🚀 go-pugleaf

Does anyone know the details of how Acorn managed to get RISCiX to work?
(Apart from a couple of guys at Acorn who did the hacking!)

There seem to be a number of conflicts between the architecture of the
ARM and the assumptions that BSD makes about the machine it will run on.
For example:

* BSD prioritises interrupts and disables all interrupts below a certain
  priority to acheive mutual exclusion (using x=spltty(); ...; splx(x);).
  The ARM doesn't prioritise interrupts unless you start twiddling bits
  in the IOC's interrupt mask.

* BSD uses one stack (if I remember rightly); the kernel and interrupt
  handlers use the user mode stack to hold temporaries and such. The ARM
  gives the kernel, IRQ and FIQ handlers separate stack pointers
  which, while looking like a good idea, forces the kernel to allocate
  memory for two or more stacks rather than one.

* The kernel executes from virtual (i.e. logically mapped) memory. On ARM
  based machines this could be inefficient as the MEMC would have to be
  reprogrammed to page the interrupt handler in (for example) and it would
  seem more logical to place the kernel in the physical memory space.

* Is MEMC's operating system mode used? (The one that stops access to IO
  devices while leaving the processor in SVC mode.)

* 32k is a huge page size for a Unix system; what effect does this have on
  the size of executables and how does the vm system cope with it?

Anyone care to illuminate the grey areas?

-Mark.

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Mark H. Wilkinson (mhw@minster.york.ac.uk)