The third access path
In the figure below, three paths to the logical child segment SEG4 exist.
- The physical path from its physical parent SEG3
- The logical path from its logical parent SEG7
- A third path from SEG4's physical dependents (SEG5 and SEG6) (because segment SEG6 is a logical parent accessible from its logical child SEG2)
These paths are called full-duplex
paths, which means accessibility
to segments in the paths is in two directions (up and down). Two delete
bits that control access along the paths exist, but they are half-duplex,
which
means they only block half of each respective path. No bit that blocks
the third path exists. If SEG4 were both physically and logically
deleted (in which case the PD and LD bits in SEG4 would be set), SEG4
would still be accessible from the third path, and so would both of
its parents.
Neither physical nor logical deletion prevents access to a segment from its physical or logical children. Logically deleting SEG4 prevents access to SEG4 from its logical parent SEG7, and it does not prevent access from SEG4 to SEG7. Physically deleting SEG4 prevents access to SEG4 from its physical parent SEG3, but it does not prevent access from SEG4 to SEG3.
