An Operating Systems Vade Mecum

68 Space Management Chapter 3bytes in physical store. The largest virtual store is also 64K bytes. If 6 bits specify thechunk, each set of 1024 bytes has its own lock, and the length of each partition must be amultiple of 1024 bytes. There are 64 chunks in all. Figure 3.4 shows how the hardwarechecks addresses.Each access must be checked, whether it is for an instruction fetch or a data fetchor store. Security introduces a slight amount of extra work on each access, but not asmuch as address translation requires. This work can often be overlapped with the mainstore access, which can be cancelled if it turns out that the security check fails. Suchoverlap is crucial to making fast computers.The kernel also has a key, and the hardware checks all its accesses, too. However,the kernel usually uses a special ‘‘master key,’’ which opens all the locks. Most attemptsto subvert the security on operating systems for the IBM 360 (many of them successful)have had the master key as their goal. Once a process is running with the master key, itcan do anything.The simple partition method described here does not provide the ability to shareparts of virtual store among processes. Either the processes have the same key, in whichcase they share entirely, or they have different keys and cannot share at all. However,each process can be given a number of keys, and finer-grained sharing is therefore possible.The logical extension of this idea is called capabilities, an important concept discussedin Chapter 6.addresschunkoffsetkeylockyesoknophysical-storedescriptor tabletrapFigure 3.4 Hardware address checking