lc3 - Are these output streams possible? -

i working on practice exam question

a the problem stack machine has input stream of abcd. possible output stream of cdba? cdab? show sequence of operations create these streams or if not possible explain why not possible.

i said both output streams possible.

for cdba, here sequence of operations create such stream

1. push a
2. push b
3. push c
4. pop c/store
5. push d

for cdab

1. push
2. pop a/store
3. push b
4. push
5. push c
6. pop c / store
7. push d
8. push c

i assumed stack machine works stack -push pop. agree 2 sequences of operations? there property of "stack machine" makes these streams impossible create/produce?

yup, looks except need push c again @ end of cdba.