Implementing C for Padauk Microcontrollers and Improving their Architecture for C Programming

Core Concepts

This paper details the implementation of the C programming language for resource-constrained Padauk microcontrollers and proposes architectural improvements to enhance their suitability for C programming, particularly in multicore settings.

Abstract

This research paper details the implementation of the C programming language for Padauk microcontrollers, tiny 8-bit devices with limited RAM. The authors successfully implemented C support for single-core Padauk architectures (pdk13, pdk14, pdk15) using the Small Device C Compiler (SDCC).

Key challenges and solutions:

• Limited Resources: The absence of general-purpose registers led to the use of a pseudo-register at a fixed memory address. To handle multiple address spaces, 16-bit pointers were used, with the top bit indicating code or data memory.
• Inefficient Stack Handling: The lack of stack-pointer-relative addressing made stack access inefficient. The authors addressed this by placing local variables at fixed memory locations, sacrificing reentrancy. The __reentrant keyword and –stack-auto compiler option were introduced to enable reentrant functions when needed.
• Multicore Challenges: Multicore devices demanded reentrant functions, necessitating locks to protect the pseudo-register. Implementing atomics, especially atomic_flag, proved complex due to the lack of lock-free primitives. The authors devised a workaround using two locks to emulate lock-free behavior.

Proposed Architectural Improvements:

To enhance the Padauk architecture for C programming, the authors propose:

• Stack Handling Instructions: Adding an spadd #k instruction to adjust the stack pointer efficiently.
• Stack-Pointer-Relative Addressing: Introducing a dedicated addressing mode or extending existing instructions (e.g., mov) to support stack-relative addressing.
• Exposing Interrupt Enable Bit: Allowing access to the global interrupt enable bit via an I/O register for efficient critical section implementation.
• Enhanced Atomic Operations: Implementing a swap instruction with indirect addressing for efficient atomic_flag and adding atomic compare-and-exchange instructions for other atomic types.
• Core Identification: Introducing an instruction to determine the currently executing core.
• Binary Coded Decimal (BCD) Support: Adding support for BCD arithmetic to optimize decimal string conversions.

Evaluation:

The authors evaluated the impact of the proposed changes on code size using benchmarks like Dhrystone, Coremark, and stdcbench. Results showed significant code size reductions, especially when compiling all functions as reentrant.

Conclusion:

The paper demonstrates the feasibility of using C for programming Padauk microcontrollers and highlights the architectural limitations hindering its efficiency. The proposed improvements offer valuable insights for future Padauk microcontroller designs, aiming to make them more suitable for C programming, particularly in multicore environments.

Stats

Padauk microcontrollers have 60 B to 256 B of RAM.
The smallest established integer benchmarks (Dhrystone 2.1, Coremark 1.0, stdcbench 0.6) require more data memory than available on Padauk microcontrollers.
Code size reductions of up to 60% were observed when compiling all functions as reentrant with the proposed architectural changes.
The program memory on a Padauk microcontroller occupies roughly the same die space as all other digital logic combined.

Quotes

"The architecture was clearly not designed to be programmed in a high-level language."
"C standards are supported to the full extent that they are supported by SDCC."
"Since nearly all instructions are single-cycle, we expect runtime and energy use to closely follow code size."

Key Insights Distilled From

C for a tiny system

by Philipp Klau... at arxiv.org 11-05-2024

https://arxiv.org/pdf/2010.04633.pdf

Deeper Inquiries

How do the proposed architectural changes affect power consumption and performance in real-world applications on Padauk microcontrollers?

The proposed architectural changes primarily center around enhancing the handling of the stack and atomic operations. These changes would lead to a reduction in code size and potentially improve both power consumption and performance in real-world applications on Padauk microcontrollers. Here's how:

Reduced Code Size: The paper demonstrates that adding stack-pointer-relative addressing and the spadd instruction significantly reduces code size, especially when compiling for reentrancy. Smaller code size translates to less program memory usage, which can indirectly impact power consumption.
Improved Performance:

Efficient Stack Access: The current lack of stack-pointer-relative addressing makes accessing local variables on the stack inefficient. The proposed changes would enable faster access to local variables, leading to faster execution of functions and potentially reducing the number of instructions required for certain operations.
Faster Atomic Operations: The paper highlights the inefficiency of implementing atomic operations beyond the basic volatile sig_atomic_t. The proposed addition of a swap instruction with indirect addressing, along with other atomic instructions, would significantly speed up common synchronization primitives like mutexes and condition variables, leading to performance gains in multi-threaded applications.

