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Acorn M4

Posted: Mon Nov 27, 2017 5:02 pm
by Pernod
What does anyone know about the M4? Why was is so big, what was it supposed to do?
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The attached ROMs are supposedly from the M4, but on inspection contain text 'Acorn RISC Second Processor', 'Brazil a500dev io DUAL memc version -.005 (18th November 1987)'. So I wonder whether they are actually from the A500 2nd processor.

Re: Acorn M4

Posted: Mon Nov 27, 2017 5:31 pm
by paulb
Obligatory Chris's Acorns link which for once doesn't have much information.

The chronology might be interesting here. Perhaps people were porting Unix to the ARM a lot earlier than we thought, in that it seemed from what people have said at various times and things like job adverts, Acorn seemed to be getting into Unix after the Archimedes was already out there. Then again, maybe it was easier to work with something closer to a second processor, particularly if they were exploring multiple MEMC capabilities for the first time.

Re: Acorn M4

Posted: Mon Nov 27, 2017 5:50 pm
by crj
I once owned the M4 that is now in the National Museum of Computing, and I did play with it a little before I donated it. I think everything below is accurate, but I might have misconstrued/misremembered somewhere along the line.

In many respects, it's very like an A680 - a stepping-stone between the A400 and the A540 ranges. So it has an ARM 2, but two MEMCs, 8Mbytes of RAM and a couple of sockets to expand it up to 16Mbytes which, so far as I can tell, are intercompatible with the A540 RAM expansion sockets.

The really shiny thing the A680 had that no mass-production Archimedes ever did was a mainboard SCSI interface with integrated fast 32-bit dual-port RAM. This meant SCSI performance was way better than with the SCSI podule. The M4 was similar, but had much more RAM. (The numbers 32Kbytes and 256Kbytes respectively spring to mind, but are way more suspect than the rest of my memory about this.)

The M4 was designed for one single purpose: to be the RISC iX build machine. Hence the need for uncharacteristically good SCSI I/O. Also, hence the need for a big box: it was empty of hard drives by the time I rescued it, but I imagine it once held several full-height 5.25" drives. The other reason the box was so large, however, was that it was an off-the-shelf enclosure rather than anything custom.

The operating system was trivial, and presumably a one-off. The EPROM labels were never meant to be read by anybody so are likely whatever stickers first came to hand. All the OS contained was a ram-test utility (it had some silly name like *ZZZZZZ), FileSwitch, FileCore and SCSIFS. The OS itself seemed pretty rudimentary - I wouldn't be surprised if it was a fork of Arthur rather than anything more recent!

Re: Acorn M4

Posted: Mon Nov 27, 2017 5:56 pm
by crj
paulb wrote:The chronology might be interesting here. Perhaps people were porting Unix to the ARM a lot earlier than we thought

The first attempt was based around the Mach microkernel, and I just bet it happened around the same time as RISC OS Gold and the Victoria project - their moonshot was a rival for the MicroVAX which could run RISC OS and Unix applications in concert.

I did once find a mysterious hard drive containing a bootable-but-not-really Mach-based RISCiX. It looked like they'd put a lot of effort into it before giving up and porting to bare metal instead. That couldn't have been quick!

(I'm unsure whether that hard drive is now in Bletchley or a cardboard box in the back of one of my cupboards. Probably Bletchley.)

Re: Acorn M4

Posted: Mon Nov 27, 2017 8:59 pm
by paulb
crj wrote:
paulb wrote:The chronology might be interesting here. Perhaps people were porting Unix to the ARM a lot earlier than we thought

The first attempt was based around the Mach microkernel, and I just bet it happened around the same time as RISC OS Gold and the Victoria project - their moonshot was a rival for the MicroVAX which could run RISC OS and Unix applications in concert.


I had heard about RISC OS Gold before but not the Victoria project. Using Mach as the basis of operating systems was quite fashionable at one time, with there being OSF/1 (DEC Tru64), NeXTStep (and subsequently Mac OS X), GNU Hurd, and (I think) also the IBM project known as SawMill that have used it.

Mach was criticised for its context-switching performance, if I recall correctly, and one now sees other microkernel families gaining use at its expense. It does make me wonder how the ARM architecture of the day would have coped, especially given what people have noted about how well it supported ARX, which was supposedly a microkernel-based operating system, too.

crj wrote:I did once find a mysterious hard drive containing a bootable-but-not-really Mach-based RISCiX. It looked like they'd put a lot of effort into it before giving up and porting to bare metal instead. That couldn't have been quick!

(I'm unsure whether that hard drive is now in Bletchley or a cardboard box in the back of one of my cupboards. Probably Bletchley.)


More treasure waiting to be unearthed!

Re: Acorn M4

Posted: Wed Nov 29, 2017 1:21 am
by Boydie
crj wrote:I did once find a mysterious hard drive containing a bootable-but-not-really Mach-based RISCiX. It looked like they'd put a lot of effort into it before giving up and porting to bare metal instead. That couldn't have been quick!

(I'm unsure whether that hard drive is now in Bletchley or a cardboard box in the back of one of my cupboards. Probably Bletchley.)


Probably got more chance if being re-found if it’s in Bletchley. Isn’t the back of your cupboards somewhere to the west of Narnia?

Re: Acorn M4

Posted: Wed Nov 29, 2017 1:52 am
by crj
If the back of my cupboards were a portal to Narnia, they wouldn't be so full!

Re: Acorn M4

Posted: Thu Jan 04, 2018 9:35 pm
by SteveBagley
Just been looking at the photo of the motherboard above and noticed it has a Z8530 SCC chip on it? Is this the only Acorn machine to use the SCC? If so, does anyone know why they would have chosen (and presumably written drivers for) this over the chip used in the Arc?

I wonder if they were considering using it to handle Econet (and/or Localtalk) as well?

Steve