 17:07 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-17 09:11 -0700
                Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-17 12:17 -0700
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-17 13:22 -0700
                Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-17 14:48 -0700
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-17 15:27 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 16:53 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 17:03 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 19:39 +0200
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 21:25 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-20 18:27 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-21 17:58 +0200
                Re: Custom CPU Designs Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-04-21 19:41 +0300
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-21 14:01 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-21 23:21 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-19 20:53 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-19 14:47 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 02:11 +0100
                Re: Custom CPU Designs Paul Rubin <no.email@nospam.invalid> - 2020-04-19 18:31 -0700
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-19 20:32 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 07:46 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-20 00:33 -0700
                Re: Custom CPU Designs Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-04-20 11:01 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 17:44 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-20 19:57 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-21 18:02 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-21 14:56 -0700
            Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-17 16:34 +0100
              Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 17:57 +0200
                Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-21 15:20 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-21 08:26 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-21 18:09 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-21 15:00 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-22 11:28 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-22 08:24 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-22 20:18 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-22 11:45 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-23 17:47 +0200
                Re: Custom CPU Designs jim.brakefield@ieee.org - 2020-04-21 15:35 -0700
            Re: Custom CPU Designs Przemek Klosowski <przemek@tux.dot.org> - 2020-04-19 18:52 +0000
              Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-19 13:42 -0700
              Re: Custom CPU Designs Clifford Heath <no.spam@please.net> - 2020-04-20 22:59 +1000
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-20 17:01 -0700
                Re: Custom CPU Designs Clifford Heath <no.spam@please.net> - 2020-04-21 10:23 +1000
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-21 03:47 -0700
                Re: Custom CPU Designs Clifford Heath <no.spam@please.net> - 2020-04-22 09:26 +1000
                Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-22 22:06 +0100
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-22 14:18 -0700
                Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-23 22:35 +0100
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-21 18:20 +0200
                Re: Custom CPU Designs Dimiter_Popoff <dp@tgi-sci.com> - 2020-04-22 18:16 +0300
                Re: Custom CPU Designs Grant Edwards <invalid@invalid.invalid> - 2020-04-22 17:06 +0000
                Re: Custom CPU Designs Dimiter_Popoff <dp@tgi-sci.com> - 2020-04-22 20:17 +0300
                Re: Custom CPU Designs Grant Edwards <invalid@invalid.invalid> - 2020-04-22 17:28 +0000
                Re: Custom CPU Designs George Neuner <gneuner2@comcast.net> - 2020-04-23 04:59 -0400
                Re: Custom CPU Designs jim.brakefield@ieee.org - 2020-04-20 17:27 -0700
              Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 17:58 +0200
                Re: Custom CPU Designs pault.eg@googlemail.com - 2020-04-21 01:00 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-21 19:21 +0200
          Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-17 17:18 +0100
    Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-16 07:40 -0700
      Re: Custom CPU Designs jim.brakefield@ieee.org - 2020-04-16 12:28 -0700
        Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-16 13:50 -0700
          Re: Custom CPU Designs jim.brakefield@ieee.org - 2020-04-16 14:19 -0700
          Re: Custom CPU Designs Theo <theom+news@chiark.greenend.org.uk> - 2020-04-16 22:33 +0100
    Re: Custom CPU Designs Clifford Heath <no.spam@please.net> - 2020-04-17 11:29 +1000
      Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-16 18:42 -0700
        Re: Custom CPU Designs Clifford Heath <no.spam@please.net> - 2020-04-17 12:35 +1000
          Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-16 20:01 -0700
            Re: Custom CPU Designs Clifford Heath <no.spam@please.net> - 2020-04-17 14:54 +1000
              Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-16 23:54 -0700
              Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-17 10:38 +0200
                Re: Custom CPU Designs Rick C <gnuarm.deletethisbit@gmail.com> - 2020-04-17 09:13 -0700
                Re: Custom CPU Designs David Brown <david.brown@hesbynett.no> - 2020-04-20 18:02 +0200

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