Re: [Tack-devel] ACK compiles on NetBSD-macppc, sort of...

[Gregory T. (t. K. <gt...@di...>]

At 4:13 PM -0400 7/21/06, Perry E. Metzger wrote:
>>>Well, this is an assembler issue, right? It should be straightforward
>>>to generate ELF instead of other formats. You pretty much *need* ELF
>>>on all modern platforms -- Linux and all the rest use ELF too. You
>>>probably also need to be able to properly embed debugging symbols...
>>
>> I can't answer that at this time.
>
>What is there to answer? :)

Whether or not it is better to break the structure and generate ELF=
 natively, or keep the ACK structure and convert ack.out to ELF.

>> It is probably important to consider that Tanenbaum's group went
>> with multiple address space segmentation for a virtual memory model,
>> while the rest of the world went with singe address space paging.
>
>Er, huh????

It would seem to me that the virtual memory model used by an operating=
 system would have some impact on choices of file formats.  Beyond what is=
 needed to get ACK running on NetBSD, I am not particularly concerned with=
 ensuring ELF is the native file format.

>> Rather than having to go through some of the very twisted hoops (and
>> at least on PPC 32-bit ELF, broken hoops) to achieve various memory
>> protections for each section, Minix just puts each section in its
>> own address space (segment).=20
>
>Er, the original Minix didn't have a choice, and modern Minix is
>demand paged, not segment swapped.=20

That statement would be in direct contradiction to "Operating Systems:=
 Design and Implementation" (Tanenbaum, Woodhull) page 420:

"Memory management in Minix 3 is simple: paging is not used at all.  Minx 3=
 memory management as we will discuss it here does not include swapping eith=
er."

Additionally, on page 422:

"In Minix 3, however, the default is to compile programs to use separate I=
 and D space."

>All modern operating systems are
>demand paged. There isn't even support for segmentation on PPC -- the
>x86 alone of modern processors still sort of has it but it is a rather
>lame holdover in the architecture.

There's a difference between operating systems lacking support for PowerPC's=
 segmentation model and PowerPC lacking support for segmentation.  While=
 almost all operating systems lack support for PPC's segmentation model, PPC=
 paged segmentation itself is quite robust (see Section 7.5 of the=
 Programming Environment Manual, in particular the discussion on virtual=
 segment IDs).  While I don't have any direct experience confirming this,=
 there are plenty of discussions on the web indicating that 32-bit PowerPC=
 ELF breaks a lot of PPC's possibilities.  There is a reason why almost all=
 PPC vendors use file formats other than ELF (PEF, IBM's modified xcoff,=
 Mach-O for all its warts), and there are reasons I am reluctant to spend=
 any time more than absolutely necessary making ACK work with ELF.

In NetBSD/macppc the default is just to map the segment registers in 256M=
 increments for the full 4G/32 bit address space.  That is only one possible=
 use of the segment registers, but expected in a single address space paged=
 operating system.  There are 16 segment registers, each capable of=
 containing a 24 bit segment id and protection state for that 256M space. =
 That's a lot of support for segmentation.  Or at least, that's my=
 understanding of the paged segmentation mode and that it is widely underuti=
lized.

tim

Gregory T. (tim) Kelly
Owner
Dialectronics.com

P.O. Box 606
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