Stories with Tag MOS Technology

• Dirty tricks 6502 programmers use

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  Posted: 2025-04-16 13:58:36

  Verbose tl;dr

  The 6502 processor, known for its limitations, inspired clever programming tricks to optimize speed and memory. These "dirty tricks" leverage quirks like the processor's behavior during undocumented opcodes, zero-page addressing, and interactions between instructions and flags. Techniques include self-modifying code to dynamically alter instructions, using the carry flag for efficient branching, and exploiting specific instruction timings for precise delays. By understanding the 6502's nuances, programmers could achieve remarkable results despite the hardware constraints.

  This blog post, titled "Dirty tricks 6502 programmers use," delves into a collection of clever and often obscure programming techniques employed by developers targeting the MOS Technology 6502 microprocessor, a popular chip used in systems like the Apple II, Atari 2600, and Commodore 64. These "tricks" leverage intimate knowledge of the 6502's architecture and instruction set to achieve optimized code size and execution speed, often at the expense of readability and maintainability.

  The author begins by explaining the 6502's zero-page addressing mode, which allows for faster and more compact code by referencing memory locations within the first 256 bytes of RAM. This is illustrated with the common practice of storing frequently used variables in this zero-page region. The post then explores more intricate techniques, such as self-modifying code. This involves manipulating the program's own instructions during runtime, a powerful but risky technique allowing for dynamic behavior, like constructing jump tables or altering opcodes based on program state. Specific examples are given demonstrating how this can be used to achieve complex logic with minimal code.

  The post further examines the exploitation of undocumented opcodes, referred to as "illegal" or "undocumented" instructions. These are operations not officially part of the 6502 instruction set, but which nonetheless execute on the hardware, sometimes with unpredictable or processor-specific results. The author emphasizes the potential dangers and portability issues of relying on these undocumented opcodes, while also acknowledging their potential for extreme optimization in certain scenarios. Examples are provided illustrating how these instructions might behave, demonstrating their ability to manipulate processor flags or perform unusual arithmetic operations.

  The use of the 6502's stack for temporary variable storage is another explored trick. While pushing and pulling values on the stack incurs overhead, it can save memory and be more efficient than zero-page access in some cases. The author elaborates on the limitations and cautions involved in managing stack usage carefully. The post also covers techniques for manipulating the processor status register (P) flags directly through specific instructions, which can bypass typical flag setting conditions and lead to smaller and faster code.

  Finally, the author discusses the concept of "bit bashing" which involves directly manipulating individual bits within memory locations. This technique, useful for controlling hardware or managing data structures efficiently, leverages logical operations like AND, OR, and XOR to manipulate bits within bytes. Examples provided demonstrate how this can be used to set, clear, or toggle specific bits. The post concludes by acknowledging the inherent trade-offs associated with these "dirty tricks," emphasizing that while they can be powerful tools for optimization, they often come at the cost of code clarity, maintainability, and portability. It encourages careful consideration of the context before employing such techniques.

  • 6502
  • Assembly Language
  • programming
  • Low-Level Programming
  • optimization
  • Tricks
  • hacks
  • Retrocomputing
  • vintage computing
  • 8-bit
  • MOS Technology
  • Microprocessor
  • Software Development

  Summary of Comments ( 2 )
  https://news.ycombinator.com/item?id=43705649

  Verbose tl;dr

  Hacker News users generally expressed appreciation for the article on 6502 programming tricks, finding it informative and nostalgic. Several commenters shared additional tricks or variations, including using the undocumented SAX instruction and manipulating the stack for efficient data storage. Some discussed the cleverness borne out of the 6502's limitations, while others reminisced about using these techniques in their youth. A few pointed out the techniques' applicability to other architectures or modern resource-constrained environments. There was some debate about the definition of "dirty" vs. "clever" tricks, but the overall sentiment was positive towards the article's content and the ingenuity it showcased. The discussion also touched on the differences between assembly programming then and now, and the challenges of optimizing for limited resources.

  The Hacker News post titled "Dirty tricks 6502 programmers use" (linking to https://nurpax.github.io/posts/2019-08-18-dirty-tricks-6502-programmers-use.html) has generated a modest number of comments, most of which express appreciation for the insights into 6502 programming techniques.

  Several commenters reminisce about their experiences with the 6502, often mentioning specific machines like the Apple II, Commodore 64, and Atari 2600. They share anecdotes about using similar techniques or encountering them in classic games. These nostalgic comments contribute a sense of shared history and community around the 6502.

  Some commenters delve deeper into specific techniques mentioned in the article. One commenter elaborates on the self-modifying code aspect, highlighting its prevalence in 6502 programming due to the architecture's limitations and the creativity it fostered. Another discusses the use of lookup tables for sine/cosine calculations, emphasizing their efficiency on the 6502. There's also discussion about the clever use of the zero page for optimization and the different addressing modes available on the 6502.

  One particular comment thread discusses the difference between "tricks" and simply using the features of the instruction set effectively. The general consensus seems to be that the techniques described are indeed clever uses of the limited resources available, qualifying as justifiable "tricks." This discussion adds a layer of nuance to the conversation, moving beyond simple appreciation to a more analytical examination of the programming practices.

  A few comments mention other resources for learning about 6502 programming and low-level optimization techniques. These recommendations provide additional avenues for readers interested in exploring the topic further.

