I have been reading Wikipedia as an introduction to Turing machines. I found a reference to John Hopcroft and Jeffrey Ullman, (1979). Introduction to Automata Theory, Languages and Computation (1st ed.), with a note that it is difficult to read. I am working hard with Wikipedia, and fear I will be unable to understand Hopcort and Ullman.
I am curious about how a Turing machine simulates a subroutine. If I understand http://en.wikipedia.org/wiki/Post%E2%80%93Turing_machine correctly, a Turing machine stores the return address and jumps to the subroutine, and jumps back to exit the subroutine, as an IBM360, IBM3* series machine did using BL (branch and link). Of course, modern machines store the return address on a stack and call the subroutine, but the two are similar. Is my assumption about storing the return address and jumping to simulate a subroutine correct?
Furthermore, has anyone simulated a subroutine by adding a symbol to the Turing machine alphabet for each subroutine? Symbols not defined within the machine would merely default to calling/jumping to the address of the subroutine. This technique is not much different from the one described in the paragraph above. I ask, because a machine I once worked on, and SDS 930 had some undefined op-codes that could be used to call subroutines. (Scientific Data Systems (SDS) was purchased by Xerox and renamed XDS).