Which ARM architectures need support? (was Re: Restructuring trapframe)

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

On 20 Sep, 2013, at 03:26 , Matt Thomas <matt%3am-software.com@localhost> wrote:
> It depends.  If a kernel can be compiled for both pre-armv6 and armv6+
> then it needs to share a common trapframe.  Otherwise they could be
> different and let the compiler deal with the differences.  One alternative
> is to only use the new trapframe for armv6 and later and default to the
> older trapframe for any kernel that incorporates pre-armv6.

On a tangentially related topic, I noticed other bits of the instruction
set which vary with the architecture version.  In particular, umull
needs to avoid having its results overwrite its source registers for
pre-armv6 CPUs, the clz instruction didn't exist prior to armv5 and
strd exists on ARMv5TE and later CPUs but not ARMv5T and earlier.

Does NetBSD code need to be written to run on armv4 CPUs?  And, if I
cared enough to want to do it, is there a way to conditionally compile for
the version of the architecture the system is being built for, in the
kernel and for applications, or does the code just need to be written
for the lowest common denominator?

Dennis Ferguson