Berkeley RISC

Berkeley RISC is one of two seminal research projects into RISC-based microprocessor design taking place under ARPA's VLSI project. RISC was led by David Patterson (who coined the term RISC) at the University of California, Berkeley between 1980 and 1984.

The other project took place only a short drive away at Stanford University under their MIPS effort starting in 1981 and running until 1984. Berkeley's project was so successful that it became the name for all similar designs to follow; even the MIPS would become known as a "RISC processor". The Berkeley RISC design was later commercialized as the SPARC processor, and inspired the landmark DEC Alpha architecture as well as the ARM architecture which by 2014 powers most mobile phones.

Both RISC and MIPS were developed from the realization that the vast majority of programs did not use the vast majority of a processor's instructions. In one calculation it was found that the entire Unix system, when compiled, used only 30% of the available instructions on the Motorola 68000. Much of the circuitry in the CPU was dedicated to decoding these instructions which were never being used. The RISC idea was to include only those instructions that were really used, using the space that had been used for the removed circuitry for other circuits that would speed the system up instead.

To do this, RISC concentrated on adding many more registers, small bits of memory holding temporary values that can be accessed at negligible cost. This contrasts with normal main memory, which might take several cycles to access. By providing more registers, and making sure the compilers actually used them, programs should run much faster. Additionally the speed of the processor would be more closely defined by its clock speed, because less of its time would be spent waiting for memory accesses. Transistor for transistor, a RISC design would outperform a conventional CPU, hopefully by a lot.

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