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