They also tried to radically solve the problem with the lack of registers: their number grew several times bigger. Itanium has 128 general purpose registers, 128 floating-point registers, 8 branch registers and 64 registers responsible for prediction algorithms. Well, so many 64bit registers will definitely be enough for storing any amount of numbers for any reasonable amount of execution units. Itanium, the first representative of the new family, has 5 of them: 2 integer units and 2 units for work with the memory (it makes 4 ALU instructions per clock), and 4 for floating-point operations. The physical memory is addressed with 44bit numbers, which actually limits the capacity at “only” 17.6 terabytes. The floating-point units work with 82bit number representations.

Intel gave up the idea of designing hardware 32bit x86 core, because they considered it to be just a waste of the die surface. That is why to make Itanium capable of performing the good old x86 code, they had to develop a special translation system transforming the x86 code into IA-64 code on the fly. Of course, the performance of such solution will be lower than that of the pure x86 working at the same core clock frequency. But to tell the truth, no one expects Itanium to be really fast with x86 applications: the support of this architecture is actually considered just an expense of the transitional period. Nevertheless, the fact is undeniable: this family is not good at solving 32bit tasks. Although, I really doubt that anyone will ever buy Itanium for this purpose.

Besides, Itanium was mostly a pilot project, like Pentium Pro, that is why we would consider it mostly a demonstration of the architecture capabilities rather than a real commercial product. A typical indication of that is the fact that the chipset for Itanium processors, 460GX, supports only PC100 SDRAM, which gives you at least some idea of the overall processor performance. On the other hand, very large L3 cache makes up for not very fast interface between the CPU and the RAM: it can be 2MB or 4MB big and it work at the full processor frequency (733MHz or 800MHz) with the bandwidth up to 12.8GB/sec.

Another Itanium’s goal was to solve the matter with the compilers, especially, since EPIC processors depend a lot on them, as we have already said. Unlike x86 compilers, which hardly affected the CPU performance at all, these compilers are fully-fledged partners of the EPIC CPUs, because they supply the processor with the vitally important information. So, the processor performance directly depends on the quality of the information supplied by the compiler.