The one type of standard call from which unwindability cannot be guaranteed is
the millicode call. This is because the assembler cannot automatically generate
the standard entry and exit sequences for millicode routines that allocate
additional stack space. Fortunately, relatively few millicode routines require
the creation of a stack frame. It is possible, however, to support unwinding
from such routines (i.e. nested millicode calls), provided that the millicode
routine which allocates the stack space is written so that it uses the correct
entry and exit sequences. It is the responsibility of the author of the
specific routine to incorporate these provisions into the actual code.
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