Efficient CPU Run Modes: Instruction, Cycle, Overflow

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Hey guys! Gianlucag and mos6502 were brainstorming, and @fmixolydian sparked a really interesting discussion about the best way to implement the ::Run() method in a CPU emulator. Let's dive into the challenges and a cool solution proposed.

The Challenge with Current ::Run() Implementation

The current setup has a couple of pain points that make it less than ideal. Here's a breakdown:

  • Overkill Cycle Method Parameter: The cycleMethod parameter in ::Run() seems a bit much. Do we really need to switch between different modes on every single call? It feels like we're adding complexity where it might not be necessary.
  • Maintenance Nightmare: Having separate ::Run and ::RunEternally methods means we have to maintain the same logic in multiple places. This can lead to inconsistencies and bugs, like the ones encountered while working on IRQ/NMI fixes. Trust me, nobody wants that!

The goal is to simplify things, make the code easier to maintain, and provide the flexibility that everyone needs.

A Proposed Solution: Three Modes of Operation

The suggestion is to introduce three distinct modes of operation, set during the constructor, to streamline the ::Run() method and improve its functionality. These modes are:

  • INSTRUCTION Mode: This mode executes the CPU one instruction at a time. The duration parameter in ::Run(int32_t duration) would specify the number of instructions to execute. The method would then return the actual number of instructions that were executed.
  • CYCLE Mode: In this mode, the CPU executes a specific number of cycles. The duration parameter would represent the number of cycles to run, and the method returns the number of cycles actually executed.
  • OVERFLOW Mode: This mode also operates in terms of cycles, but it introduces a cycleOverflow variable to handle excess cycles from previous calls. This is key to achieving the desired behavior of @fmixolydian. The duration parameter specifies the number of cycles to run, and the method always returns the value of duration. The cycleOverflow variable is initialized to zero and reset to zero whenever the ::Reset() method is called. Let's dive deeper into each of these modes.

INSTRUCTION Mode Details

INSTRUCTION mode offers precise control over CPU execution at the instruction level. When the CPU operates in INSTRUCTION mode, the duration parameter of the ::Run() method is interpreted as the number of instructions to be executed. For instance, if duration is set to 100, the CPU will attempt to execute 100 instructions. The method will then return the actual number of instructions that were executed. This is particularly useful for debugging and step-by-step analysis, allowing developers to meticulously trace the execution flow of a program. Imagine being able to step through your code line by line, instruction by instruction, to pinpoint exactly where things might be going wrong. This level of detail can be invaluable when trying to optimize performance or identify elusive bugs. The return value of the ::Run() method in INSTRUCTION mode provides critical feedback about the CPU's execution. If the return value is less than the specified duration, it indicates that the CPU encountered a halt or an interrupt before completing the requested number of instructions. This information can be used to make informed decisions about how to proceed with execution, potentially adjusting the duration or modifying the program to avoid the halt condition. Furthermore, INSTRUCTION mode enables developers to implement sophisticated debugging tools, such as instruction breakpoints and single-stepping functionality. By setting breakpoints at specific instructions, developers can pause execution at critical points in the program to examine the CPU's state, memory contents, and register values. This allows for a deep understanding of the program's behavior and facilitates the identification of complex issues.

CYCLE Mode Details

CYCLE mode provides a different level of control, focusing on the number of clock cycles the CPU executes. In CYCLE mode, the duration parameter of the ::Run() method specifies the number of clock cycles to be executed. For example, setting duration to 1000 will instruct the CPU to run for 1000 clock cycles. This mode is particularly useful for timing-sensitive operations and performance analysis. By controlling the number of clock cycles, developers can accurately measure the execution time of specific code segments or algorithms. This is crucial for optimizing performance and ensuring that the CPU meets real-time constraints. The return value of the ::Run() method in CYCLE mode indicates the actual number of clock cycles that were executed. Similar to INSTRUCTION mode, if the return value is less than the specified duration, it suggests that the CPU encountered a halt or an interrupt before completing the requested number of clock cycles. This information is vital for diagnosing timing issues and ensuring that the CPU behaves as expected under various conditions. CYCLE mode also enables developers to simulate the behavior of external hardware components that interact with the CPU. By controlling the number of clock cycles, developers can accurately model the timing of these interactions and ensure that the CPU responds correctly to external events. This is particularly important for emulating complex systems with multiple interacting components. Furthermore, CYCLE mode provides a foundation for advanced performance analysis techniques. By carefully measuring the number of clock cycles required to execute specific code segments, developers can identify performance bottlenecks and optimize their code for maximum efficiency. This can lead to significant improvements in overall system performance and responsiveness.

OVERFLOW Mode Details

OVERFLOW mode introduces a unique approach to cycle management, designed to address specific timing challenges. In OVERFLOW mode, the duration parameter of the ::Run() method also specifies the number of clock cycles to be executed. However, the key difference lies in the introduction of the cycleOverflow variable. This variable tracks any excess clock cycles from previous calls to the ::Run() method. When the ::Run() method is called in OVERFLOW mode, it first checks the value of cycleOverflow. If cycleOverflow is greater than zero, the method subtracts this value from the specified duration. This ensures that the CPU accurately accounts for any leftover clock cycles from previous operations. The cycleOverflow variable is particularly useful for handling situations where the CPU's execution is not perfectly aligned with the desired number of clock cycles. For example, if the CPU executes slightly more clock cycles than requested in one call to the ::Run() method, the excess cycles are stored in cycleOverflow. In the next call to the ::Run() method, these excess cycles are automatically accounted for, ensuring that the CPU's overall execution remains accurate. The return value of the ::Run() method in OVERFLOW mode is always equal to the specified duration. This simplifies the programming model and makes it easier to reason about the CPU's behavior. The cycleOverflow variable is initialized to zero and reset to zero whenever the ::Reset() method is called. This ensures that the CPU starts with a clean slate and that the cycleOverflow variable does not accumulate any unwanted values. OVERFLOW mode is particularly valuable for emulating systems with complex timing requirements, where precise control over the number of clock cycles is essential. By using the cycleOverflow variable, developers can ensure that the CPU's execution remains accurate and consistent, even in the face of timing variations.

Benefits of the Proposed Modes

  • Simplified ::Run() Method: By consolidating the logic into a single ::Run() method with different modes, we reduce code duplication and make maintenance easier. One place to fix bugs and add features – awesome!
  • Flexibility: The three modes provide the flexibility to control CPU execution at different levels of granularity – instruction, cycle, and overflow – catering to various use cases.
  • Addressing Specific Needs: The OVERFLOW mode directly addresses @fmixolydian's requirements by providing a mechanism to track and manage excess cycles.

In summary, this approach simplifies the ::Run() method, offers greater flexibility, and addresses specific timing requirements. What do you guys think? Let's discuss!