Risk vs Cisk

Topics: Instruction set, Reduced instruction set computer, Complex instruction set computer Pages: 4 (670 words) Published: September 10, 2013
Different Kinds of ISAs We have looked at LC3 ISA, which is a classic example of RISC type ISA

CIT 595 Spring 2007

Reduced Instruction Set Architecture (RISC) emerged around early 80s • Designers re-evaluating the current ISAs of the era • Found that ISAs had extensive instructions that were complex Complex Instruction Set Architecture (CISC)

• Need only 20% of the instructions that were used most of the time 1

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Complex Instruction Set Computer (CISC)
Memory in those days was expensive
bigger program->more storage->more money

Reduced Instruction Set Computer (RISC)
Original idea to reduce the ISA
Provide minimal set of instructions that could carry out all essential operations

Hence needed to reduce the number of instructions per program Number of instructions are reduced by having multiple operations within a single instruction Multiple operations lead to many different kinds of instructions that access memory In turn making instruction length variable and fetch-decodeexecute time unpredictable – making it more complex Thus hardware handles the complexity Example: x86 ISA CIT 595 10 - 3

Instruction complexity is reduced by 1. Having few simple instructions that are the same length 2. Allowed memory access only with explicit load and store instructions Hence each instruction performs less work but instruction execution time among different instructions is consistent The complexity that is removed from ISA is moved into the domain of the assembly programmer/compiler Examples: LC3, MIPS, PowerPC (IBM), SPARC (Sun)

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The difference between CISC and RISC becomes evident through the basic computer performance equation:

Example for RISC vs. CISC
Consider the the program fragments: mov ax, 0 mov bx, 10 mov cx, 5 add ax, bx loop Begin


mov ax, 10 mov bx, 5 mul bx, ax



RISC systems shorten execution time by reducing the clock cycles per...
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