The simulation of an offline machine M by a standard machine requires a
lengthier description. A standard machine can simulate the computation of an
offline machine by using the four-track arrangement shown in Figure 10.7. In
that picture, the tape contents shown represent the specific configuration of
Figure 10.6. Each of the four tracks of plays a specific role in the simulation.
The first track has the input, the second marks the position at which the input is
read, the third represents the tape of M, and the fourth shows the position of M’s
read-write head.
The simulation of each move of M requires a number of moves of .
Starting from some standard position, say the left end, and with the relevant
information marked by special end markers, searches track 2 to locate the
position at which the input file of M is read. The symbol found in the
corresponding cell on track 1 is remembered by putting the control unit of
into a state chosen for this purpose. Next, track 4 is searched for the position of
the read-write head of M. With the remembered input and the symbol on track 3,
we now know that M is to do. This information is again remembered by with
an appropriate internal state. Next, all four tracks of ’s tape are modified to
reflect the move of M. Finally, the read-write head of returns to the standard
position for the simulation of the next move.
lengthier description. A standard machine can simulate the computation of an
offline machine by using the four-track arrangement shown in Figure 10.7. In
that picture, the tape contents shown represent the specific configuration of
Figure 10.6. Each of the four tracks of plays a specific role in the simulation.
The first track has the input, the second marks the position at which the input is
read, the third represents the tape of M, and the fourth shows the position of M’s
read-write head.
The simulation of each move of M requires a number of moves of .
Starting from some standard position, say the left end, and with the relevant
information marked by special end markers, searches track 2 to locate the
position at which the input file of M is read. The symbol found in the
corresponding cell on track 1 is remembered by putting the control unit of
into a state chosen for this purpose. Next, track 4 is searched for the position of
the read-write head of M. With the remembered input and the symbol on track 3,
we now know that M is to do. This information is again remembered by with
an appropriate internal state. Next, all four tracks of ’s tape are modified to
reflect the move of M. Finally, the read-write head of returns to the standard
position for the simulation of the next move.