I wrote this little piece of code to calculate HL = A * 12. It seems to
be a bit faster than anything else I could find, so maybe some of you
could benefit from it, or prove me wrong! :)
It takes 46 T-states, and destroys A:
add a, a ; A * 2
ld l, a
add a, a ; A * 4
add a, l ; A * 6
ld l, a
sbc a, a
and 1
ld h, a
add hl, hl ; A * 12