8 January 2012

Instructions: Multimedia Extensions of the Desktop/ Server RISCs

Since every desktop microprocessor by definition has its own graphical displays, as transistor budgets increased it was inevitable that support would be added for graphics operations. Many graphics systems use 8 bits to represent each of the three primary colors plus 8 bits for a location of a pixel. The addition of speakers and microphones for teleconferencing and video games suggested support of sound as well. Audio samples need more than 8 bits of precision, but 16 bits are sufficient. Every microprocessor has special support so that bytes and half words take up less space when stored in memory, but due to the infrequency of arithmetic operations on these data sizes in typical integer programs, there is little support beyond data transfers. The architects of the Intel i860, which was justified as a graphical accelerator within the company, recognized that many graphics and audio applications would perform the same operation on vectors of these data.

 Although a vector unit was beyond the transistor budget of the i860 in 1989. Like a virus, over time such multimedia support has spread to nearly every desktop microprocessor. HP was the first successful desktop RISC to include such support. As we shall see, this virus spread unevenly. The PowerPC is the only holdout, and rumors are that it is “running a fever.”

The Alpha MAX operations are just byte versions of compare, min, max, and absolute difference, leaving it up to software to isolate fields and perform parallel adds, subtracts, and multiplies on bytes and half words. MIPS also added operations to work on two 32-bit floating-point operands per cycle, but they are considered part of MIPS V and not simply multimedia extensions. One feature not generally found in general-purpose microprocessors is saturating operations. Saturation means that when a calculation overflows, the result is set to the largest positive number or most negative number, rather than a modulo calculation as in two’s complement arithmetic. Commonly found in digital signal processors these saturating operations are helpful in routines for filtering.


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