SECTION VI-19: The RISC Architecture in the PIC

There are three ways available to microprocessor designers to increase the processing power of the CPU:

  1.  Increase the clock frequency of the chip. The higher the frequency, the more power and heat dissipation. Power and heat dissipation is especially a problem for hand-held devices.
  2. Use Harvard architecture by increasing the number of buses to bring more information (code and data) into the CPU to be processed. In the case of x86 and other general purpose microprocessors, this architecture is very expensive and unrealistic. In today's micro-controllers this is not a problem.
  3. Change the internal architecture of the CPU and use what is called RISC architecture.


Microchip used all three methods to increase the processing power of the PIC18 micro-controllers.

In the early 1980s, a controversy broke out in the computer design community. Since the 1960s, in all mainframes and minicomputers, designers put as many instructions as they could think of into the CPU. Some of these instructions performed complex tasks. An example is adding data memory locations and storing the sum into memory. Naturally, microprocessor designers followed the lead of minicomputer and mainframe designers. Because these microprocessors used such a large number of instructions and many of them performed highly complex activities, they came to be know as CISC (Complex instruction set computer). According to several studies in the 1970s, many of these complex instructions etched into the brain of the CPU were never used by programmers and compilers. The huge cost of implementing a large number of complex instructions into the microprocessor, plus the fact that a good portion of the transistors on the chip are used by the instruction decoder, made some designers think of simplifying and reducing the number of instructions. As this concept developed, the resulting processors came to be known as RISC (Reduce Instruction Set Complex).

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