This is true in the classic PDP-10 architecture, but would it
really be worthwhile to run Linux in an 18-bit address space?.
In the extended architecture, an int * is a 30-bit address, and the
byte offset and size is encoded in the top 6 bits. This would be a
more suitable target for Linux.
> This would mean that casting char * pointers to unsigned long and
> vice-versa wouldn't give the kind of results we expect. The kernel
> assumes in a lot of places that memory is byte-addressable and that
> casting a pointer to an unsigned long gives you the byte address of
> the first byte of the object that the pointer points to, and that it
> can do arithmetic on those byte addresses.
When a pointer-to-integer conversion is made (or vice versa), GCC
could generate code to convert between a PDP-10 hardware pointer and a
linear byte address.
> Another difficulty would be in relation to the MMU. IIRC, the KA10
> processor had a simple offset/limit memory management scheme, which
> would not be sufficient for linux, which requires support for paged
> virtual memory.
Right.
> I have forgotten what the KI10 and KL10 processors did; I recall it
> was more complex but I don't think it amounted to paged virtual
> memory.
The KI10 has limited support for paging, but I don't remember the
details. KL10 most definitely supports full paged virtual memory.
-- Lars Brinkhoff http://lars.nocrew.org/ Linux, GCC, PDP-10 Brinkhoff Consulting http://www.brinkhoff.se/ programming - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/