Andrew S. Tanenbaum (OS) Edition 4 Exercise 3 Question 1 (Page No. 254)

Question

The $IBM\:360$ had a scheme of locking $2-KB$ blocks by assigning each one a $4-bit$ key and having the CPU compare the key on every memory reference to the $4-bit$ key in the $PSW$. Name two drawbacks of this scheme not mentioned in the text.

Answer 1

Program Status Word $\mathrm{(\mathbf{PSW})}$:

The program status word is the group of data which is 8 bytes in length and provided by the operating system.

The two drawbacks are as follows:

1. The processing speed of the hardware used for doing the comparison has to be really fast.

2. As the 4-bit key is used by every reference, so maximum only $16$ keys are free in the range of $0$ to $14$. The $16$ applications can be fit in the memory at any instance of time if every key is given to an application. So, due to this, all the references get locked completely. Therefore, it will block every effort to run since each $\mathbf {PSW}$ is currently in use already.