I was embarrassed when I encountered a problem reading the “Operating System Concept” (7th edition), which:
In IBM / 370, memory is protected using keys. The key is a 4-bit quantity. Each 2K memory block has a key (storage key) associated with it. The CPU also has a key (security key) associated with it. The storage operation is allowed only if both keys are equal or equal to zero. Which of the following memory management schemes can be successfully used with this equipment?
and. Naked car
b. Single user system
with. Fixed-Process Multiprogramming
e. Multiprogramming with a variable number of processes
e. Paging
f. Segmentation
I have some problems: How were such keys calculated? Why did he provide another way “either equal to zero”, while the first way was already seemingly safe? (I mean that “either zero” is used for and under what circumstances “either zero” is effective instead of matching two keys to see if they are the same.)
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