Lecture 7 Pipeline of Instruction in C67X Processor

Summary

This lecture delves into the pipeline operations of a DSP processor, explaining the stages of instruction processing—fetch, decode, and execute. The fetch stage involves four phases (address generation, sending, waiting, and packet receiving), while the decode stage consists of two phases (dispatch and decode). The execute stage can have variable phases depending on instruction complexity, ranging up to 10 phases. The lecture emphasizes parallel processing in the DSP pipeline, where multiple instructions are processed simultaneously across functional units, significantly improving efficiency and throughput. Understanding these pipeline phases is crucial for grasping the DSP processor’s operation.

Takeaways

• 😀 The DSP processor operates with a pipeline, which allows multiple instructions to be processed in parallel.
• 😀 The pipeline consists of three main stages: Fetch, Decode, and Execute.
• 😀 The Fetch stage includes four phases: Program Address Generation (PG), Program Address Send (PS), Program Wait (PW), and Program Fetch (PR).
• 😀 The Decode stage consists of two phases: Dispatch (DP) and Decode (DC).
• 😀 The Execute stage is the most variable, with phases ranging from 1 to 10, depending on the complexity of the instruction.
• 😀 Parallel processing in the pipeline allows up to 8 instructions to be processed simultaneously, using 8 functional units.
• 😀 In the Fetch stage, the program address is generated, sent to memory, and the CPU waits for the memory to respond before fetching the instruction.
• 😀 The Decode phase involves dispatching instructions to the correct functional units and decoding the operation for execution.
• 😀 The Execute stage is where instructions are processed, with the number of cycles varying depending on the type of instruction (e.g., simple, single precision, double precision).
• 😀 Pipelining improves efficiency by allowing different stages of multiple instructions to overlap, reducing overall execution time compared to serial processing.

Q & A

What are the three main stages of the pipeline in a DSP processor?

The three main stages of the pipeline in a DSP processor are Fetch, Decode, and Execute.

How many phases are there in the Fetch stage of the pipeline?

The Fetch stage consists of four phases: PG (Program Address Generation), PS (Program Address Send), PW (Program Access Wait), and PR (Program Fetch Packet Receive).

What is the function of the PG phase in the Fetch stage?

The PG phase is responsible for generating the address of the instruction to be fetched by the CPU.

What happens during the PW phase in the Fetch stage?

During the PW phase, the CPU waits for the memory to be ready to access the instruction that has been requested.

How does parallel execution work in the DSP processor’s pipeline?

Parallel execution in the DSP processor occurs through the use of 8 functional units that process up to 8 instructions simultaneously, enabling pipelining and parallel processing.

What is the role of the DP phase in the Decode stage?

In the DP phase of the Decode stage, the fetched instruction packets are split into executable packets and dispatched to the appropriate functional units for execution.

How many phases are involved in the Decode stage of the pipeline?

The Decode stage consists of two phases: DP (Dispatch) and DC (Decode).

What does the DC phase in the Decode stage do?

The DC phase decodes the instruction to determine the operations needed, such as identifying the source and destination registers or constants to be used in the instruction.

Why does the Execute stage have a variable number of phases?

The Execute stage has a variable number of phases because different instructions vary in complexity, requiring more or fewer phases depending on factors like precision (e.g., single or double precision).

Can you give an example of instructions dispatched in the DSP processor’s pipeline?

Examples of instructions include ADD (adding two values), STW (store word in memory), ADD K (adding a constant to a register), and NOP (no operation, which does nothing).

How do functional units contribute to instruction dispatch in the DSP processor?

Functional units in the DSP processor are specialized components that handle specific types of instructions, such as multiplication or memory access, and instructions are dispatched to the appropriate functional unit based on their type.

What is the difference between serial processing and pipelining in a DSP processor?

Serial processing executes one instruction at a time sequentially, while pipelining allows multiple instructions to be processed in parallel across different functional units, improving processing speed and efficiency.