Can I know about the process of memory access?

[gihyeonjeon]

Hi. I'm student and i'm using and studying about gem5.

I just want to know about the process of memory access and I want to see about the size of memory access.

I have spent so much time on finding about memory access process... and finally found a piece of process that i want to know.

Starting with the .isa file, it goes through initiateMemRead, DataTranslation, finishTranslation, SendData, handleReadPacket in sequence.

I understand that this process involves accessing memory, but I'm having trouble understanding the sequence of decoding the instruction and determining the size of the req from it.

Can anybody help me??

Thanks! :D

[Harshil2107]

Hi,

I am not an expert in this but I recommend checking out the gem5 bootcamp 2022 YouTube videos for a deeper understanding of gem5's workings. Specifically, this video explains instructions in gem5: gem5 bootcamp 2022: Instruction execution and adding instructions. The link starts at the timestamp explaining instruction encoding/decoding.

I also asked your question to our gem5 chat gpt (link to gem5 chat gpt), here is the response from it:

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Hello,

I'm glad to hear you're exploring gem5 and delving into the details of memory access processes. Your observation about the sequence of events in memory access, starting with .isa files and proceeding through various functions like initiateMemRead, DataTranslation, finishTranslation, SendData, and handleReadPacket, is a good starting point.

To clarify your understanding of how an instruction is decoded and how the size of the memory request (req) is determined, let's break down the process:

1. Instruction Decoding: The process begins with the decoding of an instruction from the .isa file. In gem5, the instruction set architecture (ISA) files define how instructions are interpreted. When an instruction involving memory access (like a load or store) is encountered, the simulator needs to decode it to understand what operation is to be performed, which includes determining the memory address and the size of the data to be accessed.

2. Memory Access Process: The sequence of functions you mentioned (initiateMemRead, DataTranslation, etc.) is part of the memory access pipeline in gem5. Each function has a specific role:

   • initiateMemRead: This initiates a memory read request. At this stage, the simulator knows the address to read from but might not have completed all the necessary checks (like address translation or cache access).

   • DataTranslation: This step involves translating virtual addresses to physical addresses, a crucial step in modern systems where virtual memory is used.

   • finishTranslation: Once the address translation is complete, this function finalizes the translation process and prepares for the actual memory access.

   • SendData: This function is responsible for sending data as part of the memory request, such as writing data to memory in a store operation.

   • handleReadPacket: In the case of a read operation, this function handles the incoming data packet from the memory subsystem.

3. Determining Memory Request Size:

The size of the memory request (req) is typically determined during the instruction decoding phase. In gem5, when an instruction is decoded, it includes information about the operation type (e.g., load or store), the target memory address, and the size of the data to be accessed. This size is integral to forming the memory request and is usually specified by the instruction set architecture (ISA) for each type of load or store instruction. For instance, a load-byte instruction will have a different size associated with it compared to a load-word instruction.

4. Role of the CPU Model: The exact implementation of how these steps are executed can depend on the CPU model you are using in gem5 (e.g., simple CPU, O3 CPU, etc.). Different CPU models might handle the stages of memory access, like instruction decoding and memory request generation, in slightly different ways.

5. Debugging and Understanding the Process: If you want to further understand this process, you could use debugging techniques. For instance, in C++, you can use DPRINTF statements in the gem5 source code to print out information at each stage of the memory access process, including the size of the memory request. However, this requires a good understanding of the gem5 source code and should be done cautiously to avoid being overwhelmed with information.

6. Additional Resources: To deepen your understanding, you might want to refer to the gem5 documentation, particularly the sections related to the CPU models and memory system. Additionally, academic papers and technical reports on gem5 can provide insights into its architecture and how it simulates memory operations.

If you have specific questions about a certain stage in the process or need clarification on a particular aspect, feel free to ask. Understanding the intricacies of memory access in a simulator like gem5 can be challenging, but it's a rewarding part of learning computer architecture simulation.

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[gihyeonjeon]

Really thanks!
Your youtube link is really helpful for understanding about the isa, decoding process.
By that, I can solve my problem.