Debugging interrupt hangs

So, I'm trying to get some interrupt routing stuff worked out on my TAMS 3011,
and ... Suddenly the machine wedges during boot.  Specifically, it wedges
at the tail end of cpu_configure, when it tries to enable hardware interrupts.

The only difference in dmesg output, and I can't explain it, is that the
kernel used to say:

biomask 1c3c netmask 1c3c ttymask 1c3c

and now it says:

biomask 1c1c netmask 1c3c ttymask 1c3c

Unfortunately, I don't particularly understand what these numbers measure;
they're 16-bit values on a platform where many interrupts have numbers well
over 16... In fact, eliminating the casts to u_short gets me:

biomask 1c1c netmask 7f1c3c ttymask 7f1c3c

which is at least a little more plausible.

... But it doesn't explain what happened to 0x20 (that's, what, irq 6?), or
why the machine locks up now when it used to work.  Any suggestions?

-s