REDUCED INSTRUCTION SET COMPUTERS (RISC)

After the initial years of the PC (let's say 1980-1986), the search for more efficient PCs, capable of greater processing power, speed, and felexibility went into high gear.
  • RISC Computers.

    To understand the idea of RISC computers, let me introduce the CISC computer.

    First, during the 1980s, and into the 1990s, most PCs were categorized as "Complex Instruction Set Computers". The CISC computer is organized to reduce the number of instructions executed by having complex instructions which perform multiple functions in one instruction.

    The CISC development was motivated by:

    1. The desire to have a low memory bandwidth because memory was slow and expensive.
    2. The desire to offer high-level complex instructions to a compiler.
    3. Reduce the semantic gap.
    4. Facilitate compilation.

    As time passed (about 1984) the concept of "reduced instruction set computers" came about. A study was done, which indicated that approximately 80% of the work in a computer was performed by approximately 20% of the instructions. This resulted in more in depth analysis of what happens with instructions, and eventually the design of the first RISC machines.

    RISC machines usually process simple instructions much faster than CISC machines; however, they are less efficient processing complex instructions. The following table can be used as a comparison in total processing time.

    The instructions for RISC machines were chosen based on which were used most frequently, and could be accomplished in as few clock cycles as possible, preferably 1 clock cycle.

    Typically, there are approximately 50 instructions in a RISC microprocessor. Complex Instructions are usually performed by combinations of these 50 instructions.

    Most CPU manufacturers make RISC based chips, and Intel is rapidly going to RISC capabilities.

    RISC Versus CISC Machines
    Instruction CISC RISC
    Simple Instruction4 cycles1 cycle
    Complex Instructions8 cycles14 cycles
    Cycle Time100 ns75 ns
    Nr. Instructionss1 million1 million
    Simple Instructions 80% 80%
    Complex Instructions20%20%

    Given these parameters, lets compare the total processing time:

    CISC: (1,000,000) x (.80 x 4 cycles + .20 x 8 cycles) x (100 ns) = 0.48 secs.

    RISC: (1,000,000) x (.80 x 1 cycle + .20 x 14 cycles) x (75 ns) = 0.27 secs.

    (1,000,000) x (.80 x 1 cycle + .20 x 14 cycles) x (100 ns) = 0.36 secs

    So, you can see there is some improvement at either a 75 or 100 ns speed on a RISC machine.

    Examples of RISC microprocessors are the Intel 860 and 960; Motorola 88000 family, DEC Alpha chip; PowerPC, RISC 6000, and Sun SPARC.

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