Summary

The concept of an architecture, apart from implementations, has progressively pervaded the computer industry since the introduction of the IBM 360 family. As the number of independent makers and truly different architectures has contracted over time, the lifespan of successful architectural families has tended to lengthen. Over periods of a decade or more, extensions to an architecture simply become inevitable, in order to accommodate different technology, different applications, or different markets.

This chapter has contrasted the first two implementations of the Itanium architecture, which differ primarily in their programmer-visible cache structures, pipeline depths, and instruction latencies. Very few Itanium instructions operate differently on the two implementations; thus the architectural contract has been maintained.

The Itanium architecture might change in the future through selective additions of instruction subsets into initially unused opcode and subcode spaces in the encoding for its instructions. Accompanying changes in compilers and in operating system kernels should then ensure both forward and backward compatibility.