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Risk vs Cisk Essay

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

CIT 595

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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)

CIT 595

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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 instruction (i.e. simple instructions take less time to interpret) CISC systems shorten execution time by reducing the number of instructions per program CIT 595 10 – 5

The total clock cycles for the CISC version might be: (2 movs × 1 cycle) + (1 mul × 30 cycles) = 32 cycles While the clock cycles for the RISC version is: (3 movs × 1 cycle) + (5 adds × 1 cycle) + (5 loops × 1 cycle) = 13 cycles

CIT 595

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Micro-architecture Implementations
The simple instruction set of RISC machines takes less time to interpret plus less hardware Enables control unit to be hardwired for maximum speed Also allows room for performance enhancement such as pipelining Fewer instructions would mean fewer transistors, in turn less manufacturing cost

Other RISC features
Because of their load-store ISAs, RISC architectures require a large number of CPU registers These register provide fast access to data during sequential program execution They can also be employed to reduce the overhead typically caused by passing parameters on the stack Instead of pulling parameters off of a stack, the subroutine is directed to use a subset of registers 10 – 7 CIT 595 10 – 8

The more complex and variable instruction set of CISC machines require more translation takes time as well more hardware Usually implemented as microprogrammed control to tackle the variable length instructions

CIT 595


RISC vs. CISC Summary
RISC • Simple instructions, few in number • Fixed length instructions • Complexity in microcode • Complexity in compiler • Only LOAD/STORE instructions access memory • Few addressing modes • Many instructions can access memory • Many addressing modes CISC • Many complex instructions • Variable length instructions

RISC Roadblocks in the 80s
RISC chips took over a decade to gain a foothold in the commercial world This was largely due to a lack of software support Many companies were unwilling to take a chance with the emerging RISC technology Without commercial interest, processor developers were unable to manufacture RISC chips in large enough volumes to make their price competitive

Another major setback was the presence of Intel
• Had the resources to plow through development and produce powerful processors

CIT 595

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CIT 595

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