address. Offsets from a specified D-register would be checked by the hardware against the stack frame bound: rogue values would be trapped. Similarly, within a task, an array descriptor contained information on the array's bounds, and so any indexing operation was checked by the hardware: put another way, each array formed its own address space. In any case, the tagging of all memory words provided a second level of protection: a misdirected assignment of a value could only go to a data-holding location, not to one holding a pointer or an array descriptor, etc. and certainly not to a location holding machine code. Array storage Arrays were not stored contiguous in memory with other variables, they were each granted their own address space, which was located via the descriptor. The access mechanism was to calculate on the stack the index variable (which therefore had the full integer range potential, not just fourteen bits) and use it as the offset into the array's address space, with bound checking provided by the hardware. Should an array's length exceed 1,024 words, the array would be segmented, and the index be converted into a segment index and an offset into the indexed segment. In ALGOL's case, a multidimensional array would employ multiple levels of such addressing. For a reference to A(i,j), the first index would be into an array of descriptors, one descriptor for each of the rows of A, which row would then be indexed with j as for a single-dimensional array, and so on for higher dimensions. Hardware checking against the known bounds of all the array's indices would prevent erroneous indexing. FORTRAN however regards all multidimensional arrays as being equivalent to a single-dimensional array of the same size, and for a multidimensional array simple integer arithmetic is used to calculate the offset where element A(i,j,k) would be found in that single sequence. The single-dimensional equivalent array, possibly segmented if large enough, would then be accessed in the same manner as a single-dimensional array in ALGOL. Although accessing outside this array would be prevented, a wrong value for one index combined with a suitably wrong value for another index might not result in a bounds violation of the single sequence array; in other words, the indices were not checked individually. Because an array's storage was not bounded on each side by storage for other items, it was easy for the system to