bootxx needs to fit within 0th track of MBR partition?

Hi,

Recently, clang build for amd64 fails for bootxx_ffsv2:
http://releng.netbsd.org/builds/HEAD-llvm/202308230950Z/amd64.build.failed

This is because it does no longer fit within 0th track of
MBR partitions on 5inch 2HD disks:
https://nxr.netbsd.org/xref/src/sys/arch/i386/stand/bootxx/Makefile.bootxx#19

It seems difficult to hack this around by adding some
optimization flags.

If we can require at least 3.5inch 2HD disks for amd64,
BOOTXX_SECTORS can be bumped to 18, and build passes.

But, I've never found the original resource which says
"second stage loaders in MBR partitions ought/need to
fit within 0th track".

Does anyone know?

If not, I will bump BOOTXX_SECTORS for amd64 in few days.

Thanks,
rin