sigframe
CTASSERT((sizeof (struct sigframe) % 16) == 8);
minstacksz = sizeof (struct sigframe) + SA(sizeof (*uc));
tuc->uc_xsave = (unsigned long)(sp + sizeof (struct sigframe));
uc = (ucontext_t *)(sp + sizeof (struct sigframe) + xsave_size);
struct sigframe frame;
sp = (caddr_t)fp + sizeof (struct sigframe);
struct sigframe *volatile fp;
minstacksz = sizeof (struct sigframe);
fp = (struct sigframe *)
fp = (struct sigframe *)(tos - SA(minstacksz));