B. Reduced-Instruction-Set Computing (RISC Processors RISC is a style of computer architecture that emphasizes
the processor simplicity and efficiency. RISC processors have been developed to enhance the execution speed by
using a pipelined architecture and a reduced instivctions set containing a few simple instructions and by migrating
complex operations to software. The term IRISC is used by contrast to CISC (Complex Instruction Set Computing)
which is usually associated to conventional micrciprocessors. In general, RISC architecture is characterized by a
large register file and instruction cache, and absence of data cache. To illustrate typical RISC architecture, block
diagrams of two RISC processors are shown in Fig:. 8 and
9. They are, respectively, the Motorola MC88100 and the Integrated Device Technology 79R3000A. (This processor is based on the MIPS Technologies R3000 architecture.) Typical characteristics of current RISC processors are listed below:
- Reduced instruction set (50 to 75 instructions).
- Single-cycle execution.
- Instructions implemented directly in hardware, precluding the need for microcoded operations.
- Simple fixed-format instructions (32-bit opcodes, two formats maximum).
- Simplified addressing modes (three modes maximum).
- Register-to-register operation for data manipulation instructions.
- Memory access by load/store operations.
- Large register file (greater than 32 registers).
- Simple efficient instruction pipeline visible to compilers.