Date: Tue, 8 Mar 2016 15:23:28 -0800 From: John Clark <jeclark2006@aim.com> To: Ian Lepore <ian@freebsd.org> Cc: Markus Hitter <mah@jump-ing.de>, freebsd-embedded@freebsd.org Subject: Re: ? about kernel size.. Message-ID: <06F91A0C-648B-49B2-88C1-86FDD5D8B30F@aim.com> In-Reply-To: <1457478889.1406.50.camel@freebsd.org> References: <CAPKZHbVyPji-bZwDzM77TN6qybjRcf%2BZe5r6WZmbG98LkhT-rg@mail.gmail.com> <CANCZdfriqr24Lh9ZuptaC0gEm6gAV6LN9XHcVAJtbyaBejEgNg@mail.gmail.com> <CAPKZHbW%2BG7WnSU__yeYBVPqs8MPmFm-5q_wM4sm9FxHhEEgPDg@mail.gmail.com> <1457473674.1406.46.camel@freebsd.org> <CAPKZHbX8BXKC_=8PPvtasqE%2BRj96_mPQkqdRt=hqU6fazxpPfA@mail.gmail.com> <56DF513E.9000405@jump-ing.de> <938133EE-CCEE-4F08-BB78-8227D9D0C6EC@aim.com> <1457478889.1406.50.camel@freebsd.org>
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On Mar 8, 2016, at 3:14 PM, Ian Lepore <ian@freebsd.org> wrote: > Jeez, really? This thread is going to be hijacked to discuss the > theoretical possiblity of running on 40 year old hardware? > > Unbelievable. > > — Ian Some one trying to run something like FreeBSD on a non-MMU processor, is going … 40 years back in time(technically the 286 is a 35 or so year old design…)… Demand the project has at least a serviceable MMU… A few years ago, the boss got the big idea to use an 8051 based processor board for a system controller unit… ugh… I started my career using the 8048 and the ‘graduated’ to the 8051… then moved to a different company to work on ‘real’ machines… PDP-11s… In way the system controller was an not so pleasant walk down memory lane… John Clark. > > On Tue, 2016-03-08 at 15:09 -0800, John Clark via freebsd-embedded > wrote: >> Not to barge in to your discussion… but yes there were Unix variants >> that worked on the 286. Xenix was one such OS. Xenix being >> Microsoft’s idea of avoiding the Unix name… >> >> Originally Xenix ran on PDP-11, and was ported to other machines that >> have long passed into the oblivion of technological Hell. >> >> As for ‘preemetive multitasking… of course one can run a preemptive >> scheduler on almost any CPU that has a clock interrupt. >> >> I wrote one for the 286 based on the kernel described in Douglas >> Comer’s “Xinu” book. >> >> Products with that hack where produced for about 8 years… in the >> early-mid-80s. I wrote it such that it would run in a DOS box and >> allow for machine control of various robotic systems for industrial >> inspection machines. >> >> In any case one can develop a multitasking kernel to run in a non-MMU >> based system… just lends itself to crapping out on the least >> provocation… >> >> What an MMU provides is hardware ‘address translation’ such that the >> application can run in an ‘virtual absolute’ addressing space, >> have memory protection such that errant code can’t clobber other >> tasks or the kernel, and also given appropriate devices, >> have ‘swapping’ for larger than real memory applications. >> >> The 286 ‘segment’ registers gave a bit of ‘translation’ capability in >> that one could address relative to the segment registers, and so, >> code and data could be place in available member and context switched >> would update the segment registers. >> >> The segment registers were a crappy way of accessing memory if one >> had long linear arrays of data to process… such as image processing… >> which happened to be the application of my work at the time… >> >> And one could always implement an external MMU which was popular with >> the Motorola M68K which was sort of the contemporary alternative to >> the Intel x86 line. >> >> John Clark. >> >> On Mar 8, 2016, at 2:25 PM, Markus Hitter <mah@jump-ing.de> wrote: >> >>> Am 08.03.2016 um 22:56 schrieb Brad Walker: >>>> But, are you saying that no engineering has been done on this yet >>>> OR no >>>> amount of engineering could make it work? >>> >>> If I recall correctly from some 25 years ago, memory address >>> mapping >>> (which is what a MMU does) is mandatory for preemtive multitasking. >>> An >>> i286 can't run a Unix-like OS either. >>> >>> >>> In 2008 I tried to get FreeBSD down to its minimum, too. The >>> success >>> post is about all what's left today: >>> >>> https://lists.freebsd.org/pipermail/freebsd-embedded/2008-October/0 >>> 00604.html >>> >>> The task to get there is simple and straightforward, but time >>> consuming: >>> go step by step through the kernel configuration to disable >>> whatever you >>> can spare. Configure, build, try, repeat. If you need a small >>> entire >>> system, do the same for packages and every single file you copy >>> into >>> your system image. >>> >>> >>> Markus >>>
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