RISC vs CISC Architectures

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Questions and Answers

Which of the following statements best describes the evolution of processor design?

Modern processors exclusively use either pure RISC or pure CISC architectures.
Modern x86 processors strictly adhere to the original CISC specifications without any influence from RISC.
Modern x86 and ARM processors have integrated features from both RISC and CISC designs. (correct)
Modern ARM processors strictly adhere to the original RISC specifications without any influence from CISC.

What is the primary function of the operand within an assembly language instruction?

To indicate the priority level of the instruction in the execution queue.
To identify the memory location where the data required by the instruction is stored. (correct)
To define the size of the data being processed by the instruction.
To specify the operation to be performed (e.g., add, subtract).

How does the allocation of bits for the opcode and operand differ between RISC and CISC architectures, and what is a direct consequence of this difference?

There is no difference; both RISC and CISC architectures allocate bits equally between the opcode and operand.
RISC allocates more bits to the operand, enabling it to directly access more memory locations, while CISC uses addressing modes due to fewer bits being allocated to the operand (correct)
CISC allocates more bits to the operand, leading to a simpler instruction set and the ability to directly address more memory locations.
RISC allocates more bits to the opcode, enabling more complex instructions at the cost of fewer directly accessible memory locations.

Which statement accurately compares the characteristics of RISC and CISC architectures regarding instruction complexity and execution?

<p>CISC instructions are more complex, requiring more clock cycles but fewer instructions to complete a task, while RISC instructions are simpler, requiring fewer clock cycles but more instructions. (B)</p> Signup and view all the answers

Imagine a hypothetical processor architecture, 'Approximate Instruction Set Computing' (AISC), that optimizes solely for minimal code size, regardless of execution speed or hardware complexity. Given that modern CPUs balance various design goals, what would be the MOST significant trade-off in an AISC design compared to existing RISC and CISC architectures?

<p>AISC would require extremely complex individual instructions, potentially leading to increased processor fabrication costs and more difficult debugging due to reliance on complex addressing modes. (D)</p> Signup and view all the answers

Flashcards

Assembly Language Instructions

Instructions with mnemonics (load, add) and symbols (X, Y, Z); includes opcode and operand.

Machine Code Instructions

Binary instructions the computer understands, fetched from memory into the CPU.

CISC

Complex Instruction Set Computing; more complex instructions, fewer instructions needed, more clock cycles per instruction.

RISC

Reduced Instruction Set Computing; simpler instructions, more instructions needed, less clock cycles per instruction.

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Memory Access (RISC vs. CISC)

RISC processors allocate more bits to the operand while CISC relies on addressing modes due to allocating fewer bits to the operand.

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Study Notes

RISC and CISC represent distinct processor design methodologies.
PCs initially adopted Intel's x86 architecture, based on CISC.
Apple computers originally utilized the ARM (Advanced RISC Machines) processor architecture, based on RISC.
Current x86 and ARM processors have evolved to incorporate aspects of both RISC and CISC designs.

Assembly Language Instructions

Assembly language instructions use mnemonics (e.g., load, add, store, halt) and symbols for memory addresses (e.g., X, Y, Z).
Instructions include an operation code (opcode) that defines the operation to be performed.
The operand indicates the memory location of the data needed for the instruction.

Machine Code Instructions

Assembly language instructions become binary machine code for execution.
Operations have binary codes and operands are converted to numeric memory addresses before being loaded into RAM.
Machine code instructions are retrieved from memory into the CPU's current instruction register for execution.
Within that register, bits are allocated for both the opcode and the operand.
The allocation of bits to the opcode and operand dictates the number of possible operations and accessible memory locations.

Key differences between RISC and CISC

RISC allocates fewer bits to the operation code compared to CISC.
Modern RISC processors generally include 70-80 instructions, while CISC processors may have up to 150.
CISC instructions are more complex, with a single CISC instruction generally accomplishing more than a single RISC instruction.
RISC-based machines need more instructions but fewer clock cycles per instruction, whereas CISC requires fewer instructions but more clock cycles per instruction.
RISC programs typically occupy more memory than CISC programs.
Compilers for RISC are more complex than those for CISC processors.

Considerations and Tradeoffs

Clock cycles per second, clock cycles per instruction, and the total instructions for a task are important considerations.
High-level programming code closely resembles CISC assembly code.
Compilers for CISC processors are simpler and faster than those for RISC processors.
RISC assembly programmers typically spend more time programming than CISC programmers.
CISC processor chips are more complex and costlier to design and manufacture than RISC processor chips.
RISC processors can directly access more memory locations due to more operand bits, while CISC relies on addressing modes due to fewer operand bits.

Benefits of processors based on RISC architecture

RISC architecture is often employed in high-end applications that demand rapid processing of a limited number of instructions, such as video, audio, and image processing.