Many base bound pairs

Figure 6 diagrams the situation for a system call with an indirect reference to the operation, a direct reference to an object and a single datum. There are 10 pointers involved, each of which is relative to some implied base address and must be checked against some implied bound. Furthermore, there are three capabilities involved, each of which must be checked for the correct type and sufficient access bits, as well as continued existence of the object (correct unique name in the appropriate MOT entry). Finally, any data to be manipulated within the defined object will be addressed relative to a base address (the ECS address of the object) and checked against a bound (the size of the object). On the CDC 6400, given that the necessary data is already in the central registers, the time required to access a word in central memory by a pointer relative to a base bounds pair is twice that required to follow a direct pointer. Thus, the addition of hardware instructions which provide memory access by pointers through base bounds pairs could greatly reduce system overhead. However, additional time would still be spent checking types, access bits and unique names.

 

Figure 6: Pointer structure for a typical ECS system call, with an indirect reference to the operation.

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