  While not a large volume of comments, the discussion on Hacker News around this article provides valuable context and insights from experienced programmers, enriching the original article with personal anecdotes, technical explanations, and further learning resources.

• I believe 6502 instruction set is a good first assembly language

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  Posted: 2025-02-06 01:28:17

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  The 6502 assembly language makes a great first foray into low-level programming due to its small, easily grasped instruction set and straightforward addressing modes. Its simplicity encourages understanding of fundamental concepts like registers, memory management, and instruction execution without overwhelming beginners. Coupled with readily available emulators and a rich history in iconic systems, the 6502 offers a practical and engaging learning experience that builds a solid foundation for exploring more complex architectures later on. Its limited register set forces a focus on memory operations, providing valuable insight into how CPUs interact with memory.

  The blog post "6502 is a good starting point for learning assembly" by Nemanja Trifunovic argues that the 6502 assembly language presents an ideal entry point for individuals venturing into the world of low-level programming. Trifunovic posits that the 6502's relative simplicity, compared to more modern architectures like x86_64, makes it less daunting for beginners. Its limited instruction set, small number of registers (only three 8-bit registers and a status register), and lack of complex features like memory segmentation contribute to a more manageable learning curve.

  The author emphasizes the educational value of grasping the fundamental concepts of assembly language, such as direct manipulation of memory addresses, registers, and the CPU's operation, which are often obscured by higher-level languages. The 6502's straightforward architecture allows learners to quickly develop a concrete understanding of these foundational principles without being overwhelmed by the intricacies of modern processors.

  Furthermore, the post highlights the availability of accessible emulators and readily available documentation for the 6502, which further lowers the barrier to entry. The author notes that the 6502's use in iconic retro computers and gaming consoles provides a tangible context for learning, potentially increasing motivation and engagement. This historical relevance, coupled with the active online communities dedicated to these retro platforms, provides ample opportunities for learners to explore, experiment, and seek assistance.

  Trifunovic contrasts the 6502 with more complex architectures, arguing that starting with a simpler system fosters a deeper understanding of the core concepts of assembly language. He believes this strong foundation will prove beneficial when transitioning to more sophisticated architectures later on. The author acknowledges that while the 6502 may not be directly applicable to modern software development, the acquired knowledge of low-level programming principles will be transferable and invaluable for anyone seeking a deeper understanding of computer systems. In essence, the 6502 serves as a stepping stone towards a more comprehensive understanding of computer architecture and programming.

  • 6502
  • Assembly Language
  • Beginner
  • learning
  • instruction set
  • programming
  • Retro Computing
  • vintage computing
  • Low-Level Programming
  • Computer Architecture
  • MOS Technology
  • Microprocessor
  • 8-bit

  Summary of Comments ( 184 )
  https://news.ycombinator.com/item?id=42957823

  Verbose tl;dr

  Hacker News users generally agreed that the 6502 is a good starting point for learning assembly language due to its small and simple instruction set, limited addressing modes, and readily available emulators and documentation. Several commenters shared personal anecdotes of their early programming experiences with the 6502, reinforcing its suitability for beginners. Some suggested alternative starting points like the Z80 or MIPS, citing their more "regular" instruction sets, but acknowledged the 6502's historical significance and accessibility. A few users also discussed the benefits of learning assembly language in general, emphasizing the foundational understanding it provides of computer architecture and low-level programming concepts. A minor thread debated the educational value of assembly in the modern era, but the prevailing sentiment remained positive towards the 6502 as an introductory assembly language.

  The Hacker News post titled "6502 instruction set is a good first assembly language" generated a modest discussion with several commenters sharing their perspectives on learning assembly and the 6502 specifically.

  Several commenters agreed with the premise, citing the 6502's simplicity and ease of understanding as key benefits for beginners. One user mentioned its small instruction set, simple addressing modes, and lack of complex features like memory segmentation, making it easier to grasp fundamental assembly concepts. They also pointed out the availability of emulators and readily accessible documentation as further advantages. Another commenter recounted their positive experience learning 6502 assembly on an Apple II, highlighting the practical, hands-on nature of learning with older hardware. Someone else chimed in to say that working with limited resources like those on the 6502 encourages efficient coding practices.

  Some commenters suggested alternative starting points for learning assembly. One proposed the Z80, arguing it has a cleaner instruction set than the 6502 while still being relatively simple. They specifically mentioned the orthogonal instruction set design as a significant advantage. Another commenter recommended MIPS assembly, citing its prevalence in education and its clean, RISC-based architecture. They highlighted the availability of good learning resources, especially for beginners.

  A few commenters offered more general advice on learning assembly. One stressed the importance of understanding the underlying hardware, regardless of the chosen architecture. They recommended learning about registers, memory addressing, and the fetch-execute cycle as foundational concepts. Another emphasized the value of practical projects, suggesting that building something, even simple, is crucial for solidifying understanding.

  One user expressed a different perspective, cautioning that learning assembly on older, simpler architectures might not translate well to modern systems. They suggested that while these older architectures are good for understanding fundamental concepts, the complexity of modern systems requires additional learning.

  In summary, while some disagreement existed on the optimal first assembly language, many commenters acknowledged the 6502's merits as a beginner-friendly option due to its simplicity and accessibility. The discussion also highlighted the broader importance of understanding hardware fundamentals and engaging in practical projects when learning assembly language, regardless of the chosen architecture.