Lower Power Consumption: While the paper focuses on code size reduction, the performance improvements indirectly contribute to lower power consumption. Fewer instructions and faster execution times mean the microcontroller can potentially finish tasks quicker and spend more time in low-power modes, leading to overall energy savings.
However, it's important to note that the paper primarily focuses on code size as a proxy for performance and power consumption. Real-world impact would depend on the specific application and usage patterns. Factors like the frequency of stack access, the prevalence of atomic operations, and the overall system design would influence the actual benefits observed.

Could alternative programming paradigms, such as event-driven programming or state machines, be more suitable for the resource constraints of Padauk microcontrollers than C?

Yes, alternative programming paradigms like event-driven programming or state machines can be more suitable for the resource constraints of Padauk microcontrollers than traditional imperative programming in C, particularly for certain applications. Here's why:

Reduced Memory Footprint: Event-driven programming and state machines often result in a smaller memory footprint compared to C programs. This is because they typically involve less code overhead for managing program flow and data structures. In resource-constrained environments like Padauk microcontrollers, where RAM and program memory are limited, this reduced footprint is highly beneficial.
Efficient Resource Utilization: These paradigms are inherently designed for efficient resource utilization. Event-driven systems only execute code in response to specific events, minimizing unnecessary processing and making the most of limited CPU cycles. State machines, with their well-defined states and transitions, offer a structured and predictable way to manage program logic, which can be advantageous in resource-limited systems.
Simplified Complexity: For applications that are inherently event-driven or can be easily modeled as state machines, these paradigms can simplify the development process. They provide a more intuitive and manageable way to represent the system's behavior, potentially leading to more maintainable and less error-prone code.
However, it's important to consider the trade-offs:

Learning Curve: Event-driven programming and state machines can have a steeper learning curve for developers accustomed to traditional imperative programming.
Debugging Challenges: Debugging event-driven systems can be more challenging due to the asynchronous nature of event handling.
Limited Applicability: These paradigms might not be the most efficient or suitable for all types of applications. Complex computations or algorithms might be better suited for the expressiveness of C.
Ultimately, the choice between C and alternative paradigms depends on the specific application requirements, developer expertise, and the trade-offs between code size, performance, and development complexity.

Given the increasing prevalence of interconnected devices, how can the development and optimization of C compilers for resource-constrained microcontrollers contribute to the advancement of the Internet of Things (IoT)?

The development and optimization of C compilers for resource-constrained microcontrollers play a crucial role in advancing the Internet of Things (IoT) due to the following reasons:

Enabling Efficient Resource Utilization: IoT devices often operate with limited battery power, processing capabilities, and memory. Optimized C compilers can generate highly efficient machine code that minimizes energy consumption, reduces latency, and maximizes the use of scarce resources. This efficiency is paramount for extending battery life, improving responsiveness, and enabling complex computations on resource-constrained devices.
Expanding Device Capabilities: As C compilers become more sophisticated, they can support more complex data structures, algorithms, and communication protocols, unlocking new possibilities for IoT applications. This enables the development of feature-rich devices capable of handling tasks like edge computing, machine learning, and real-time data analysis, pushing the boundaries of what's possible in the IoT ecosystem.
Reducing Development Costs: Efficient C compilers can significantly reduce development costs by simplifying the programming process, enabling code reuse, and improving code portability across different microcontroller platforms. This allows developers to focus on application logic and innovation rather than low-level optimizations, accelerating the development cycle and bringing new IoT solutions to market faster.
Enhancing Security: Optimized C compilers can incorporate security features and mitigations at the code level, hardening IoT devices against vulnerabilities and cyberattacks. This is crucial for protecting sensitive data, ensuring device integrity, and building trust in the IoT ecosystem.
In conclusion, the development and optimization of C compilers are essential for unlocking the full potential of the IoT. By enabling efficient resource utilization, expanding device capabilities, reducing development costs, and enhancing security, these compilers pave the way for a future where billions of interconnected devices can operate seamlessly and securely.