Resource Trajectories (See Fig. 1)

• The horizontal (vertical) axis represents the number of instructions executed by process A (B)
• Every point in the diagram represents a joint state of the two processes
• If the system ever enters the box bounded by $I_1$ and $I_2$ on the sides and $I_5$ and $I_6$ top and bottom, it will eventually deadlock when it gets to the intersection of $I_2$ and $I_6$
• At this point, A is requesting the plotter and B is requesting the printer, and both are already assigned
• The entire box is unsafe and must not be entered
• At point $t$ the only safe thing to do is run process A until it gets to $I_4$. Beyond that, any trajectory to $u$ will do
• At point $t$ B is requesting a resource. The system must decide whether to grant it or not
• If the grant is made, the system will enter an unsafe region and eventually deadlock

Figure 1: Two process resource trajectories.