Microprocessor Characteristics

• Fundamentals of Microprocessors 2 by Ing. Dr. Eliel Keelson covers characteristics, classification, and performance factors of microprocessors.

Characteristics of Microprocessors

• Microprocessors are generally characterized by instruction set, word length, and clock speed.

The Instruction Set

• Complete set of machine-level instructions that a microprocessor can execute.
• Instructions act as commands for the microprocessor.
• Used to perform arithmetic operations, data transfer, control flow, and logical comparisons.
• The number of instructions that the CPU understands is fixed.
• Instruction sets can range from 10 (SIMP computer) to 150-200 (advanced computers).
• Examples include ADD, SUB, MOV, JUMP, and LOAD.
• CPUs do not all use the same instruction set, so the same instruction numbers may be interpreted differently.
• Instructions are classified into data transfer, arithmetic, logical, and control flow instructions.
• Data Transfer Instructions are for moving data between registers, memory, and I/O.
• Arithmetic Instructions are for mathematical calculations (e.g., ADD, SUB).
• Logical Instructions are for bit-wise operations (e.g., AND, OR, NOT).
• Control Flow Instructions are for altering the program flow (e.g., JUMP, CALL).
• The Instruction Set Architecture (ISA) consists of the instructions a CPU understands and the known registers.
• ISA includes specifications for data types, addressing modes, memory organization, and I/O mechanisms.
• ISA serves as a bridge between hardware (processor design) and software (programming).
• Common ISA types include RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer).
• Other types include Minimal instruction set computers (MISC), Explicitly parallel instruction computing (EPIC), Very long instruction word (VLIW), Zero instruction set computer (ZISC), and One instruction set computer (OISC)

The Word Length

• Word length refers to the number of bits a microprocessor can process in a single instruction.
• Word length is directly proportional to the processing power of the microprocessor.
• An 8-bit microprocessor processes 8 bits at a time, with internal registers holding 8 bits of data.
• A 16-bit microprocessor processes 16 bits at a time, with registers holding 16 bits.
• Examples of word lengths include 8-bit, 16-bit, 32-bit, and 64-bit microprocessors.

The Clock Speed

• Clock speed determines how quickly a microprocessor executes instructions.
• Clock speed synchronizes all operations between the microprocessor and connected components.
• It generates clock pulses, which are measured in MHz or GHz.
• Higher clock speeds enable the microprocessor to execute more instructions in less time.
• Modern microprocessors operate at clock speeds of several GHz.

Microprocessor Classification

• Microprocessors are classified based on their instruction set architecture (ISA).
• Main types are RISC, CISC, and special purpose processors.
• RISC Processors: Reduced Instruction Set Computing processors.
• CISC Processors: Complex Instruction Set Computing processors.
• Special Purpose Processors: Designed for specific tasks or applications.

CISC (Complex Instruction Set Computer)

• CISC is designed to reduce the number of instructions per program using complex instructions.
• Complex instructions perform multiple operations in a single command.
• They minimize the number of instructions per program and ignore the number of cycles per instruction.
• CISC reduces program size.
• CISC requires fewer memory cycles (fetch, read/write).
• CISC has a large number of instructions of variable sizes.
• Key features include a large, complex instruction set, multiple tasks executed in one cycle, variable-length instructions, and less software effort due to hardware complexity.
• Advantages include being easier for programmers, reducing the load on compilers, and efficient use of memory.
• Disadvantages include increased hardware complexity and slower execution due to complex instruction decoding, and instructions may take multiple clock cycles.
• Example processors are Intel x86 and AMD x86-64.

RISC (Reduced Instruction Set Computer)

• RISC is designed to simplify instructions, where each instruction performs a single operation.
• Each instruction executes in a single clock cycle.
• RISC emphasizes reducing execution time per instruction.
• RISC has a small, simple set of instructions of uniform length.
• RISC reduces execution time using the simplified instruction set.
• Key features include a simple, limited instruction set, fixed-length instructions, focus on software optimization, and pipelining for parallel execution.
• Advantages include faster execution, lower hardware complexity, lower power consumption, and efficient performance for modern and embedded applications.
• Disadvantages include the need for more instructions to perform the same task as CISC and reliance on advanced compilers and software optimization.
• Example processors are ARM, Apple M1/M2, IBM PowerPC, and RISC-V.

Factors Affecting CPU Performance

• Processor performance is affected by clock speed, cache size, and number of cores.

Factors Affecting CPU Performance: Clock Speed

• Clock speed is the number of pulses the CPU clock generates per second, measured in hertz (Hz).
• Faster clock speeds allow the computer to run fetch-decode-execute cycles faster.
• Faster clock speeds leads to processing more instructions in the same amount of time.
• CPU clocks can be sped up via overclocking.
• Increasing the number of pulses per second allows more instructions to be processed.
• This increases performance, but requires more power, heat dissipation, and can strain battery life.
• Overclocking causes the CPU to work harder and produce more heat, leading to long-term hardware damage.

Factors Affecting CPU Performance: Cache

• Cache is a small amount of high-speed RAM built into the processor.
• It temporarily holds data and instructions likely to be reused.
• Allows for faster processing because data and instructions do not have to be fetched from RAM.
• Larger cache sizes reduce the time a processor has to wait for instructions, improving performance.

Factors Affecting CPU Performance: Core

• A core is a processing unit within a CPU is known as a core.
• Each core can fetch, decode, and execute its own instructions.
• A CPU with more cores leads to processing more instructions in a given time.
• Sometimes a core has to wait for output from another before continuing, causing a delay.
• Multiple cores increase the processor cost.
• Many modern CPUs are dual-core or quad-core, providing superior processing power.