The programmer has a reasonable expectation that when a solution has been developed and coded in Fortran or C, that same code will be transportable to other environments. This diagram indicates how the programmer is removed from directly working with the particular hardware currently available. This makes the programmer's code writing skills valuable and makes moving from one compute environment to another reasonably straight forward. The expertise a programmer would develop would not directly transfer to other environments. With the new compute platforms of High Performance Computing, new concerns are addressed and the compiler cannot take care of everything.
Registers hold one word of main memory storage.
The functional units can operate on information in the registers to produce a new result, which is stored in a register and then, perhaps in memory. Address computations are integer computations and are performed in the IFUs. Characterizing RISC RISC is more of a design philosophy that a set of goals. Of course every RISC processor has its own personality. This list highlights the differences between RISC and CISC processors.