Messages in this thread | | | Subject | Re: [PATCH] C undefined behavior fix | From | Lars Brinkhoff <> | Date | 04 Jan 2002 10:52:02 +0100 |
| |
Paul Mackerras <paulus@samba.org> writes: > Lars Brinkhoff writes: > > > [Linux] won't get very far on a PDP-10, I can assure you. :) > > Any particular reason why? > As to the question whether such code could be developed, it would > depend a lot on how gcc did things. I would expect an int * to be > just an 18-bit address but a char * to be a byte pointer, i.e. a > 36-bit word with the byte offset and size in the top 18 bits and the > word address in the lower 18 bits.
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/
| |