6502 Assembly Optimization Tips for NES

Master key strategies to boost speed and minimize code size

NES console hardware, 6502 assembly coding on vintage computer

Key Highlights

Speed Enhancements: Optimize loops, jump tables, and branching for faster execution.
Code Size Reductions: Inline subroutines and split data structures for memory efficiency.
General Best Practices: Use profiling, comment extensively, and understand the 6502 architecture.

Introduction

When working with 6502 assembly on the NES, performance and memory usage are at a premium. The limitations of the NES hardware require programmers to employ a range of strategies to ensure code runs efficiently while minimizing memory footprint. This document outlines advanced tips and best practices for optimizing 6502 assembly code on the NES, with a focus on both speed and size. You will find detailed strategies that include loop optimizations, structured jump tables, data organization techniques, and other best practices that are crucial when writing low-level code for this platform.

Optimizing for Speed

Speed is crucial when developing for the NES, as many in-game operations must be carried out within tight time constraints. The 6502 processor is powerful for its time, but it has inherent limitations that require careful tuning.

Efficient Loop Management

Optimizing loops is one of the most effective ways to increase execution speed. A common strategy is to count down instead of up. The advantage of counting down is that the 6502 processor can often use status flags (such as the zero flag) to detect loop termination without issuing a compare instruction. Typically, looping by decrementing a counter toward 0 or -1 helps save extra cycles. Additionally, semi-unrolling loops—where multiple iterations are effectively combined into one—reduces the iterative overhead associated with loop control instructions.

In your assembly code, ensure inline assembly routines do not introduce unnecessary JSR-RTS chains (Jump to Subroutine and Return from Subroutine). If a subroutine is only used once, incorporating its functionality directly into the main code path eliminates the overhead of making the subroutine call, thus saving valuable cycles.

Jump Tables and Conditional Branching

Jump tables are a time-tested method for controlling code flow. Instead of performing multiple comparisons or branching operations, a jump table directs execution using a single indexed jump which can be combined with the RTS instruction. This optimization is particularly beneficial when handling multiple conditions that lead to different branches of code.

Furthermore, optimally positioning branch instructions is vital. Keep the branch targets close to the branch instructions; the NES hardware executes nearby branches faster, reducing the wait time associated with fetching instructions from discontinuous memory spaces. Always analyze your branch distances using profiling tools available in popular emulators like FCEUX.

Using Illegal Instructions

Some advanced compilers and assembly enthusiasts explore the use of "illegal" instructions. These can combine the behavior of two valid instructions into a single operation, offering time savings under highly specific circumstances. It is crucial, however, to ensure that these instructions are well-tested across all target hardware variants, as reliance on non-standard behavior can lead to unpredictability.

Optimizing for Size

Memory is another critical consideration when developing for the NES. With only a limited amount of onboard RAM and ROM available, every byte counts. Optimizing code size not only saves memory but can also contribute to execution speed, as tighter code can improve cache locality and reduce fetch times.

Inlining Subroutines

Rather than calling subroutines with JSR and returning with RTS for one-time operations, it is often more efficient to inline the code. Doing so removes the overhead associated with the call and return sequence. Where code repeatability is low, embedding the routine directly in your main flow can streamline execution. This approach is especially useful in performance-critical routines such as those in the Non-Maskable Interrupt (NMI) period.

Splitting Word Tables

Many NES programs use tables to store multiple 16-bit words. By splitting such tables into separate high and low byte arrays, you can reduce the complexity of working with these words across your program. This organization allows for more efficient memory access and can reduce the size of the compiled code. Managing data in split tables also simplifies arithmetic operations on these values by separating concerns and enabling the optimizer to generate more efficient addressing code.

Utilizing the Stack

The 6502 stack is an underutilized resource that can help reduce local variable usage in your code. By storing temporary values on the stack, you can reduce the burden on primary memory and make your code inherently more size-efficient. The jumping in and out of subroutines often involves stack operations, so ensuring optimal use of this storage can be a double-edged sword—benefiting both speed and reducing overall code size.

General Best Practices and Further Optimizations

Beyond direct performance tweaks, several general best practices in 6502 assembly coding can contribute to both speed and size optimizations.

Addressing Modes and Instruction Selection

Understanding the various addressing modes of the 6502 is crucial. Different addressing modes require varying numbers of cycles and have different memory footprint implications. Carefully select the addressing mode that fits your specific use-case for improved performance. Experiment with zero-page addressing, which is the fastest mode available, whenever possible—especially when working with frequently accessed variables.

Control Flow and Loop Unrolling

Loop unrolling is a valuable technique where you expand the body of a loop to decrease the frequency of loop control operations. By partially unrolling loops, you trade a slight increase in code size for significant improvements in execution speed. For operations that require repetitive tasks in time-critical sections, such as rendering or NMI routines, unrolling can provide the necessary cycle savings.

