Itanium Rising: Breaking Through Moore's Second Law of Computing Power

Data Speculation

EPIC also speeds up processing by a technique know as data speculation. Speculation works to reduce the memory latency, which is the delay time to access and move data to and from memory. Typically, memory speed is slower than the processor speed, which causes the processor to wait or “stall” for several clock cycles each time it must access memory. As a general rule, today's systems have memory that runs at about half the speed of the processor.

With speculation, the program is able to put all the “load instructions” at the start of a routine. As the loads are processed, other set up or processing can be performed so that when the program is ready to use the load results, the data is ready to use. As an example, see Figure 8-3 ...