Ensamblador X86 Parte 4 Ciclo de Fetch

Understanding the Execution Cycle of x86 Processors

Overview of Processor Operation

• A processor operates in clock cycles, where all registers involved in the execution cycle require a clock cycle to determine their actions at specific moments.

Instruction Execution Cycle

• The episode introduces the instruction execution cycle within processors, also referred to as the fetch-execute cycle. This process can be simplified into two main phases: fetching and executing instructions.

Fetch Phase

• The fetch phase involves retrieving an instruction from memory, which is essential since instructions are stored there for sequential execution by the processor.

Execute Phase

• The execute phase consists of three tasks:

• Determine the type of instruction (e.g., addition or subtraction).

• Execute the identified instruction based on previous determinations.

• Store results either in main memory or a register after processing data.

Program Counter Functionality

• An example is provided involving a program counter (PC), which is a set of flip-flops that store the address of the next instruction to be executed.

Initial State and Addressing

• Upon powering on, if PC starts at 0, it will execute the instruction located at memory address 0 first. After executing this instruction, it moves to retrieve subsequent instructions.

Memory Word Size Considerations

• The discussion highlights that memory has a defined word size; for instance, with a 64-bit architecture, each instruction occupies one row in memory. However, practical implementations may vary based on manufacturer specifications.

Instruction Storage Requirements

• If an instruction requires more space than available in one row (e.g., needing four rows for a 64-bit instruction), then PC increments accordingly (by four).

Memory Address Register Role

• To access memory efficiently, an intermediate register called Memory Address Register (MAR) is used to hold addresses temporarily before accessing actual data from memory.

Data Retrieval Process

• When requesting data from memory:

• The value from PC is transferred to MAR.

• Control unit sends signals indicating whether to read or write data at that address.

Memory Response Time and Data Handling

• Once memory receives requests:

• It retrieves values based on specified addresses and sends them back to CPU.

• There’s inherent latency between sending requests and receiving responses known as response time.

Temporary Data Storage

• Retrieved data is temporarily stored in a register called Memory Data Register (MDR), which must match the width of the word size for proper handling of larger datasets.

Final Steps in Instruction Processing

• Finally, when reading an instruction:

Understanding CPU Instruction Execution

The Role of Intermediate Registers

• The instruction register, also known as the Gervais register, is crucial for copying data and managing operations. It highlights the importance of understanding which process is needed based on the type of operation being performed.

Phases of Instruction Processing

• The first phase concludes with fetching the instruction, transitioning into the execution phase where different micro-operations are executed depending on the instruction type.

Special Registers in Execution Phase

• A special register known as Phase III may vary by manufacturer, containing 10 to 11 registers that play a role during execution. This emphasizes how hardware design can influence processing capabilities.

Instruction Encoding and Micro-Operations

• During execution, encoding instructions is essential; different types require distinct micro-operations. For example, addition differs from multiplication in terms of electronic components involved.

Control Unit Functionality

• The control unit reads operation codes to determine necessary actions. In a multiplication example, it identifies operands and connects them through multiplexers to facilitate data flow between registers.

Data Handling in Operations

• The control unit sends operation codes to multiplexers to connect inputs correctly. For instance, it links data from Register X and a constant (3), ensuring accurate operand selection for calculations.

Storing Results Post Operation

• After performing an operation like multiplication, results can be stored either in a register or main memory. Implicit instructions dictate where outputs should be saved without additional commands.

Managing Output Storage

• In complex x86 instructions, results are often implicitly stored back into specific registers (e.g., AX). This streamlines processes by reducing explicit storage commands required from programmers.

Finalizing Execution Cycle

• Once results are stored in Register AX, the execution cycle ends. The control unit signals an increment in Program Counter (PC), preparing for the next instruction fetch from memory.

Synchronization of Registers

• All registers must synchronize their read/write operations using clock signals due to their flip-flop nature. This coordination ensures proper timing for data transmission across components during processing cycles.

Understanding CPU Operation and Instruction Execution

The Role of Clock Cycles in CPU Functionality

• A CPU operates using clock cycles, which are essential for coordinating the execution of instructions. Each register involved in the execution cycle requires a specific clock cycle to function correctly.

• If data is sent to memory at an incorrect moment within the clock cycle, it will not be processed or read properly, highlighting the importance of timing in CPU operations.

Instruction Complexity and Memory Access

• When executing complex instructions, such as multiplying values from registers with those stored in memory, additional steps are required. This involves fetching data from memory (MAR and MDR), complicating the process.

• Complex processors necessitate more electronic components to manage these intricate processes effectively compared to simpler architectures like RISC (Reduced Instruction Set Computing).

Simplification in RISC Architecture

• In RISC architecture, operations are streamlined; for instance, loading data directly into registers simplifies instruction handling by reducing unnecessary steps.