Leveraging Macros and Code Structuring

Macros can be particularly powerful in assembly language, allowing you to inline frequently used code without the run-time penalty of a subroutine. However, use them judiciously to avoid overly long branch distances and ensure that your macros do not interfere with the structured flow of your program. Clear and consistent commenting is invaluable as your program grows in complexity. Each function and operation should be logically separated, which improves maintainability and assists later optimization passes.

Using Profiling Tools

Profiling is a critical step in any optimization process. Tools like FCEUX and other NES emulators provide debugging capabilities that allow you to measure the performance of your code. Identify hotspots—sections of code that take up a disproportionate amount of time—and apply targeted optimizations. Profiling helps you understand whether inlining subroutines, reordering branches, or unrolling loops offers the best performance gains.

Comparative Analysis: Performance vs. Code Size

A key challenge when optimizing 6502 assembly on the NES is balancing performance with code size. In many cases, an optimization that improves performance might slightly increase code size, while aggressive size optimizations could inadvertently slow down the program. It is essential to evaluate the trade-offs based on the specific needs of your NES project.

Performance Optimizations

Performance techniques like counting down in loops and using jump tables can greatly enhance the speed of your code. For example, using the RTS-based jump tables avoids the overhead of a conventional JSR/RTS chain, allowing you to save critical cycles during runtime. Furthermore, efficient branch placement reduces delays caused by longer branch distances.

Another strategy involves restructuring your control flow to minimize redundant operations. By avoiding unnecessary comparisons or intermediate storage, you reserve processor cycles for the core functionality of your game or application.

Size Optimizations

On the other hand, size optimizations require a different set of techniques. Splitting word tables and inlining subroutines can lead to a more compact binary, which is often necessary given the limited memory available on an NES cartridge. Utilizing the stack for temporary storage not only reduces RAM usage but can also help in reducing the overall binary size when used appropriately.

It is also helpful to avoid redundant code. In addition to inlining, always review your code for operations that can be combined or optimized. Bitwise operations such as AND, OR, and XOR are generally more efficient than their logical counterparts and should be used wisely.

A Comprehensive Comparison Table

The following table summarizes the primary techniques for both performance and size optimizations in 6502 assembly programming on the NES:

Optimization Area	Techniques	Benefits
Speed	Count down loops Semi-unrolled loops Jump tables with RTS Optimized branch instructions	Faster execution, reduced cycle counts
Size	Inline subroutines Split word tables Effective stack usage Avoid redundant code	Smaller code footprint and efficient memory usage
General	Use macros wisely Profile with debugging tools Choose optimal addressing modes Comment and structure code	Improved maintainability and overall efficiency

Additional Considerations

Beyond the primary strategies, several additional best practices can further elevate your 6502 assembly programming on the NES:

Use of Macros

Macros provide a convenience that can reduce code repetition. By inlining frequently used operations, the code not only becomes more elegant but also reduces the overhead associated with function calls. However, it is important to verify that the inline code does not exceed branch distance limitations or interfere with other timing-sensitive parts of your code.

Testing and Profiling

Implement a robust testing strategy using emulators and profiling tools. Tools such as FCEUX offer cycle counting and debugging capabilities, allowing you to pinpoint performance bottlenecks. Always validate your optimizations by checking that your changes do not compromise game logic or cause unintended side effects.

Data Management

Effective memory and data management can have a profound impact on performance. Organize data into structures that facilitate rapid access, and use well-planned memory layouts to reduce the number of needed instructions to access data. By splitting word tables and using separate arrays for each field in composite data types, you can maximize the efficiency of your memory accesses.

Balancing Trade-offs

In low-level programming, optimizations are rarely free; improvements in one area can sometimes lead to compromises in another. Always analyze whether the performance benefits of an optimization justify any increase in code size. Evaluate your project requirements carefully and choose the optimizations that align with your overall development goals. Employ iterative development cycles, allowing you to refine optimizations by continuously profiling and benchmarking your code.

Tools and Resources

Leveraging the right tools can make a significant difference in the optimization process. Here are some tools you may find indispensable:

Assemblers and Emulators

Popular assemblers such as NESASM3, ca65, and ld65 are essential for compiling your code. Each has its own quirks, but they provide robust support for debugging and optimization. Emulators like FCEUX allow you to simulate NES hardware, providing insights into cycle counts and enabling real-time debugging.

Community Forums and Wikis

The NES development community is rich with shared knowledge and best practices. Resources like the NESdev Wiki, AtariAge Forums, and various Reddit threads offer practical tips, detailed explanations, and sample code snippets that can further your understanding of both basic and advanced optimization techniques.

Benchmarking Tools

Always profile your code. Benchmarking tools help determine which sections of your code are the most critical. Whether you are trying to improve frame rates or reduce load times, these tools give quantitative data that guide your optimization efforts.